Department of Physics • University of Cologne

Antibiotics are a mainstay of modern medicine, but there is huge global concern about the emergence of antibiotic resistant infections. Despite their importance, we understand surprisingly little about how antibiotics actually inhibit bacterial infections. This is a complex problem because of the interplay between antibiotic action and bacterial growth, which connects with physics via nonlinear dynamics, mechanics and stochasticity. I will discuss, with examples, how bacteria grow in different scenarios. I will further discuss how statistical physics approaches can provide new understanding of how antibiotics work in the context of growing bacteria, with implications for antibiotic resistance evolution.

In solid-state matter, topological properties of the band structure can enforce the appearance of conducting one-dimensional edge states at the boundaries of the material, while the bulk remains insulating. These states are a fascinating playground to study the physics of one-dimensional quantum coherent conductors. They also provide a support system for ‘topological quantum bits’, robust against local perturbations, and envisioned to solve decoherence problems in the realization of quantum computers. I will discuss how one can utilize GHz excitations and current correlations to generate, manipulate and study excitations in these systems down to the elementary level of single electrons.

In the NISQ era, we are looking to approach quantum advantage with shallow algorithms, which are often variational in nature. In those cases, proofs of speedup are hard to come by and a lot of research is empirical. A particularily relevant case here is the Quantum Approximate Optimization Algorithm (QAOA). I will present its semiclassical limit, mean-field QAOA, which can be classically simulated and present a criterion for its applicability, hence allowing to identify cases when full QAOA is not promising. Another relevant case is the study of quantum manybody systems with digital algorithms. Here, I will show an improved, physics-inspired algorithm to sample the key observable for studying theses systems, the single-particle Green’s function. Work with Dmitry Bagrets, Aditi Misra-Spieldenner, Tobias Stollenwerk, Tim Bode, Peter Schuhmacher, David Headley, and Gino Bishop

We are all made from nuclear matter, with carbon, nitrogen and oxygen being among the most essential to form all life on earth. Although Earth’s complex organic life is dictated by the rules of biology and organic chemistry, the fact that these elements are available in large quantities is a direct consequence of their nuclear properties. Nuclear astrophysicists have a detailed understanding about how lighter elements are formed by nuclear fusion reactions in dying stars. Thus, Carl Sagan was correct when he famously said, “We are made of starstuff.” Fusion reactions are however not able to explain the creation of heavier elements, whose exact origin is a complex puzzle with many open questions. In this presentation I will provide an overview about current experimental efforts in the field of explosive nucleosynthesis. I will discuss the origin of the rare 60Fe isotope detected via Accelerated Mass Spectrometry at the University of Cologne, as well as our efforts to enhance the sensitivity for detection of this key isotope in the future. I will also show recent results from Radioactive Ion Beam facilities which enable us to study neutron-rich nuclear matter, playing a pivotal role in neutron star merger events.

We explore the use of methods from statistical inference to study Bell nonlocality

Unconventional superconductivity with high critical temperatures, topologically non-trivial phases, frustrated magnetism, spin-liquids or the intensively discussed Kitaev phases are a few examples of exotic states in quantum materials. One of the big challenges in quantum physics is the microscopic description of such systems. Moreover, being able to understand them implies the possibility of predicting compounds with desirable properties. In this talk, I will present and discuss strategies for designing quantum materials from first principles and by using statistical methods, and will motivate their possible use for present technological applications.

In the past few years, van der Waals heterostructures have emerged as a new platform for studying frustrated magnetic systems, both in structures with layers exhibiting intrinsic magnetism, as well as "Hubbard model simulators" such as moiré transition metal dichalcogenides (TMD). I will first discuss bilayer U(1) Dirac spin liquids, with the homogenously stacked system being unstable towards spontaneous symmetry breaking. We show that adding a relative twist between the two layers softens the ordering instability, and the resulting ordered phase features a tuneable lattice of magnetic vortices. In the second part of my talk, I will present ongoing work on correlated insulating states at fractional fillings in moiré TMD bilayers, where the formation of self-organized charge lattices has been observed experimentally. We describe these ordered states and explore the possibility of spin-liquid states within a slave-rotor mean-field theory.

We generalize the notion of quantum state designs to infinite-dimensional spaces. We first prove that, under the definition of continuous-variable (CV) state t-designs from Comm. Math. Phys. 326, 755 (2014), no state designs exist for t≥2. Similarly, we prove that no CV unitary t-designs exist for t≥2. We propose an alternative definition for CV state designs, which we call rigged t-designs, and provide explicit constructions for t=2. As an application of rigged designs, we develop a design-based shadow-tomography protocol for CV states. Using energy-constrained versions of rigged designs, we define an average fidelity for CV quantum channels and relate this fidelity to the CV entanglement fidelity. As an additional result of independent interest, we establish a connection between torus 2-designs and complete sets of mutually unbiased bases.

Memory formation and ageing are abundant in many soft matter systems. The disorder underlying these systems gives rise to a rich energy landscape, consisting of a large number of metastable states. These landscapes are accompanied by a plethora of pathways, along which such systems can evolve when exposed to a varying temperature or mechanical load. The resulting dynamics can be rather complex, giving rise to dynamically critical phenomena such as irreversibility and yielding. At the same time, such system exhibit parallels with the adaptive evolution of biological populations in time-varying environments. In this talk I will present a general framework to analyze the dynamics and memory formation of driven disordered systems. I will then show how this framework can be applied to understand both the response of a cyclically sheared amorphous solid, as well as the antibiotic resistance evolution of a microbial population subject to time-varying drug concentrations.

Device-independent quantum cryptography In this talk I will introduce the device-independent framework, and explain how the violation of a Bell inequality can be used to certify the security of cryptographic protocols. I will discuss how device-independent cryptography leads to security even when the underlying system and measurement setups are not well characterized or provided by an untrustworthy provider. Finally I will comment on the theoretical and experimental challenges and the progress in implementing device-independent quantum key distribution.

Integrable systems typically exhibit non-generic transport properties. The spin 1/2 Heisenberg model is particularly rich, with ballistic transport of energy and different regimes of spin transport, including ballistic, diffusive, and super-diffusive ones. I will discuss the fate of the diffusive and super-diffusive regimes under different types of Hamiltonian perturbations and show that symmetry of perturbations can play an important role. For example, perturbations that respect the SU(2) symmetry of the isotropic Heisenberg model can cause anomalous transport behaviour even away from exact integrability. In the second part, I will focus on Markovian perturbations and propose a new Krylov space-type approach for an efficient steady-state description of weakly driven and open, nearly integrable systems. Lindblad dissipator will be used to add the most necessary conservation laws iteratively and thus increase the interpretability of stabilized non-thermal steady states.

Extensive quantitative experiments in the past decades have established simple empirical laws of resource allocation obeyed by exponentially growing bacteria subjected to different environmental or genetic perturbations. Combinations of these laws, together with their kinetic extensions, have led to quantitative account of a number of long-standing phenomena in microbiology. In this talk, I will briefly recount how we embarked on the path of top-down phenomenological studies, exhibit the striking universality of the law of ribosome allocation across diverse bacterial lineages, and describe a recent study establishing how this law is implemented molecularly via simple regulatory processes. The findings provide a rare view of “dimensional reduction” by a living cell, i.e., how a cell manages to collapse the complex, high-dimensional dynamics of metabolic reactions underlying cell growth to quantitatively “perceive” the growth rate, and allocate resources in accordance to the growth rate. Overall, these studies showcase how the basic methodology of classical physics can be used to discover simple organizing principles of living systems and construct quantitative, predictive theories linking molecules to cellular behaviors.

A remarkable feature of Weyl semimetals (WSMs) is Fermi arcs - anomalous surface states that connect Weyl nodes of opposite chirality. At the interface betweem two WSMs, they can instead connect Weyl nodes of the same chirality, leading to perfect transmission of electrons across the interface. In this talk, I will describe some recent work on the magnetic breakdown of electron transport in this setup. For an interface between WSMs twisted relative to one another, we show that a longitudinal magnetic field leads to a suppression of conductance owing to Landau-Zener tunneling between the interface modes of opposite chirality.

Quantum Repeaters for Quantum Communication Quantum channels enable the implementation of communication tasks inaccessible to their classical counterparts. The most famous example is the distribution of secret key. However, in the absence of quantum repeaters, the rate at which these tasks can be performed is dictated by the losses in the quantum channel. In practice, channel losses have limited the reach of quantum protocols to short distances. Quantum repeaters have the potential to significantly increase the rates and reach beyond the limits of direct transmission. This talk will provide a high-level overview of different types of the proposed quantum repeater architectures. We will then look at the specific techniques that these schemes utilise in order to achieve long distance quantum communication, analyse their strengths and limitations as well as discuss the required hardware needed to implement them. Finally, I will introduce a novel type of quantum repeater schemes which make efficient use of the quantum communication channels by encoding quantum information in the bosonic error-correcting codes.

Topological spin textures, such as two-dimensional skyrmion lattices (SkLs) and three-dimensional hedgehog lattices (HLs) are approximately represented by superpositions of multiple spin density waves, and hence, called multiple-Q spin textures. In such spin structures, the phase degree of freedom of the superposed waves plays an important role in the topological properties as well as the symmetry of the magnetic textures, but the systematic investigation has not been performed thus far. In this study, we theoretically investigate the evolution of magnetic and topological properties in SkLs composed of three spin density waves (3Q-SkL) and HLs composed of four spin density waves (4Q-HL) while changing the phases as well as the magnetization. By introducing a hyperspace to deal with the phases systematically, we find that the 3Q-SkLs change their skyrmion number among -2, -1, 0, 1, and 2 depending on the phase and magnetization. In the case of the 4Q-HLs, we clarify that the density and configuration of topological objects called hedgehogs and antihedgehogs change in a wide range. Our results provide good references to discuss how the actual systems experience the magnetic and topological transitions in an applied magnetic field.

Since its birth roughly 60 years ago, the field of Nuclear Astrophysics strives to provide a comprehensive description of element synthesis in the Universe. While parts of stellar nucleosynthesis are well understood, others remain elusive to this day. Especially, the production of elements heavier than iron has been one of the major open questions in the field. Multiple processes and stellar sites contribute to the complex puzzle of heavy element production. Interpreting the astronomical observations requires the understanding of the nuclear processes that drive stellar explosions. This colloquium will focus on the critical nuclear properties needed to explain heavy element nucleosynthesis. I will discuss recent experimental results, as well as new initiatives and future plans at the next generation rare isotope facility, the Facility for Rare Isotope Beams (FRIB) at Michigan State University.

Twisted moire materials have gained much attention since the recent discovery of superconductivity in twisted bilayer graphene. These materials offer an exciting new experimental platform to study the physics of strongly correlated electrons. The moire Hubbard model describes correlations in certain homobilayer twisted transition metal dichalcogenides. In this talk, I will present our recent results on the essential properties of this system. Using exact diagonalization and density matrix renormalization group methods, we find magnetic Mott insulating and metallic phases, which, upon doping exhibit intertwined charge and spin ordering and, in some regimes, pair binding of holes. The phases are highly tunable via an interlayer potential difference. Remarkably, the hole binding energy is found to be highly tunable revealing an experimentally accessible regime where holes become attractive. Moreover, I will present a novel approach to diagnosing superconducting Cooper condensation based on the structure of the two-body density matrix. I find that a superconducting condensate can be fragmented into partial condensates living on the charge stripes of a system. References: [1] Tunable Stripe Order and Weak Superconductivity in the Moire Hubbard Model, A. Wietek, J. Wang, J. Zang, J. Cano, A. Georges, A. Millis, Phys. Rev. Research 4, 043048 (2022) [2] Fragmented Cooper pair condensation in striped superconductors, A. Wietek, Phys. Rev. Lett. 129, 177001, Editors suggestion (2022)

We prove a generic spin-statistics relation for the fractional quasiparticles that appear in quantum Hall planar setups. The proof is based on an efficient way for computing the Berry phase acquired by a generic quasiparticle translated in the plane along a circular path, and on the crucial fact that once the gauge-invariant generator of rotations is projected onto a Landau level, it fractionalizes among the quasiparticles and the edge. Using these results we define a measurable quasiparticle fractional spin that satisfies the spin-statistics relation. As an application, we predict the value of the spin of the composite-fermion quasielectron proposed by Jain; our numerical simulations agree with that value. We also show that the difficulties encountered when studying the statistical properties of Laughlin’s quasielectron can be traced back to a spin that does not satisfy the spin- statistics relation. We conclude with some remarks on the total angular momentum of the system as a function of the quasiparticle positions and on an ambiguity in the definition of the spin. Further results on the non-Abelian anyons of the Moore-Read wavefunction will be presented. References [1] A. Nardin, E. Ardonne and LM, unpublished [2] Comparin, Biella, Opler, Macaluso, Polychronakos, LM, PRB 105 085125 (2022) [3] Macaluso, Comparin, LM, Carusotto, PRL 123 266801 (2020)

In its many variants, randomized benchmarking (RB) is a broadly used technique for assessing the quality of gate implementations on quantum computers. A detailed theoretical understanding and general guarantees exist for the functioning and interpretation of RB protocols if the gates under scrutiny are drawn *uniformly* at random from a compact group. In contrast, many practically attractive and scalable RB protocols implement random quantum circuits with local gates randomly drawn from some gate-set. Despite their abundance in practice, for those *non-uniform* RB protocols, general guarantees under experimentally plausible assumptions are missing. In this work, we derive such guarantees for a large class of RB protocols for random circuits that we refer to as *filtered RB*. Prominent examples include linear cross-entropy benchmarking, character benchmarking, Pauli-noise tomography and variants of simultaneous RB. Building upon recent results for random circuits, we show that many relevant filtered RB schemes can be realized with random quantum circuits in linear depth, and we provide explicit small constants for common instances. We further derive general sample complexity bounds for filtered RB. We show filtered RB to be sample-efficient for several relevant groups, including protocols addressing higher-order cross-talk. Our theory for non-uniform filtered RB is, in principle, flexible enough to design new protocols for non-universal and analog quantum simulators. On the technical side, we develop a general perturbative description of noise in random circuits in terms of harmonic analysis that can also be used to analyze the noise-robustness of random circuit protocols beyond RB.

The mitigation of climate change requires a comprehensive transformation of our energy system. Power plants based on fossil fuel must be replaced by renewable energy sources, which challenges the operation and stability of the electric power system. In my talk, I will review the key challenges and discuss how methods and ideas from theoretical physics can contribute to their solution. I will focus on two topics: (i) Renewable power fluctuates on many time scales, making it increasingly difficult to balance generation and load. Methods from stochastic time series analysis are essential to quantify these fluctuations and to understand their impact on power system operation. (ii) Damages of transmission and generation infrastructures are the biggest threat for system stability. Network science helps to understand how failures spread and enables the design of resilient grid structures.

Liquids are usually described within classical physics, whereas solids require the tools of quantum mechanics. I will show how in nanoscale systems this distinction no longer holds. At these scales, liquid flows may in fact exhibit quantum effects as they interact with electrons in the solid walls. I will first discuss the quantum friction phenomenon, where charge fluctuations in the liquid interact with electronic excitations in the solid to produce a hydrodynamic friction force. Using many-body quantum theory, we predict that this effect is particularly important for water flowing on carbon-based materials, and we obtain experimental evidence of the underlying mechanism from pump-probe terahertz spectroscopy. I will then show how the theory can be pushed one step further to describe hydrodynamic Coulomb drag - the generation of electric current by a liquid in the solid along which it flows. This phenomenon involves a subtle interplay of electrostatic and electron-phonon interactions, and suggests strategies for designing materials with low hydrodynamic friction.

Active matter is matter driven out of equilibrium by its microscopic constituents. Common examples include the cytoskeleton of the cell, bacteria swimming in a fluid, or herds of wildebeest on the African planes. In these systems, the constituents create forces and motion driving the system out of equilibrium. But now imagine cells dividing or a tumor growing. Then activity does not enter the dynamics via the forces, but by material conservation. The material generates itself. In order to grow, cells must exert mechanical pressure on the neighboring tissue. In turn, mechanical stress influences growth and may play a role in cell competition. This insight has led to the notion of homeostatic pressure – the pressure exerted by a tissue in homeostasis. We use particle based computer simulations to model growing tissues. Surprisingly, we find that when cross interactions are taken into account tissues with different homeostatic pressure can coexist and the evolution can favor the weaker tissue, or even results in tumor heterogeneity.

I will introduce a new type of symmetry-breaking order called "Incommensurate Kekule Spiral (IKS)" order, and argue that it is ubiquitous in the experimentally observed phase diagram of magic-angle graphene

As part of the "Meet The Alumni" lecture series, our former PhD student Timo Knispel gives insights into his recent start at his new employer d-fine. d-fine is a European consulting firm with a focus on quantitative topics and the development of future-proof technological solutions. The consulting approach of d-fine is based on many years of practical experience and around a team with a strong analytical and technological background. A combination that has proven its worth through tailored, efficient and sustainable implementations for more than two hundred clients from all economic sectors. Together with his colleague Thomas Bschorr, Timo will give us exciting insights into the project work and his start at d-fine as well as answer any questions you might have. Afterwards, Timo and Thomas will be available for further questions and in-depth discussions in the get together with drinks and snacks.

Workshop MENTAL HEALTH IN MASTER/PHD STUDIES - CHALLENGES AND COPING STRATEGIE

There are few candidates for quantum algorithms which might outperform classical approaches while being amenable to noisy quantum computing devices. Beside quantum simulation algorithms, quantum algorithms for approximate optimization are promising candidates. The target application for these approaches is finding approximate solutions to combinatorial optimization problems. In this talk I will give an overview on quantum approximate optimization approaches, while mainly focusing on the quantum approximate optimization algorithm (QAOA), quantum annealing and variations thereof. I will demonstrate these concepts by considering a particular real-world planning problem, the flight-gate assignment problem [1]. In addition, I will cover novel approaches for the diagrammatic analysis of parametrized quantum circuits, like QAOA. Here I will demonstrate how such methods can be employed to help with the performance investigation of QAOA and other variational quantum algorithms. [1] https://ieeexplore.ieee.org/document/9222273 [2] https://arxiv.org/abs/2204.01307

Several lattice models display a condensation transition in real space when the density of a suitable order parameter exceeds a critical value. At equilibrium there are well established results and we discuss different scenarios of the condensation transition. In the out-of-equilibrium setup we consider one of such model with two conservation laws, attached to two external reservoirs, R_1 and R_2. Both reservoirs impose subcritical boundary conditions: When R_1=R_2 the system is in equilibrium below the localization threshold and no condensate appears. Instead, when R_1 differs from R_2 localization may arise in an internal portion of the lattice. This phenomenon is due to the coupled transport of two conserved quantities. In collaboration with Gabriele Gotti and Stefano Iubini.

The rich phase diagrams of many transition metal oxides (TMOs) is the result of the intricate interplay between electrons, phonons, and magnons. This makes TMOs very susceptible to external parameters such as pressure, doping, magnetic field, and temperature which in turn can be used to finely tune their properties. The same susceptibility makes TMOs the ideal playground to design experiments where the interaction between tailored electromagnetic fields and matter can trigger the onset of new, sometimes exotic, physical properties. This aspect has been explored in time domain studies [1] and has led to the demonstration that ultrashort mid-IR light pulses can “force” the formation of quantum coherent states in matter, disclosing a new regime of physics where thermodynamic limits may be bridged and quantum effects can, in principle, appear at ambient temperatures. In this presentation, I will review our recent results in archetypal strongly correlated cuprate superconductors and demonstrate the feasibility of a light-based control of quantum phases in real materials [2,3,4]. I will then introduce our new approaches to time domain spectroscopy going beyond mean photon number observables [5-10] and show that the statistical features of light can provide richer information than standard linear and non-linear optical spectroscopies[11,12]. Finally, I will elaborate on the possibility offered by embedding complex materials into resonant optical cavities[13] and show how properties for the light-matter assembly different from those of its constituents can emerge. [1] Advances in physics 65, 58-238, 2016 [2] Science 331, 189-191 (2011) [3] Phys. Rev. Lett. 122, 067002 (2019) [4] Nature Physics 17, 368–373 (2021) [5] Phys. Rev. Lett. 119, 187403 (2017) [6] New J. Phys. 16 043004 (2014) [7] Nat. Comm. 6, 10249 (2015) [8] Nat. Comm. 13, 2667 (2022) [9] J. of Physics B 53, 145502 (2019) [10] Optics Letters 45, 3498 (2020) [11] Light: Science & Applications, 11, 44 (2022) [12] PNAS March 19, 116 (12) 5383-5386 (2019) [13] Rev. Sci. Inst. 93, 033102 (2022)

Main subject I will talk is ultrafast photocurrent generation and transport in high-quality graphene photodetector. The current generated by femtosecond laser pulse through photo-thermoelectric effect was detected in time domain with the resolution of faster than 1 picosecond by on-chip THz spectroscopy. We could comprehensively understand how incident light is converted to electrical current in graphene, which is crucial information for developing ultrafast graphene optoelectronics. I will also briefly talk about two other subjects in our group: Growth and transport in InAs/InGaSb two-dimensional topological insulator, and Coherent transport of fractional quasiparticle at fractional-integer quantum Hall junction. Reference: Yoshioka, NK et al., Nature Photonics (accepted), arXiv:2203.05752

In this talk, I will present recent results on the statistical properties of active particles. I will focus on the run-and-tumble particle (RTP) model, which describes the persistent motion of a class of bacteria, including E. coli. In the first part of the talk, I will show that many properties of this model, including the survival probability and the record statistics, are completely universal, i.e., independent of the system's dimension and the speed fluctuations of the particle. In the second part of the talk, I will investigate the position distribution of a single RTP at late times. I will show that, under certain conditions, a condensation transition can be observed in the large-deviation regime where the particle is far from its starting position.

Huang, Kueng, Preskill introduced the learning task now known as “classical shadows”: given few copies of an unknown state ρ, construct a classical description of the state from independent measurements that can be used to predict certain properties of the state. Specifically, they show Θ(B/epsilon^2) samples of ρ suffice to approximate the expectation value Tr(Oρ) of any Hermitian observable O to within additive error epsilon provided Tr(O^2) ≤ B and the eigenvalues of O are contained in [-1,1]. We consider the task of constructing classical shadows with joint measurements and pure unknown states. We show Θ(√B/epsilon + 1/epsilon^2) copies are necessary and sufficient to construct classical shadows in this setting. https://uni-koeln.zoom.us/j/99938061817?pwd=bk5hemQzbmY4ZVd6dFFhTnlsb0lEUT09

When sufficiently deformed, amorphous solids exhibit irreversible, or plastic, response and eventually fail. Although such behaviour is broadly common to all solids, amorphous solids, or glasses, display disordered microscopic structure, leading to significant differences in how these phenomena may be understood, and to distint statistical mechanical approaches to address them. After a brief overview of glass phenomenology, recent approaches to yielding of amorphous solids is described, with specific focus on the response to cyclic shear deformations. Results from computer simulations of model glasses, as well as theoretical approaches and results will be discussed.

Floquet or periodically driven systems show topological phases that are qualitatively different from their static counterparts. In this talk I will first introduce the new kinds of topological phases that can be realized in free-fermion Floquet systems. I will then show that the edge modes encountered in certain free fermion Floquet systems are remarkably robust to adding interactions, even in disorder-free systems where generic bulk quantities can heat to infinite temperatures due to the periodic driving. This robustness of the edge modes to heating can be understood in the language of strong modes for free fermion chains, and almost strong modes for interacting chains. I will then outline a tunneling calculation for extracting the long lifetimes of these edge modes by mapping the Heisenberg time-evolution of the edge operator to dynamics of a single particle in Krylov subspace.

https://uni-koeln.zoom.us/j/95031559873?pwd=d1gwRlhlaXYrK085S0lqU295ZlNkZz09 Meeting ID: 950 3155 9873 Password: 745603

(Antrittsvorlesung) Remote attendance possible via: https://uni-koeln.zoom.us/j/7641714854 Kennwort: 209699

Within the description of correlation functions in dense fluids, the dynamical laws arise from glass-transition singularities. The simplest such singularity yields a master function with a two-step decay. Higher-order singularities allow for logarithmic decay laws, while multiple singularities give rise to complex master functions. While the scenarios with multiple singularities appear relevant for colloids, liquid water, and granular matter, the simple two-step scenario may be identified within simple models that allow for analytical calculations.

Graphene--the one-atom-thick sheet of carbon--is the most famous of 2D materials due to its unique electronic properties and mechanical strength. However, its chemical inertness makes graphene also an excellent nearly electron-transparent support for other materials and nanostructures. In this presentation, I will give an overview of our recent work enabled by a unique interconnected vacuum system containing an aberration-corrected scanning transmission electron microscope (Nion UltraSTEM 100) with a unique objective area that allows sample cleaning via laser, in situ chemical experiments, and direct vacuum transfer to an atomic force microscope (AFSEM by GeTEC Microscopy), an argon glove box, target chamber with a plasma ion source and evaporators, and long term vacuum sample storage. In this presentation, I will demonstrate that defect-engineering of graphene enables its substitutional heteroatom doping and growth of nanoclusters with a well-defined concentration and a narrow size distribution, as well as the direct correlation of its atomic structure and mechanical properties.

In this talk, I will discuss some of our recent results on the quantum spin Hall effect in HgTe-based two-dimensional topological insulators. In the first part, I will show that the conductance quantization due to the quantum spin Hall effect can be observed even in the presence of magnetic impurities. Our experiments on (Hg,Mn)Te quantum wells with an inverted band structure reveal that the quantum spin Hall quantization is observed only at low temperatures (T < 400 mK), where Kondo screening of the magnetic impurities suppresses backscattering. The second part of the talk will focus on understanding the fluctuations in the quantized spin Hall conductance that has been observed in almost all realizations of the quantum spin Hall effect - including the first paper in 2007. To examine these fluctuations, we fabricate high-quality quantum spin Hall microstructures using a carefully optimized wet-etching process. We perform temperature and gate-dependent measurements in the regime of quantized conductance. The fluctuations in conductance show a characteristic temperature dependence that is related to Kondo interactions of helical edge channels with small puddles. These charge puddles - acting as Kondo correlated quantum dots due to small dimensions - arise due to fluctuations in the potential landscape in narrow gap semiconductors.

I will discuss the effects of local measurements on critical quantum ground states. These states are highly entangled and feature algebraic correlations between local observables. As a consequence, local measurements can have highly nonlocal effects. Our focus is on Tomonaga-Luttinger liquid (TLL) ground states, a continuous family of critical states in one dimension whose structure is parameterized by a Luttinger parameter K. We show that arbitrarily weak local measurements, performed over extended regions of space, can conspire to drive transitions in long-wavelength correlations. Conditioning first on a particular measurement outcome we show that there is a transition in the character of the resulting quantum state for K<1, and highlight a formal analogy with the effect of an impurity on transport. To study the full ensemble of measurement outcomes we consider averages of physical quantities which are necessarily nonlinear in the system density matrix. We show how their behaviour can be understood within a replica field theory, and for the measurements that we consider we find that the symmetry of the theory under exchange of replicas is broken for K<1/2. For K<1/2 there is a suppression of quantum fluctuations of measured observables which becomes more prominent at long wavelengths.

We review some recent results on the development of efficient unconstrained tree tensor network algorithms and their applications to high-dimensional many-body quantum systems and machine learning problems in High Energy Physics. In particular, we present results on topological two-dimensional systems, two dimensional Rydberg atom systems, and two and three-dimensional lattice gauge theories in presence of fermonic matter. Finally, we present their application to LHCb event classification and to the study of open many-body quantum systems, specifically to the computation of the entanglement of formation in critical many-body quantum systems at finite temperature.

(Antrittsvorlesung) Remote attendance possible via: https://uni-koeln.zoom.us/j/7641714854 Kennwort: 209699

Remote attendance possible via: https://uni-koeln.zoom.us/j/7641714854 Kennwort: 209699

Remote attendance possible via: https://uni-koeln.zoom.us/j/7641714854 Kennwort: 209699

The SARS-CoV-2 pandemic has resulted in unprecedented genomic and epidemiological surveillance as well as virological characterization in record time. The volume of data and requirement for rapid feedback to decision makers required continuous data analysis and method development to make sense of the data. Multiple variants of SARS-CoV-2 have arisen independently from each other and swept the globe over the course of two years. In contrast, endemic RNA viruses we often see co-circulation of variants and gradual emergence of one variant from another. This endemic circulation is characterized by an approximate equilibrium between the build-up and decay of immunity. I will discuss models of the transition from pandemic to endemic circulation, the possibility of speciation, and the drivers of antigenic evolution in SARS-CoV-2 and other viruses. Remote attendance possible via: https://uni-koeln.zoom.us/j/7641714854 Kennwort: 209699

Understanding the response of a disordered solid to an externally imposed forcing or deformation is important in order to characterize the transitions between rigid and flowing states in a wide variety of soft matter systems, such as the yielding transition of an amorphous solid under shear. Such complex phenomena are already present in the "AQS" regime where thermal effects are negligible, the response to forcing is quasi-static, and the dynamics proceeds via triggering of mechanical instabilities. A key insight underlying my recent research has been the observation that the AQS conditions permit a rigorous representation of the driven dynamics in term of a directed state-transition graph. This AQS graph represents the response of the system to any deformation protocol, providing thereby a bird's-eye (or map-like) view of all the possible dynamics, which in turn is encoded in the topology of the AQS graph. At the same time, the AQS graph is a discrete random structure that lends itself to a mathematical treatment. In this talk I will describe the energy landscape of a sheared amorphous solid via AQS transition graphs, highlighting key features, such as hysteresis, and showing how various topological properties of this graph can be captured in terms of stochastic models.

Topological edge states are protected states that form between two insulators with different topological properties. The dimensionality of the interface, as well as the type of insulators that form it, endow the topological edge state further properties. I will focus on situations where one of the materials is a superconductor --a quasiparticle insulator. In the case where the edge is zero dimensional, the topological states are Majorana zero-modes localized in the core of a vortex or bound to the end of a nanowire. They are anyons with non-Abelian braiding statistics, but when they are immobile, one cannot demonstrate this by exchanging them in real space and therefore indirect methods are usually needed. I will talk about how to use the chiral motion along the boundary of the superconductor to braid a mobile vortex, either with another mobile vortex in the edge channel or with an immobile vortex in the bulk. When two vortices are fused, they transfer charge into a metal contact. We calculate the time dependent current profile for the fusion process, which consists of ±e/2 charge pulses that flip sign if the world lines of the vortices are braided prior to the fusion. This is an electrical signature of the non-Abelian exchange of Majorana zero-modes. Next, I will discuss how one can deconfine the Majorana zero-modes localized in the vortex cores by applying a spatially oscillating pair potential. In the deconfined phase at zero chemical potential the Majorana fermions form a dispersionless Landau level, protected by chiral symmetry against broadening due to vortex scattering. The Majorana Landau level also has a signature that distinguishes it from a regular Landau level: the coherent superposition of electrons and holes in the Majorana Landau causes a local density of states oscillation with a wave vector set by the Cooper pair momentum as well as the inverse coherence length. This striped density of states pattern also provides a means to measure the chirality of the Majorana fermions. I will conclude by discussing the quantal dynamics in the deconfined phase.

Extracting important information from a quantum system as efficiently and tractably as possible is an important subroutine in most quantum technologies. We present an efficient method for constructing an approximate classical description of a quantum state using very few measurements of the state. This description, called a classical shadow, can be used to predict many different properties. The required number of measurements is independent of the system size and saturates information-theoretic lower bounds [arXiv:2002.08953]. These quantum-to-classical converters pave the way for new synergies between (near-term) quantum computing and classical machine learning [arXiv:2106.12627] in the context of quantum many-body physics. Conversely, instances where classical shadows fail constitute promising candidates for new types of quantum advantage (quantum-enhanced learning) [arXiv:2112.00778].

Predicting the fate of an interacting system in the limit where the electronic bandwidth is quenched is often highly non-trivial. The complex interplay between interactions and quantum fluctuations driven by the band geometry can drive a competition between various ground states, such as charge density wave order and superconductivity. In this work, we study an electronic model of topologically-trivial flat bands with a continuously tunable Fubini-Study metric in the presence of on-site attraction and nearest-neighbor repulsion, using numerically exact quantum Monte Carlo simulations. By varying the electron filling and the spatial extent of the localized flat-band Wannier wavefunctions, we obtain a number of intertwined orders. These include a phase with coexisting charge density wave order and superconductivity, i.e., a supersolid. In spite of the non-perturbative nature of the problem, we identify an analytically tractable limit associated with a `small' spatial extent of the Wannier functions, and derive a low-energy effective Hamiltonian that can well describe our numerical results. If time permits, I will also discus some preliminary quantum Monte Carlo results for twisted bilayer graphene at charge neutrality.

Topological phases of matter have been the subject of intense interest since the discovery of the fractional quantum hall effect. More recently they have come to underpin leading approaches to quantum error correction. To date, the focus has remained on topological phases in two dimensions and the anyon models that describe them. In three dimensions our understanding is significantly less developed, the existence of immobile quasiparticles known as fractons opens up a landscape of intricate possibilities. The immobility of fractons leads to new possible symmetry actions, subdimensional phase transitions, and glassy dynamics that can be harnessed to produce more robust quantum codes. In this talk I will give an overview of recent efforts to develop a systematic understanding of fracton topological phases and what this might tell us about three dimensional topological codes.

Zoom PW: 573506

Laser-induced optical excitation of hydrogen-like alkaline and alkalineearth atoms can be used to create arrays of highly controllable Rydberg states, which are promising candidates for atom-based quantum computation and for realizing new many-body phenomena. In a gas of atoms, the interplay between the laser-drive and strong interactions between Rydberg states yields new cooperative effects, including either the complete blocking or the facilitation of excitations by a nearby Rydberg state. I will discuss theoretical and experimental approaches to the so-called facilitation regime, in which the creation of excitations is conditioned on the presence of a nearby Rydberg state. This gives rise to a kinetically constrained dynamics, supporting the avalanche-like spreading of excitations and a non-equilibrium phase transition into an absorbing state with zero excitations. The interplay of interactions, laserdrive, dissipation and atomic motion, each acting on their individual timescale, gives rise to a multifaceted dynamics. It ranges from an epidemiclike spreading of excitations on an emergent complex network at short times to a long-lived critical regime of repeated scale invariant excitation avalanches, reminiscent of self-organized criticality, at late times.

The average affinity of specific antibodies increases dramatically over the course of an immune response. This increase is the result of a Darwinian process in which B lymphocytes undergo iterative cycles of random hypermutation of their immunoglobulin genes, followed by selective proliferation of clones bearing affinity- enhancing mutations. This evolutionary process takes place in highly dynamic microanatomical structures known as germinal centers, which arise within secondary lymphoid organs upon infection or immunization. Our work combines intravital multiphoton microscopy with mouse genetics to study how the dynamics of B and T lymphocytes within germinal centers shapes the evolution of the high-affinity antibodies that are crucial to protection from infectious disease.

A central result in the foundations of quantum mechanics is the Kochen-Specker theorem. In short, it states that quantum mechanics cannot be reconciled with classical models that are noncontextual for ideal measurements. This phenomenon is also known as quantum contextuality. The first explicit derivation by Kochen and Specker was rather complex, but considerable simplifications have been achieved thereafter. We propose a systematic approach to find minimal Hardy-type and Greenberger-Horne-Zeilinger-type (GHZ-type) proofs of the Kochen-Specker theorem, these are characterized by the fact that the predictions of classical models are opposite to the predictions of quantum mechanics. Based on our results, we show that the Kochen-Specker set with 18 vectors from Cabello et al. [A. Cabello et al., Phys. Lett. A 212, 183 (1996)] is the minimal set for any dimension, verifying a longstanding conjecture by Peres. In this talk, we will give an introduction to quantum contextuality, Kochen-Specker theorem and our proof of Peres conjecture. Zoom: https://uni-koeln.zoom.us/j/97462167683?pwd=RFE3VzhsWkIzUkxWUXJCS1dIN3Q0dz09

Motivated by a recent reporting of a first of a kind experimental realization of a gapless quantum spin liquid state on the novel square-kagome lattice geometry realized in the spin S=1/2 system KCu6AlBiO4(SO4)5Cl [1], we discuss the possible microscopic origins of this state, and more broadly highlight this novel geometry as an ideal playground for stabilizing a diverse array of exotic quantum phases [2]. Employing state-of-the-art quantum many-body numerical techniques such as variational Monte Carlo (VMC) with versatile Gutzwiller-projected Jastrow wave functions, unconstrained multi-variable variational Monte Carlo (mVMC), and pseudo-fermion/Majorana functional renormalization group (PF/PM-FRG) methods, we establish the presence of a quantum paramagnetic ground state and investigate its nature, by classifying symmetric and chiral quantum spin liquids, and inspecting their instabilities towards competing valence-bond-crystal (VBC) orders. Our analysis reveals that a VBC with a pinwheel structure of spin correlations emerges as the lowest-energy variational ground state, and it is obtained as an instability of the U(1) Dirac spin liquid. We argue that further neighbor Heisenberg and/or Dzyaloshinkii-Moriya interactions play a crucial role towards explaining the experimental observations. References: [1] Nat. Commun. 11, 3429 (2020), Gapless spin liquid in a square-kagome lattice antiferromagnet, Masayoshi Fujihala, Katsuhiro Morita, Richard Mole, Setsuo Mitsuda, Takami Tohyama, Shin-ichiro Yano, Dehong Yu, Shigetoshi Sota, Tomohiko Kuwai, Akihiro Koda, Hirotaka Okabe, Hua Lee, Shinichi Itoh, Takafumi Hawai, Takatsugu Masuda, Hajime Sagayama, Akira Matsuo, Koichi Kindo, Seiko Ohira-Kawamura & Kenji Nakajima [2] Phys. Rev. B 104, L220408 (2021), Pinwheel valence-bond-crystal ground state of the spin-1/2 Heisenberg antiferromagnet on the shuriken lattice, Nikita Astrakhantsev, Francesco Ferrari, Nils Niggemann, Tobias Müller, Aishwarya Chauhan, Augustine Kshetrimayum, Pratyay Ghosh, Nicolas Regnault, Ronny Thomale, Johannes Reuther, Titus Neupert, Yasir Iqbal Zoom-Link: https://uni-koeln.zoom.us/j/9524918125?pwd=dWN1RW1mMUpDRWdyMWwzWGtURWVIZz09

Rude colleagues say that solid states chemists follow for their reactions only the principle "shake and bake". Of course, high temperatures help to overcome activation barriers. However, we all know that this is not enough. A multitude of other parameters play an essential role for efficient solid-state reactions. I will present you how we use extremely strong oxidizing agents, such as sulfuric acid or its anhydride SO3, to synthesize silicate analogous materials such as borosulfates. In these, the charge compensating heteropolyanionic subunits are composed of vertex-linked (SO4)- and (BO4)-tetrahedra. In contrast to the immense structural diversity of silicates, the number of borosulfates is yet very limited and the extent of their properties is still unknown. Furthermore, allow the extreme reaction conditions for the stabilization of cationic species with unprecedented properties, like 1D metal behavior in [Au2Cl4](B(S2O7)2](SO3). In an effort to expand the knowledge on oxoanionic networks even further, we have also gone and tested our systems on rhenates, manganates and technetates. In this context, the handling of technetium and its compounds is another challenge. In addition to the acids, strong alkaline media can be a perfect medium for the synthesis of oxidic and oxoanionic materials, and the application of pressure in O2 gas autoclaves or the use of a Multianvil press (pmax = 26 GPa) allows for investigations of the afore mentioned reactions in a regime that is still almost unexplored. Using a wide array of analytical methods such as optical spectroscopy, magnetochemistry and photoelectron spectroscopy we try to investigate all samples as detailed as possible and corroborate our findings by quantum chemical calculations. Herein, I will therefore present an overview of our investigations on borosulfates and oxometallates, the effects of the counter cations on the chemistry and some high pressure approaches that we have access to at the University of Cologne. It will turn out that “shake and bake” does not always hold true, and that there is more to solid state chemistry than meets the eye.

Bell tests are the prime example of device-independent experiments where hence the specific design of the experimental devices does not influence the conclusions of the experiment. However, using statistical analysis of data from Bell tests one can show, for example, that an explanation of the data requires superluminal communication. Is this a breakdown of special relativity? In this talk I will discuss why one should not jump to the latter conclusion and how data analysis could be used to eventually improve experimental setups---if desired by the research community. https://uni-koeln.zoom.us/j/6233837377

Parallels between biological evolution and statistical physics are well-known and have led to fruitful exchanges and concepts. One such concept is the fitness landscape which, analogous to the energy landscape of a disordered system, is an abstract representation of the environment in which a biological population evolves. Evolution in changing environments is still poorly understood. In this talk we consider a recently introduced model for antibiotic resistance evolution in bacteria. The fitness landscape changes with the antibiotic concentration, giving rise to tradeoffs between adaptation to low and high antibiotic concentrations. We show that the adaptive evolution under slowly changing antibiotic concentration can be mapped to the dynamics of a class of athermal driven disordered systems, which in turn are paradigmatic models for memory formation in matter.

Frustrated three dimensional quantum magnets bear a rich phenomenology but are notoriously hard to treat theoretically. We show how a Majorana representation of spin operators, in combination with the functional renormalization group allows for quantiative simulations at finite temperatures. Focusing on Heisenberg magnets, we establish a finite-size scaling approach and extract accurate results for critical temperatures and -exponents. Further, we quantify and discuss the improvements introduced by two-loop contributions in the flow equations applied to the Pyrochlore lattice. We then investigate the accuracy of its prominent spin-ice rule under the influence of both quantum and thermal fluctuations.

With rapid progress across platforms for quantum systems, the problem of many-body quantum state reconstruction for noisy quantum states becomes an important challenge. There has been a growing interest in approaching the problem of quantum state reconstruction using generative neural network models. In this talk, I will present the 'attention-based quantum tomography' (AQT), a method of quantum state reconstruction that uses an attention mechanism-based generative network. The results demonstrate not only that AQT outperforms earlier neural-network-based quantum state reconstruction on identical tasks but that AQT can accurately reconstruct the density matrix associated with a noisy quantum state experimentally realized in an IBMQ quantum computer. In addition, I will highlight broader applications of AQT currently being developed.

A theory for the nonlocal shear stress correlations in supercooled liquids and colloidal suspensions is derived from first principles. It captures the crossover from viscous to elastic dynamics at an idealized liquid to glass transition and explains the emergence of long-ranged stress correlations in glass, as expected from classical continuum elasticity. Whereas Eshelby‘s elasticity pattern is recovered independently of the dynamics, the Goldstone modes differ drastically: Newtonian dynamics leads to propagating transverse sound, while the Goldstone modes of a colloidal glass are diffusive. Precursors of the long-ranged anisotropic stress correlations in the fluid can be observed in both cases. Their spatial extent can be characterized by a correlation length, which grows like the shear viscosity for a Newtonian fluid and like its square root in a colloidal fluid. These results are derived within a hydrodynamic theory, generalising Maxwell‘s model to finite wavenumbers, after the dynamics has been decomposed into potentially slow modes, associated with conservation laws and/or the order parameter, and fast microscopic degrees of freedom.

Engineering flat electronic bands from first mathematical principles Alexander Kruchkov Institute of Physics, EPFL, Lausanne, Switzerland Department of Physics, Harvard University, USA Branco Weiss Society in Science, ETH Zurich, Switzerland The concept of flat electronic bands is a change of paradigm in condensed matter physics. In this talk, I will address the origin of band flatness and its connection to the quantum geometry of electronic states.1 My work focuses on engineering the flat topological bands from the first mathematical principles, namely the Riemannian metrics and quantum geometry, with consequent holomorphicity and meromorphicity of the flat band states,1,2,3 enforced with accurate atomistic compu­ tations, DFT and tight binding with lattice relaxation effects.4,5 It provides deep insights on the origin of band flatness, which should be looked for in the real space (rather than in momentum space as conventionally it had been). In the complicated and messy domain of band structure computations, the developed approach provides an aesthetical tool for engineering flat electronic bands in realistic materials4 with consequent experimental confirmation.6,7 Time permitting, I will address the novel formalism of flat­band quantum transport built on the concepts of quantum geometry.8 References: [1] A. Kruchkov, Origin of band flatness and constraints of higher Chern numbers, arXiv 2105.14672 (2021). [2.] G. Tarnopolsky, A. J. Kruchkov, A. Vishwanath, Origin of magic angles in twisted bilayer graphene, Physical Review Letters 122, 106405 (2019). [3.] E. Khalaf, A. J. Kruchkov, G. Tarnopolsky, A. Vishwanath, Magic angle hierarchy in twisted graphene multilayers, Physical Review B 100, 085109 (2019). [4.] S. Carr, C. Li, Z. Zhu, E. Kaxiras, S. Sachdev, A.J. Kruchkov, Ultraheavy and ultrarelativistic Dirac quasiparticles in sandwiched graphenes, Nano Letters (2020). [5.] F. Haddadi, Q. Wu, A. J. Kruchkov, O.V. Yazyev, Moiré flat bands in twisted double bilayer graphene, Nano Letters (2020). [6.] J.M. Park, Y. Cao, K. Watanabe, T. Taniguchi, P. Jarillo­Herrero, Tunable strongly coupled superconductivity in magic­angle twisted trilayer graphene, Nature volume 590, (2021). [7.] Z. Hao, A. M. Zimmerman, P. Ledwith, E Khalaf, D. Najafabadi, K. Watanabe, T. Taniguchi, A. Vishwanath, P. Kim, Electric field–tunable superconductivity in alternating­twist magic­angle trilayer graphene, Science 371, 6534 (2021). [8.] A. Kruchkov, Quantum transport in dispersionless electronic bands, in preparation (2021).

When cold electrons move in a perfect, static, and stable lattice, according to conventional wisdom, they move like almost free quasiparticles in flat space. Here, we present evidence that this is not entirely correct, and quasiparticles actually move in an emergent curved spacetime. To this end, we discuss the second order electrical conductivity in materials lacking inversion and time-reversal symmetry, which exhibits a mixed axial-gravitational anomaly. We can explain this surprising result in terms of dynamical deformations of the semiclassical wavepacket as it moves through the periodic lattice potential, thereby establishing a common framework for the appearance of anomalous terms in many response functions. Our proposition that quasiparticles behave essentially like quantum cats has powerful implications for all types of quantum transport and may allow to probe synthetic gravitational fields in a bulk condensed matter setting.

Quantum systems are generally extremely hard to isolate from their environment, even more so if we want to exert control or perform measurements. It is therefore important to understand the effect of the coupling to the environment. I will discuss a random matrix approach to quantum many-body systems with strong dissipation and show that the locality of dissipation processes gives rise to a hierarchy of relaxation timescales, which can be experimentally observed. This theory can also be applied to the case of a strong variance in dissipation strengths, which can lead to an emerging set of metastable states.

A record is an entry in a time series that is greater or smaller than all previous entries. The public perception of climate change is particularly sensitive to record-breaking weather events, which often receive extensive media coverage. As a consequence, a number of recent studies have been aimed at detecting and quantifying the effects of a warming climate on the frequency of temperature records. The mathematical theory of records makes universal and sometimes counter-intuitive predictions about the temporal statistics of record events, but it is largely restricted to stationary time series. After an introduction to the classic theory, I will explain the extensions needed to deal with trends, and the effect of warming on temperature records will be described at different levels of resolution. In the last part of the talk I will discuss time series of precipitation records, where the presence of dry days of zero precipitation induces nontrivial correlations between record-breaking events.

The genetic heterogeneity found within a tumor also extends to the spatial dimension of clonal architecture, but only recently has high-depth multi-region sequencing data become available. We use spatially resolved genetic data to study a tumor’s mode of evolution and growth as well as population genetic theory of mutant type extinction to infer cancer cell turnover rates and mutation rates.

This talk includes two works that have relation to the Kitaev model on the honeycomb lattice. In the first part, I will focus on the properties of the spin-1 Kitaev model, and discuss the controversial results about whether the spin-1 model is gapless. In the second part, I will talk about the spin-orbital generalizations of the Kitaev model, and transitions between topological phases featuring emergent fractionalized excitations. Reference: 1. Physical Review B 102, 121102 (R) (2020) 2. Physical Review Letters 125, 257202 (2020)

The talk will be given in the HS III. You can all attend in presence. As an alternative you can also participate via zoom: https://uni-koeln.zoom.us/j/4225866762

Microbes often colonize spatially-constrained habitats, such as pores in the skin or crypts in the colon. The resulting micro-communities can be very stable and contribute to the long-term function of our microbiomes. Due to a lack of dynamical observations, it is however unclear how these communities and their ecological functions arise. By monitoring and modeling microbial populations in microfluidic channels of systematically varied size, we find a rich spectrum of scale-dependent dynamical patterns that are controlled by the competition between density-dependent outflow and population growth. These results elucidate how the injection of degrees of freedom, driven by cell proliferation, can drive a non-equilibrium phase transition (different from MIPS) and suggest that the mechanics jammed cellular packings can influence the evolutionary dynamics of dense microbial populations

By quenching electron kinetic energy, one can reveal correlated phases in previously weakly correlated materials, such as in twisted bilayer graphene. Disorder typically obscures correlated phases; for instance, it might just lead to localized electron wave functions. Incommensurability-driven phases provide a platform to observe both phase transitions typically associated with disorder (Anderson-like transitions) as well as flat bands and correlated phases. In this talk, I will discuss our work to understand this, particularly within the context of topological band structures. In both two-dimensional semimetals and topological insulators, we see that disorder induces a delocalization eigenstate transition in momentum space. The wave functions exhibit multifractality in this phase, and in the case of two-dimensional semimetals, it occurs at the "magic-angle" as for twisted bilayer graphene. Concomitantly, there is a precise sense in which the "bands" become increasingly flat at these transitions. In the case of the topological insulator, the Berry curvature also becomes more uniform as the wave functions near criticality: clearing the way for potential fractional Chern insulators in these systems.

Artificial molecular motors that convert external energy into controlled motion have seen great developments in the last decades [1]. While many studies exist in solution, little is known how such functional molecules behave on a surface. However, such a solid support can be advantageous as it offers fixed points of reference as well as confinement in two dimensions, making it easier to study the directionality of their motion. We have studied so-called Feringa motors on various surfaces by low-temperature scanning tunnelling microscopy (STM). On Cu(111) these molecules adsorb in three different conformations, which are related to the states of the motor cycle in solution. The relative stability of the in-solution motor states is reversed on the surface. It was found that rotations of individual molecules can be induced over rather long distances up to about 100 nm by voltage pulses with the STM tip. Importantly, these rotations show high directionality (clockwise or anticlockwise), which will be discussed in view of their specific chemical structure and adsorption. [1] W. R. Browne and B. L. Feringa, Nat. Nanotech. 1, 25 (2006)

The search for novel topological phases of matter is on-going, with many recent efforts focusing on study of non-electronic and other analogues of the foundational electronic topological phases of matter of the ten-fold way. We return to study of the original electronic topological phases by introducing the topological skyrmion phases of matter. These are symmetry-protected topological phases defined by skyrmions, or topological spin textures, in momentum-space for Bloch Hamiltonians with more than two bands. These topological phases are protected by a generalized particle-hole symmetry C' present in some centrosymmetric superconductors. We first present a method for constructing various four-band toy models for skyrmion phases in two-dimensions, including models describing superconductors, and discuss some experimental signatures. We then introduce three-dimensional topological skyrmion phases and show that they display a bulk-boundary correspondence, trapping exotic localized "cross" zero-mode states at defects in the bulk and on the boundary of the system. Finally, we consider a more complex tight-binding model for spin-triplet superconductivity in transition metal oxides, which can realize topological skyrmion phases of matter in individual mirror subsectors, even for a parameter set previously used to model superconducting Sr2RuO4. Importantly, we find two types of topological phase transitions by which the skyrmion number can change. In a superconductor, the first kind occurs with a closing of the superconducting gap. The second kind, however, occurs when spin is not conserved and without the closing of direct energy gaps. As this type-II topological phase transition occurs without breaking the protecting symmetry of the topological phase in an effectively non-interacting system, it contradicts the "flat band" limit assumption of the ten-fold way classification scheme of topological phases of matter.

For this opening colloquium of the CRC183 summer school on "Machine Learning in Condensed Matter Physics" Lei Wang is going to give an overview talk on the topic.

This is the contributed session of the CRC 183 Summer School "Machine Learning in Condensed Matter Physics" We will have the following short talks: Geometry of learning neural quantum states Chae-Yeun Park (University of Cologne) Unsupervised Learning Universal Critical Behavior via the Intrinsic Dimension Tiago Mendes Santos (MPI PKS) Convolutional restricted Boltzmann machine aided Monte Carlo: An application to Ising and Kitaev models Daniel Alcalde Puente (Forschungszentrum Jülich) AI-assisted quantum many-body computation beyond Markov-chain Monte Carlo Hongyu Lu (University of Hong Kong) Learning Many-body Hamiltonians from Dynamical Data Frederik Wilde (FU Berlin) From observations to complexity of quantum states: an unsupervised learning approach Zala Lenarčič (Jožef Stefan Institute)

I will describe different mechanisms by which gravity emerges from 2D conformal field theory. This includes holography and kinematic space, both examples of emergent gravity from entanglement, as well as a more recently discovered mechanism that involves an irrelevant deformation of the conformal field theory.

2D materials are an exciting host to engineer atomic quantum systems by chemical design rules. In this colloquium, I will give an overview on our efforts to design and probe defect systems in monolayer transition metal dichalcogenides (TMDs) by means of chemical doping, Helium ion beam bombardment and atomic manipulation. By means of low-temperature scanning probe microscopy, we directly resolve the discrete electronic spectrum of single dopants in a charge neutral or ionized state and map out their associated defect orbitals [1-5]. Different types of defects reveal the interplay between chemical impurity states [1-3], multi-valley hydrogenic bound states [4,6], and electron-phonon coupling [1,7] at reduced dimensions. In particular, we will discuss the atomically controlled generation of carbon radical ions (CRIs) by hydrogen depassivation [7] and electrically driven photon emission from individual defects [8]. References: [1] B. Schuler et al., Phys. Rev. Lett. 123, 076801 (2019) [2] S. Barja et al., Nat. Commun. 10, 3382 (2019) [3] B. Schuler et al., ACS Nano 13, 10520 (2019) [4] M. Aghajanian et al., Phys. Rev. B 101, 081201(R) (2020) [5] E. Mitterreiter et al., Nano Lett. 20, 4437 (2020) [6] K. Cochrane et al., 2D Mater. 7, 031003 (2020) [7] K. Cochrane et al., arxiv: 2008.12196 [8] B. Schuler et al., Sci. Adv. 6, eabb5988 (2020)

The classical Heisenberg model in two spatial dimensions constitutes one of the most paradigmatic spin models, taking an important role in statistical and condensed matter physics to understand magnetism. Still, despite its paradigmatic character and the widely accepted ban of a (continuous) spontaneous symmetry breaking, controversies remain whether the model exhibits a phase transition at finite temperature. Importantly, the model can be interpreted as a lattice discretization of the O(3) non-linear sigma model in (1+1) dimensions, one of the simplest quantum field theories encompassing crucial features of celebrated higher-dimensional ones (like quantum chromodynamics in (3+1) dimensions), namely the phenomenon of asymptotic freedom. This should also exclude finite-temperature transitions, but lattice effects might play a significant role in correcting the mainstream picture. In this work, we make use of state-of-the-art tensor network approaches, representing the classical partition function in the thermodynamic limit over a large range of temperatures, to comprehensively explore the correlation structure for Gibbs states. By implementing an SU(2)-symmetry in our two-dimensional tensor network contraction scheme, we are able to handle very large effective bond dimensions of the environment up to 1500, a feature that is crucial in detecting phase transitions. With decreasing temperatures, we find a rapidly diverging correlation length, whose behaviour is apparently compatible with the two main contradictory hypotheses known in the literature, namely a finite-T transition and asymptotic freedom.

As physical implementations of small quantum computers become a reality, we investigate two common bottlenecks: limited memory size and noise caused by heat dissipation. In this talk I will go over the theoretical fundamentals behind heat production in quantum information processing, and how it can be alleviated without a large cost in terms of quantum memory requirements. The talk will be accessible to non-experts, familiar with quantum mechanics. Landauer posited that irreversible information-processing operations (like the erasure of a classical or quantum memory at the end of a computation) have an intrinsic thermodynamic work cost, which is associated with heat dissipation to the environment [1]. In the case of a quantum computer this heat dissipation is particularly undesirable, as the computer relies on cooling to keep coherence of quantum states. Bennett proposed a solution: if we run the circuit up until the final measurements and then reverse it, we keep heat production to a theoretical minimum [2]. However, this proposal has a cost in terms of quantum memory: none of the quantum registers that are only needed for parts of the computation can be freed until the very end. In the intermediate term, when we have only a few hundreds of qubits in a quantum memory, this is a bottleneck for parallelization of quantum computations: we have to wait to finish an algorithm before we can free the memory to run another. In recent work, we try to ameliorate this problem by investigating "online erasure" of quantum registers that are no longer needed for a given algorithm. That way, we could free up quantum memory already halfway through a computation. We study the minimal cost of erasure in these scenarios, using results on the erasure of entangled quantum registers [3]. For a specific class of algorithms (the hidden subgroup problem) we find optimal online erasure protocols, and see how we could use them to optimize the algorithms [4].

Everyone who is good enough in physics can work in the academic field, no matter what their identity is besides that, right? We do not have any issues with discrimination, or do we? Is our study and work environment really benefiting everyone equally? Or are people marginalized here as well – and if so, what is going on? And what can we do about it? The event will be with Sherard Robbins, a development consultant, author, and CEO of Visceral Change. He will address the topic of “Racism and Physics” and present us some background information and numbers about it. There will also be space for discussions: How does it affect our work environment and what can we do about it?

The rich phenomenology of twisted bilayer graphene (TBG) near the magic angle is believed to arise from electron correlations in topological flat bands. An unbiased approach to this problem is highly desirable, but also particularly challenging, given the multiple electron flavors, the topological obstruction to defining tight binding models and the long-ranged Coulomb interactions. While numerical simulations of realistic models have thus far been confined to zero temperature, typically excluding some spin or valley species, analytic progress has relied on fixed point models away from the realistic limit. Here we present for the first time unbiased Monte Carlo simulations of realistic models of magic angle TBG at charge-neutrality. We establish the absence of a sign problem for this model in a momentum space approach, and describe a computationally tractable formulation that applies even on breaking chiral symmetry and including band dispersion. Our results include (i) the emergence of an insulating Kramers inter-valley coherent ground state in competition with a correlated semi-metal phase, (ii) detailed temperature evolution of order parameters and electronic spectral functions which reveal a `pseudogap' regime, in which gap features are established at a higher temperature than the onset of order and (iii) predictions for electronic tunneling spectra and their evolution with temperature. Our results pave the way towards uncovering the physics of magic angle graphene through exact simulations of over a hundred electrons across a wide temperature range. This work has been recently uploaded to ArXiv: 2105.12112

The discovery of the quantum Hall effect (QHE) in 1980 marked a turning point in condensed matter physics. The measurement of the Hall conductivity of a two-dimensional electron system showed that electrical conduction can be precisely defined only in terms of fundamental constants. But, what happens to the QHE in three dimensions? At a first glance, the answer seems straightforward and sobering: The quantization of the Hall conductivity gets destroyed. This has led to the general conjecture that quantum-Hall physics is exclusive to two dimensions. However, we show that the Hall effect of genuine three-dimensional (3D) metals is actually deeply related to the QHE and can be understood from quantum Hall physics. We propose that - as a general rule - plateau-like features in the Hall conductivity of 3D electron systems are precisely given by the conductance quantum, scaled by the Fermi wave vector of the electrons along the magnetic field direction.

Topological phases were discovered in condensed matter physics and recently extended to classical physics such as topological mechanical metamaterials. Their study and realization in soft-matter and biological systems has only started to develop. In this talk we discuss how topological phases may determine the behavior of nonlinear dynamical systems that arise, for example, in population dynamics. We have shown that topological phases can be realized with the anti-symmetric Lotka-Volterra equation (ALVE). The ALVE is a paradigmatic model system in population dynamics and governs, for example, the evolutionary dynamics of zero-sum games, such as the rock-paper-scissors game [1], but also describes the condensation of non-interacting bosons in driven-dissipative set-ups [2]. We have shown that for the ALVE, defined on a one-dimensional chain of rock-paper-scissors cycles, robust polarization emerges at the chain’s edge [3]. The system undergoes a transition from left to right polarization as the control parameter passes through a critical value. At the critical point, solitary waves are observed. We found that the polarization states are topological phases and that this transition is indeed a topological phase transition. Remarkably, this phase transition falls into symmetry class D within the “ten-fold way” classification scheme of gapped free-fermion systems, which also applies, for example, to one-dimensional topological superconductors. Beyond the observation of topological phases in the ALVE, it might be possible to generalize the approach of our work to other dynamical systems in biological physics whose attractors are nonlinear oscillators or limit cycles. [1] J. Knebel, T. Krüger, M. F. Weber, and E. Frey, Phys. Rev. Lett. 110, 168106 (2013). [2] J. Knebel, M. F. Weber, T. Krüger, and E. Frey, Nature Communications 6, 6977 (2015). [3] J. Knebel, P. M. Geiger, and E. Frey, Phys. Rev. Lett. (in press) [arXiv:2009.01780].

Only the Speed of Light limits You? - Discrimination and its impact on the academic field of physics Everyone who is good enough in physics can work in the academic field, no matter what their identity is besides that, right? We do not have any issues with discrimination, or do we? Is our study and work environment really benefiting everyone equally? Or are people marginalized here as well – and if so, what is going on? And what can we do about it? The talk will be given by Prof. Tomas Brage. He has a very successful career in physics and is currently a physics professor at the Division of Mathematical Physics of the Department of Physics at Lund University. Besides all his publications in this field, he also does research concerning “Gender and Physics”. He will inform us about numbers, bias, and stereotypes regarding gender, and their influence on physics. After his talk, we can also ask questions and have a discussion about the topic.

Textbook quantum criticality refers to phenomena which originate in states which scale-invariant in both space and time, occurring at continuous zero-temperature phase transitions. Various experimental findings have raised interest in local forms of quantum criticality, where scale invariance in space in either absent or irrelevant. Prominent examples include variants of the Mott metal-to-insulator transition in a number of strongly correlated electron compounds. After an extended introduction, I will discuss one of the simplest cases, namely the bandwidth-tuned Mott transition in a single-band setting. Recent transport studies, both theoretical and experimental, near this transition have uncovered - surprisingly - apparent quantum critical scaling of the electrical resistivity at elevated temperatures, despite the fact that the actual low-temperature phase transition is of first order. This raises the question whether there is a hidden Mott quantum critical point. I will argue that the dynamical mean-field theory of the Hubbard model admits, in the low-temperature limit, scale-invariant (i.e. power-law) solutions which had been overlooked before. It is this incoherent power-law regime, corresponding to approximate local quantum criticality, which is continuously connected to and responsible for the apparent quantum critical scaling above the classical critical end point. I will discuss the role of magnetic frustration in these phenomena, non-local corrections, and broader implications for the phenomenology of bad metals.

Quantum Simulation promises insight into quantum physics problems which are beyond the ability to calculate with conventional methods. Quantum simulators can be built either using a ‘digital’ Trotter decomposition of the problem or by directly building the Hamiltonian in the lab and performing ‘analogue’ experiments. I will present here a different approach, by which the model to simulate emerges naturally from a completely different microscopic Hamiltonian. I will illustrate this in the example of the emergence of the Sine-Gordon quantum field theory from the microscopic description of two tunnel coupled super fluids. Special emphasis will be put on how to verify such emergent quantum simulators and how to characterize them. Thereby I will present two tools: High order correlation functions and their factorization [1], the evaluation of the quantum effective action and the momentum dependence of propagators and vertices (running couplings, renormalization of mass etc ..) of the emerging quantum field theory [2] and quantum field tomography that points to a new way to read out quantum simulators [3]. Together they establish general methods to analyse quantum systems through experiments and thus represents a crucial ingredient towards the implementation and verification of quantum simulators. Work performed in collaboration with the groups of Th. Gasenzer und J. Berges (Heidelberg), Jens Eisert (FU Berlin) and E. Demler (Harvard). Supported by the DFG-FWF: SFB ISOQUANT: and the EU: ERC-AdG QuantumRelax [1] T. Schweigler et al., Nature 545, 323 (2017), arXiv:1505.03126 [2] T. Zache et al. Phys. Rev. X 10, 011020 (2020) [3] M. Gluza et al., Communication Physics 3, 12 (2020)

We have proposed to realize the Kitaev spin liquid state in 3d honeycomb cobaltates, due to the more localized wave functions of 3d ions compared with that of 4d and 5d ions, and also the easy tunability of the exchange Hamiltonian in favor of Kitaev interaction. In this talk, I will discuss the key parameters which have very large impacts on the exchange constant, and the crystal field effect is emphasized as an efficient phase control method driving the magnetically ordered cobaltates into the spin liquid state. In good agreement with the experimental data, our theoretical predications verify the great potential of realizing the Kitaev spin liquid in 3d transition metal compounds.

Magnetooptical observations of thin magnetic cylinders: theory and experiment (K. Richter1*, O. Vahovsky3, A. Thiaville2, R. Varga3 and J. McCord1) 1 University of Kiel, Kaiserstraße 2, 24143 Kiel, Germany 2University Paris Sud XI, 914 00, Orsay, France 3 University of Pavol Jozef Safarik, 040 01 Kosice, Slovakia *e-mail: kornel.richter@gmail.com Thin cylindrical wires attract considerable attention due to the interesting features of domain wall motion like absence of the Walker breakdown that prevents very high domain wall velocities [1]. While several experimental techniques allow for well controllable deposition of magnetic structures with complex geometries [2], a reliable determination of the surface magnetization usually meets several obstacles in case of samples with curved surface. Here, we provide a full analytical calculation of the Magneto-Optical Kerr Effect (MOKE) contrast for cylinders with reduced diameter. It is shown that the cylindrical shape of sample surface gives rise to a spatial distribution of the planes of incidence that are all tilted each to other [3]. Such mutual orientation of incident rays in combination with circumferential dependence of a local angle of incidence gives rise to apparent magneto-optical contrasts [3,4] that cannot be interpreted well without considering the curved surface of a cylinder. Theoretical calculations [3] are tested experimentally on amorphous glass-coated microwires with well-defined cylindrical shape. Finally, we use our framework to study shape of the domain wall in FeSiB microwire characterized by very fast domain wall propagation. The setup is optimized for characteristic optical and magneto-optical features of thin magnetic cylinders. Time-resolved measurements of a magnetic contrast along the circumference of wire are used to reconstruct a three-dimensional image of a surface domain wall shape. Our measurements confirm a tilted shape of domain wall [5, 6], in agreement with previous observations of microwires with positive magnetostriction alloy. The experimental technique represents a new way for direct visualizations of a surface magnetism of thin cylinders in optical range. [1] M. Yan, et al., Phys. Rev. Lett. 104, 057201 (2010). [2] A. Fernández-Pacheco, et al., Sci. Rep. 3, 1492 (2013). [3] K. Richter, et al., Phys. Rev.B 96, 064421 (2017). [4] K. Richter, et al., IEEE Trans. Magn. 50, 2501404 (2014). [5] K. Richter, et al., J. Appl. Phys.127, 193305 (2020). [6] O. Vahovsky, et al., J. Magn. Magn. Mater. 483, 266 (2019).

Open quantum systems are ubiquitous in the contexts of atomic and molecular physics, circuit QED and optomechanics. Couplings to an environment can also have very interesting effects on the dynamics of many-particle quantum systems. In order to arrive at a tractable description of such problems, it is customary to work within the Markovian approximation, whereby the system is described by a Lindblad master equation. While much progress has been made in analysing Lindblad equations for many-particle systems by employing, for example, perturbative and matrix product states methods, it is clearly highly desirable to have exact solutions in specific, and hopefully representative, cases. This talk aims to show that for a number of interacting open quantum systems, it is possible to obtain exact analytic solutions through a connection with integrable models. I will describe how a correspondence between a Lindblad equation and an integrable Hamiltonian can be established and what information about the open systems it provides. In particular, I will discuss the equivalence in structure of generalised Hubbard models and vectorised Lindblad equations, which are already visible in the R-matrix of the integrable model. I will also mention how this construction can be extended by focusing purely on the integrability of the invariant subspaces of the Lindblad equations. The talk is based on the work presented in SciPost Phys. 8, 044 (2020) (doi: 10.21468/SciPostPhys.8.3.044).

Environmental coupling typically drives a quantum system to a unique steady state with very little coherence, which is a major obstacle for quantum information processing. I will talk about a simple experimental setting of an array of two-level systems with localised pump and loss that has multiple highly coherent steady states, including maximally entangled states of nonlocal Bell pairs. Such states originate from a hidden symmetry that conserves Bell pairs over long distances, leading to controllable long-range entanglement. I will discuss how to selectively prepare and observe these states in a broad range of present-day setups.

The talk is in three parts. Part 1: Universal spectral signatures of many-body-localization The notion of universality is enormously powerful in understanding equilibrium phases and phenomena. At the most practical level, it allows us to substitute a complicated problem for a much simpler one. A surprising notion of universality has been observed for quantum chaotic systems, which are said to constitute a non-equilibrium 'eigenstate phase' of matter. Correlations in the spectrum are observed to be reproduced by a particularly simple system- random matrix ensembles, which predict universal signatures, such as a `ramp' in the spectral-form-factor. I will argue the existence of analogous universal signatures for a different eigenstate phase constituted by many-body-localized systems. Based on joint work with Manas Kulkarni (ICTS-TIFR) and Jedediah Pixley (Rutgers). Part 2: Eigenstate phases with non-Abelian symmetry. A long-standing open question is whether there exist any eigenstate phases beyond the now-well-known examples of many-body-localized or quantum-chaotic systems. A promising possibility presents itself for systems with finite non-Abelian symmetries. By numerically studying a spin-chain with such a symmetry in the presence of disorder, we see some evidence for the presence of a novel eigenstate phase of matter. Based on joint work with Tzu-Chieh Wei (Stony Brook), Sriram Ganeshan (CCNY) and Lukasz Fidkowski (University of Washington). Part 3: Emergent supersymmetry in fermionic symmetry-protected-topological phases. Symmetry-protected topological phases are characterized by an almost trivial insulating bulk but exotic boundaries due to the presence of 't Hooft anomalies. These anomalies present themselves in manifold ways- protected gapless phases, Landau-forbidden deconfined quantum critical transitions and the absence of a trivial order. I will demonstrate that for a large class of fermionic SPT phases, the boundary anomaly presents itself in a fascinating manner- emergent supersymmetry with no fine tuning of any kind! This might present the most robust realization of supersymmetry in solid-state and cold atomic systems. Based on joint work with Juven Wang (Harvard)

It has been predicted that topological magnon insulators are realized in kagome plane of pyrochlore magnetic insulators with Dzyaloshinskii-Moriya interaction. Bosonic topological insulators can exhibit a topological band structure. Although bosonic system ground state is trivial condensate or the vacuum, their excited states may carry signatures of the topology. In this presentation, I will show the distinct features of excitations in kagome lattice within the framework of the spin-wave theory: (a) finite lifetime at zero temperature due to spontaneous two-magnon decays, (b) the renormalization of magnon energy depending on the magnetization direction, (c) singularities in the decay rate. Those features are shown in the form of dynamical structure factors. Their effect in experimentally detecting edge modes will also be discussed.

Superconducting nanowire single-photon detectors (SNSPDs) provide sufficient sensitivity to enable laser induced fluorescence (LIF) experiments in the mid-infrared, an exciting technical development for physical chemistry given the importance of vibrational spectroscopy to molecular science. In this talk, I will describe how an SNSPD works and how to use it. I will also present results of experiments on the vibrational dynamics of monolayers and multilayers of solid CO adsorbed at the surface of a NaCl crystal, to show the capabilities of time-resolved infrared LIF spectroscopy and provide observations of astonishing phenomena arising from dipole-dipole coupling between molecules. In these experiments, a laser light pulse excites about half of the CO to its

The characterization of quantum chaos in many-body systems has been an active area of research in the past several years. There have been several proposed diagnostics of quantum chaos, like the out-of-time-ordered correlators and the spectral form factor, to name a few. The focus of this talk is another quantity known as the operator entanglement. A particularly interesting measure of operator entanglement is the tripartite operator mutual information which is a quantitative measure of the amount of information delocalization. By studying the tripartite operator mutual information of both unitary operators and local operators in a variety of systems ranging from conformal field theories to spin chains, we demonstrate the utility of operator entanglement in distinguishing chaotic and non-chaotic theories.

Typically, a generic quantum many-body system under isolation is known to thermalize. However, there are exceptions where thermalization is violated. For instance, while a strong violation of thermalization occurs in the well known many-body localizing systems (MBL), a violation in the weaker sense occurs through the phenomenon of quantum many-body scars (QMBS). In this talk, I will first review both MBL and QMBS. I will then describe our recent work where we showed that an emergent symmetry in a single eigenstate of a disordered model protects it from MBL while not preventing the rest of the states in the spectrum from many-body localizing. I will motivate how such an example calls for exploring "inverted quantum scars", namely, non-area law states embedded in a spectrum of localized states. Time permitting, I will also walk you through our other recent works where we construct topologically ordered scarred models.

Much insight has come from the idea that non-equilibrium systems can exhibit phenomena not found in thermal equilibrium. Understanding these non-equilibrium phases and the transitions between them is a major research frontier of modern many-body physics. In my talk I will outline two examples of novel physics found in non-equilibrium quantum systems. The first, discrete time crystals (DTCs), are a prototypical example of a non-equilibrium phase of matter, defined by the fact that they spontaneously break time-translation symmetry. I will describe the results of an experiment involving the observation of a DTC in a doped semiconductor platform, where both interactions and dissipation play a significant role in stabilizing the DTC order. Next I will discuss measurement-induced transitions, which are a new class of phase transition in open quantum systems. I will focus on two aspects of these transitions: the effect of the underlying unitary dynamics on the nature of the phase transition, and the role of dimensionality in determining the universal properties at criticality.

As a potential window on transitions out of the ergodic, many-body-delocalized phase, we study the dephasing of weakly disordered, quasi-one-dimensional fermion systems due to a diffusive, non- Markovian noise bath. This bath is generated by inelastic scattering due to short-ranged interactions. We calculate the dephasing of weak localization perturbatively through second order in the bath coupling. However, we find that instabilities in the calculation signal a failure of the self-consistent Born approximation. We also consider a many-channel quantum wire where short-ranged, spin-exchange interactions coexist with screened Coulomb interactions. We calculate the dephasing rate, treating the short-ranged interactions perturbatively and the Coulomb interaction exactly. The latter provides a physical infrared regularization that stabilizes perturbation theory at long times, giving the first controlled calculation of quasi-1D dephasing due to diffusive noise. Our results are relevant to the search for precursors to the MBL transition that can be understood in the high-T ergodic phase. As a bonus, the calculation also provides a mechanism for the enhancement of dephasing at low temperatures in spin SU(2)-symmetric quantum wires, beyond the Altshuler-Aronov-Khmelnitsky result. The enhancement is possible due to the amplification of the triplet-channel interaction strength, and provides an additional mechanism that could contribute to the experimentally observed low-temperature saturation of the dephasing time.

In this presentation, the emergence of localised, stable wave packets (solitons) is investigated in the context of the repulsive one-dimensional Bose gas. In the weakly interacting limit, the gas is approximated by the classical non-linear Schrödinger equation, of which it is known that such classical localisation occurs. A natural question to ask would be whether these classical wave packets could be reproduced starting from the quantum non-linear Schrödinger equation, and if so, which quantum mechanical wave function would produce such density profiles. Previous research has indeed attempted to do so, however, these studies had two main limitations: 1) only the attractive regime was investigated, which has limited physical significance because there is no saturation, and 2) localisation was shown only in limiting cases. In the talk, some of the mathematical steps are outlined in an attempt to overcome these two shortcomings. Besides introductory material, the talk is split into three parts according to the results obtained: 1) numerical results are discussed that show whether the desired localisation even exists outside the weakly interacting regime, 2) analytical efforts are discussed that show that Dirichlet kernels arise from the density form factor and finally 3) I will share some of the analytical efforts on simplifying general density form factors, which are of relevance when comparing magnitudes between form factor terms. It is found that mutually repulsive particles experience attraction at the macroscopic level, outside of the weakly and maximally interacting limits.

The talk will cover spectroscopic investigations on the catalytic properties of iron nanoparticle arrays and details on the formation of nanographene in nano-heterostructures. The increased complexity of real-life catalysts as compared to surface science leads to the “materials gap”. To close this gap, studies of adsorption and reaction experiments on nanoparticles supported on 2D materials were conducted under in situ conditions with the particular aim to elucidate the reactivity of such more “realistic” nanoparticles. It is shown that the use of a quantitative spectroscopy allows for gaining a detailed insight even to these complex systems, thereby facilitating a further step into bridging the materials gap from fundamental science to applied sciences. The use of the 2D materials as a chemically “innocent” substrate gives the intriguing possibility to study the system without the additional influence of side processes such as spill over and reverse spill over. Such studies allow for characterization of novel cluster systems towards their reactivity and adsorption behavior, e.g. for iron nanoclusters with the probe molecule CO. Such reactivity studies are also possible for the more complex chemistry of ethylene on Pt nanoclusters where detailed insights to the reaction pathway become available. Thereby the changes when going from single crystals to nanoclusters become evident. As a second topic, the formation of van-der-Waals nano-heterostructures is discussed. Insights from spectroscopic and microscopic studies allowed us to characterize the individual preparation steps and the final nano-heterostructure consisting of graphene / metal / h-BN stack. Experiments with different cluster sizes are shown to obtain insights to the pore filling of the h-BN substrate. This gives a direct measure of the agglomeration of the nanoclusters during preparation. Literature F. Düll, et al. ACS Applied Nano Materials 2 (2019) 7019. F. Düll, et al. Journal of Chemical Physics 152 (2020) 224710. F. Düll, et al. Physical Chemistry Chemical Physics 21 (2019) 21287.

Spins can be represented in terms of complex fermionic operators. This allows for a direct application of many-body techniques like the functional RG approach to frustrated quantum magnets. However, as the representation above introduces unphysical Hilbert space sectors, the applicability of this approach at finite temperatures is questionable. Here, I review an alternative Majorana fermion representation of spins which does not introduce any unphysical states. We develop a general functional RG formalism for Majoranas and apply it to the Pseudo-Majorana representation of Heisenberg models on small spin clusters as well as frustrated square and triangular lattices. We compute the finite temperature behavior of spin correlations and thermodynamic quantities and compare to exact diagonalization and the high-temperature series expansion.

Machine learning is becoming widely used in analyzing the thermodynamics of many-body condensed matter systems. Restricted Boltzmann machine (RBM) aided Monte Carlo simulations have sparked interest recently, as they manage to speed up classical Monte Carlo simulations. In the talk, based on my recently published paper (Phys. Rev. B 102, 195148), I will explain how we used the convolutional restricted Boltzmann machine (CRBM) method to reduce the number of parameters to be learned drastically by taking advantage of translation invariance. Furthermore, I will show that it is possible to train the CRBM at smaller lattice sizes, and apply it to larger lattice sizes. To demonstrate the efficiency of CRBM, I show the application to the Ising and honeycomb Kitaev models.

The Kitaev honeycomb model is well-known to host a quantum spin liquid at zero temperature, exhibiting topological order and fractionalization of spin into emergent Majorana fermion minimally coupled with Z2 gauge field. The exact solvability of this model stems from the conservation of local gauge fluxes, which also has dramatic impact upon the nonequilibrium dynamics when the system is brought out of equilibrium and isolated from any environment. The latter issue has been widely studied in the quest of emergent ergodicity in generic quantum manybody systems, or ergodicity breaking in manybody localization or other mechanisms. In this talk, I will show that the homogeneous Kitaev model exhibits the phenomenon of disorder-free localization, where the fractionalized Majorana fermion is partially localized by the Z2 gauge flux disorder dynamically generated in a highly excited state. As a result, the system exhibits exotic quench dynamics that is neither MBL nor ergodic: (1) algebraic lightcone of dimer correlation spreading; (2) algebraic decaying critical correlation in late time steady state; (3) algebraic growth of projective bipartite entanglement entropy to a volume law. Further, I will show numerical evidence that the long time feature is stable against solvability breaking perturbation. Our results may possibly imply nontrivial dimer correlation for Kitaev spin liquids at finite temperature.

In the last five years, a new field has emerged at the intersection between Condensed Matter and Quantum Optics, denominated “Quantum Magnonics”. This field strives to control the elementary excitations of magnetic materials, denominated magnons, to the level of the single quanta, and to interface them coherently to other elementary excitations such as photons or phonons. The recent developments in this field, with proof of concept experiments such as a single-magnon detector, have opened the door for hybrid quantum systems based on magnetic materials. This can allow us to explore magnetism in new ways and regimes, has the potential of unraveling quantum phenomena at unprecedented scales, and could lead to breakthroughs for quantum technologies. A predominant role in these developments is played by cavity optomagnonic systems, where an electromagnetic cavity, either in the optical or microwave regime, is used to enhance and control the interaction between photons and magnons. In this talk, I will introduce the field of Quantum Magnonics with emphasis on Cavity Optomagnonics, and present some theoretical results from our group which aim to push the boundaries of the current state of the art.

In this talk, I will show that the defects of the valence plaquette solid(VPS) order parameter, in addition to possessing non-trivial quantum numbers, have fracton mobility constraints in the VPS phase. The spinon inside a single vortex cannot move freely in any direction, while a dipolar pair of vortices with spinon pairs can only move perpendicular to its dipole moment. These mobility constraints, while they persist near QCP, can potentially inhibit the condensation of spinons and preclude a continuous transition from the VPS to the Néel antiferromagnet. Instead, the VPS melting transition can be driven by the proliferation of spinon dipoles. In particular, we argue that a 2d VPS can melt into a stable gapless phase in the form of an algebraic bond liquid with algebraic correlations and long-range entanglement. Such a bond liquid phase yields a concrete example of the elusive 2d Bose metal with symmetry fractionalization.

Topological quantum chemistry (TQC) framework has provided a complete description of the universal properties of all possible atomic band insulators in all space groups considering the crystalline unitary symmetries. It links the chemical and symmetry structure of a given material with its topological properties. While this formalism filled the gap between the mathematical classification and the practical diagnosis of topological materials, an obvious limitation is that it only applies to weakly interacting systems. It is an open question to which extent this formalism can be generalized to correlated systems that can exhibit symmetry protected topological Mott insulators. In this talk I will first introduce TQC and its application and then I will address this question by combining cluster perturbation theory and topological Hamiltonians within TQC. This simple formalism will be applied to calculate to the phase diagram of a representative model. The results are compared to numerically exact calculations from density matrix renormalization group and variational Monte Carlo simulations together with many-body topological invariants. References 1. M.G. Vergniory, L. Elcoro, C. Felser, N. Regnault, B.A. Bernevig, Z.Wang, “A complete catalogue of HighQuality Topological Materials“, Nature 566, 480-485 (2019) 2. B. Bradlyn, L. Elcoro, J. Cano, M.G. Vergniory, Z. Wang, C. Felser, M.I. Aroyo, B.A. Bernevig, “Topological quantum chemistry”, Nature 547 (7663), 298-305 (2017). 3. M. Iraola et al, in preparation

Bell inequalities are central tools for studying nonlocal correlations and their applications in quantum information processing. Identifying inequalities for many particles or measurements is, however, difficult due to the computational complexity of characterizing the set of local correlations. We develop a method to characterize Bell inequalities under constraints, which may be given by symmetry or other linear conditions. This allows to search systematically for generalizations of given Bell inequalities to more parties. As an example, we find all possible generalizations of the two-particle inequality by Froissart [Il Nuovo Cimento B64 , 241 (1981)], also known as I3322 inequality, to three particles. For the simplest of these inequalities, we study their quantum mechanical properties and demonstrate that they are relevant, in the sense that they detect nonlocality of quantum states, for which all two-setting inequalities fail to do so.

In (quantum) information theory and statistics it is very useful to know inference relations between random variables. These relations are generally difficult to find from the data of the random variables, for example because there are random variables that cannot be observed. However, in the relatively new study of causal inference through causal structures, we aim to find such inference relations. Causal structures are Directed Acyclic Graphs that depict correlations and causation between random variables. By analysing the entropy inequalities that are implied by such a structure, one can exclude certain causal relations by comparing the given data to the inequalities. In this talk, I will introduce a non-standard entropy inequality, known as a Zhang-Yeung inequality. We will see that there also exists a partially quantum version of this inequality. I will explain the effects of this inequality, or rather its unexpected lack thereof, on certain quantum causal structures. To conclude, I will pose a question that is still unanswered and for which I'm open to new ideas.

In this talk, I show that when the time reversal symmetry is broken in a multi-component superconducting condensate without inversion symmetry the resulting Bogoliubov quasiparticles generically exhibit mini Bogoliubov-Fermi (BF) surfaces, for small superconducting order parameter. The absence of inversion symmetry makes the BF surfaces stable with respect to weak perturbations. With sufficient increase of the order parameter, however, the Bogoliubov Fermi surface may disappear through a Lifshitz transition, and the spectrum this way become fully gapped [1]. Furthermore, an explicit example for the emergence of BF surfaces in non-centrosymmetric multicomponent superconductors is given by the discussion of d-wave superconductivity in Rarita- Schwinger-Weyl semimetals [2]. The three-dimensional Rarita-Schwinger-Weyl semimetals host pseudospin-3/2 fermions at a fourfold linear band crossing point and the spin-3/2 nature of the fermions allows for the formation of spin-2 Cooper pairs. [1] Julia M. Link and Igor F. Herbut, Arxiv:2006.10899 [2] Julia M. Link and Igor F. Herbut, Phys. Rev. B 101, 184503 (2020)

We compare the BFGS optimizer, ADAM and Natural Gradient Descent (NatGrad) in the context of Variational Quantum Eigensolvers (VQEs). We systematically analyze their performance on the QAOA ansatz for the Transverse Field Ising Model as well as on overparametrized circuits with the ability to break the symmetry of the Hamiltonian. The BFGS algorithm is frequently unable to find a global minimum for systems beyond about 20 spins and ADAM easily gets trapped in local minima. On the other hand, NatGrad shows stable performance on all considered system sizes, albeit at a significantly higher cost per epoch. In sharp contrast to most classical gradient based learning, the performance of all optimizers is found to decrease upon seemingly benign symmetry-preserving and more complex symmetry-breaking overparametrization of the ansatz class, with BFGS and ADAM failing more often and more severely than NatGrad. Additional tests for the Heisenberg XXZ model corroborate the accuracy problems of BFGS in high dimensions, but they reveal some shortcomings of NatGrad as well. Our results suggest that great care needs to be taken in the choice of gradient based optimizers and the parametrization for VQEs. This is joint work with Michael (Kastoryano) and Christian (Gogolin).

Kitaev proposed an interacting model of spins (1/2) on a honeycomb lattice exactly solvable by mapping onto free Majorana fermions coupled to a Z_2 gauge field, hosting vortices (pi flux in a hexagon). In the presence of a time-reversal symmetry-breaking term, the ground state is gapped, characterized by a Chern number v in Z and the system hosts anyonic excitations. The Chern number ν mod 16 specifies the type of anyonic excitations, hence the sixteen-fold way. Our investigation in many well-chosen triangular vortex configurations (and their duals) shows 14 out of 16 of these phases. The phases with v = +-7 are missing. In a general case, we prove that any periodic vortex configuration with an odd number of vortices per geometric unit cell can only host even Chern numbers whereas odd Chern numbers can be found in other cases. Reference: J.N. Fuchs, S. Patil, J. Vidal, https://arxiv.org/abs/2005.03655 People interested in chatting with the speaker please let it know to Matteo Rizzi, who will organize a schedule accordingly.

In this study, we seek to provide a new way of studying classical fluctuation theorems. Fully quantum fluctuation theorems employ machinery that involves explicit modeling of all degrees of freedom in a quantum setting- the system that delivers the work, the heat bath, the system that imposes the control, as well as the energy reservoir or battery that fuels the dynamics. We intend to study the effects of introducing this explicit modeling in a purely classical phase space setting. To meet this end we construct the required mathematical machinery in classical phase space and derive four versions of fluctuation theorems in terms of evolution induced on the energy reservoir by the system. One of the aims is to eliminate the implicit assumptions counted upon in standard approaches and study the effects of explicitly modeling batteries and external controls.

The efficient numerical simulation of nonequilibrium real-time evolution in isolated quantum matter constitutes a key challenge for current computational methods. This holds in particular in the regime of two spatial dimensions, whose experimental exploration is currently pushed forward using quantum simulators. In this talk I will discuss our recent efforts towards the development of versatile variational approaches, which are efficiently applicable also to two-dimensional systems. On the one hand, a perturbatively motivated variational ansatz enabled us to identify disorder-free localization in an interacting two-dimensional lattice gauge theory [1]. On the other hand, leveraging large scale computational resources, we were able to demonstrate that neural network wave functions can exceed the capabilities of tensor networks on the task of computing quench dynamics in the two-dimensional quantum Ising model [2]. [1] P. Karpov et al., arXiv:2003.04901 [2] M. Schmitt and M. Heyl, arXiv:1912.08828

This thesis explores one of the most prominent objects in quantum computing and fault-tolerance, the Clifford group, from a representation theory perspective. In particular, we look at the structure of the t tensor power oscillator representation of the symplectic group, intimately connected to the Clifford group, for multi-qudit systems -with odd local dimensions- described by the phase space formalism. The motivation of our work stems from the characterization of tensor power representations of the Clifford group. These representations appear in many applications of Quantum Information, from randomized benchmarking to quantum state distinction, as well as in quantum state tomography. Working with qudits, we are able to parametrize a Clifford unitary with the metaplectic representation of the symplectic group, which provides us with the capacity to investigate such representations in order to gain valuable information about the Clifford group. In parallel, the Clifford group and its related oscillator representation of the symplectic group are also important in representation theory, particularly in the study of the Howe duality, which involves a well-known notion in mathematical community, the so called theta correspondence. These representations are well understood in the case t < n in terms of self-orthogonal Calderbank-Shor-Steane (CSS) quantum codes and a correspondence termed "eta correspondence". The regime t > n was left unexplored. This thesis is an effort to investigate this regime. The objective is to decompose the t tensor power oscillator representation into irreducible ones, a goal that entailed two main levels. In the first, we decompose the representation into Sp irreducible ones, and the second, we restrict it to a special Abelian subgroup of the symplectic group, and we perform a further decomposition of the restricted oscillator representation into the subgroup’s irreducible ones (known as the N-spectrum in the literature). The analysis was carried out by implementing analytical formulas into the GAP System for Computational Discrete Algebra along with Sage Math and the proof techniques are based on mathematical tools from number theory as well as representation and character theory. The work done in this master thesis is a step forward in the characterization of the oscillator representation of the symplectic group, in an attempt to give relevant insights about the Clifford group.

Abstract In the context of tumor growth, niches can play an important role in the evolutionary process. In this thesis we will investigate the possible consequences of niches on the allele frequency distribution of mutations which constitutes a fundamental quantity in standard genetics. Somatic mutations in tumor tis- sue are cell–intrinsic properties whose variety constitutes to the intra–tumor heterogeneity. This can result in different phenotypes throughout the disease, that decrease the success of homogeneous therapy strategies. Intra–tumor heterogeneity can change during the outgrowth of a tumor. Here we will describe the dynamics of this evolution with a statistical model including a niche environment and will later investigate the heterogeneity that occurs within the spatial expansion of a tumor. Spatial patterns of allele frequency distributions can teach us about the underlying evolutionary processes and moreover show rules and limitations for the interpretation on cancer sequencing data.

Novel computational paradigms in combination with proper hardware solutions are required to overcome the limitations of our state-of-the-art computer technology, in particular regarding energy consumption. Due to the inherent complex and non-linear nature, spintronics offers the possibility towards energy-efficient, non-volatile hardware solutions for various unconventional computing schemes.[1-3] In this talk, I will discuss the possibility of using magnetic skyrmions for in particular two unconventional computing schemes – reservoir computing and stochastic computing. Reservoir computing is a computational scheme that allows to simplify spatial-temporal recognition tasks. We have shown that random skyrmion fabrics provide a suitable physical implementation of the reservoir[4,5] and allow to classify patterns via their complex resistance responses either by tracing the signal over time or by a single spatially resolved measurement.[6] Stochastic computing is a computational paradigm that allows to speed up a calculation while trading for numerical precision. Information is encoded in terms of bitstreams as a probability. A key requirement and simultaneously a challenge is that the incoming bitstreams are uncorrelated. The Brownian motion of magnetic skyrmions allows creating a device that reshuffles the bit-streams.[7,8] [1] J. Grollier, D. Querlioz, K. Y. Camsari, KES, S. Fukami, M. D. Stiles, Nat. Electron. (2020) [2] E. Vedmedenko, R. Kawakami, D. Sheka, ..., KES, et al., J. of Phys. D (2020) [3] G. Finocchio, M. Di Ventra, K.Y. Camsari, KES, P. K. Amiri and Z. Zeng, arXiv:1907.04601 [4] D. Prychynenko, M. Sitte, et al, KES, Phys. Rev. Appl. 9, 014034 (2018) [5] G. Bourianoff, D. Pinna, M. Sitte and KES, AIP Adv. 8, 055602 (2018) [6] D. Pinna, G. Bourianoff and KES, arXiv:1811.12623 [7] D. Pinna, F. Abreu Araujo, J.-V. Kim, et al, Phys. Rev. Appl. 9, 064018 (2018) [8] J. Zazvorka, F. Jakobs, D. Heinze, ..., KES, et al., Nat. Nanotech. 14, 658 (2019)

While molecular gases are well described by established kinetic theories, when macroscopic granular particles replace molecules as the constituents in a granular gas, new challenges arise due to the dissipative nature of their collisions. Including dissipation into the theories leads to three main predictions: 1. non-Maxwellian velocity distributions, 2. homogeneous cooling following the algebraic Haff's law and 3. emergence of cluster instabilities. Experimental test of the theories requires (i) a low-gravity environment, and (ii) external excitation of the granular particles that can drive them into a homogeneous spatial distribution. An experiment - Magnetically Excited Granular Matter (MEGraMa) - was developed at DLR to meet these demands and flown on board of parabolic flights, in a drop tower, and on a sounding rocket. While the first two theoretical predictions can be demonstrated convincingly, quantitative deviations between theory and experiment are apparent.

In this talk I will describe how topology emerges in crystalline environments from the perspective of local orbitals and highlight subtleties which appear when characterizing topology protected by crystalline symmetry. I will introduce a new notion of a topological obstruction which is not protected by bulk energy gap closings in periodic boundary conditions, but only in systems with open boundaries, which we call boundary obstructed topological phases. We will describe in this context on a celebrated model - the quantized quadrupole insulator in two-dimensions.

Networks based on entangled quantum systems enable interesting applications in quantum information processing and the understanding of the resulting quantum correlations is essential for advancing the technology. We show that the theory of quantum coherence provides powerful tools for analyzing this problem. For that, we demonstrate that a recently proposed approach to network correlations based on covariance matrices can be improved and analytically evaluated for the most important cases.

Systems consisting of a large number of interacting bosons or fermions in two dimensions can in some situations form anyonic quasiparticles that are neither bosons, nor fermions. Such systems are topologically ordered, and they have many special properties. They are, for instance, not characterized by a local order parameter, which makes it challenging to detect the phases. Here, we demonstrate that anyons are an interesting tool to detect topological order in new types of systems and to detect phase transitions involving topological phases. The phase transitions are detected based on the idea that the system does not support the same types of quasiparticles on the two sides of the transition. We test the method on three rather different examples, and in all cases we find that it is sufficient to study a simple property of the anyons, requiring low numerical costs, to accurately detect the phase transition point. A further advantage of the method is that it does not require particular boundary conditions, and we utilize this to demonstrate that topological order can also occur in fractal dimensions and on quasicrystals. References: arXiv:1909.02046, arXiv:1907.03193, PRB 101, 115413 (2020).

Abstract: A major goal for theoretical physics is the unification of gravity with quantum mechanics. An important tool for tackling this question is holography, or the duality between a theory of gravity and a conformal field theory in one lower dimension. In this talk I will address the question of how to reconstruct the geometry of a gravitational spacetime with field theoretic tools, relying on observables such as Wilson lines that are amenable to quantization. I will invoke a varied toolkit drawing on techniques from many areas of physics and mathematics, from quantum information to symplectic geometry. Importantly, my focus will be on de Sitter spacetime, which closely resembles our actual universe, and whose quantization could give insight into non-perturbative phenomena in cosmology.

A major goal of modern physics is to understand and test the regime where quantum mechanics and general relativity both play a role. However, its distinctive effects are typically relevant at high energies or strong gravitational fields, beyond the reach of present-day experiments. In this seminar I will discuss a different approach, focused on low-energy but composite quantum systems. Internal dynamics of composite systems can act as a sensitive probe of relativistic effects even at low-energies and in weak gravitational fields -- I will explain what quantum and general relativistic effects can arise for such systems and how they can be tested. I will outline the broader relevance of this approach for the research on joint foundations of quantum mechanics and general relativity, and for the future quantum-enhanced technologies such as metrology and sensing, or information processing.

There is evidence that spacetime and gravity emerge from the dynamics of more fundamental, strongly coupled degrees of freedom. The most concrete realization of this idea is the AdS/CFT paradigm, where gravity is holographically encoded in the physics of certain conformal field theories. I will describe some of my contributions towards understanding fundamental aspects of AdS/CFT, in particular the search for the dual description of the volume of the emergent space, and general results about chaos and the quantum butterfly effect in strongly coupled quantum systems.

In this talk, I will review the recent advances in the understanding of intriguing infinite-dimensional symmetries that emerge at the boundary of spacetimes. These symmetries play an important role in the description of many phenomena such as gravitational memory effects, scattering amplitudes in flat space and black hole physics. By connecting seemingly unrelated topics in theoretical physics, infinite-dimensional symmetries have shed a new light on the fascinating universal features of gauge theories and gravity.

In the search for a theory of quantum gravity that is able to resolve singularities in classical general relativity and explain the first moments of the universe, string theory delivers powerful tools: the “holographic” AdS/CFT duality allows us to study quantum gravity using much better understood field theory methods. I will explain some of the progress towards an explicit decoding of relevant aspects of gravity from a holographic quantum field theory. I will particularly focus on the emergence of spacetime dynamics from quantum entanglement, and on a new way to think about quantum chaos which serves as a useful diagnostic of black hole physics.

A non-Hermitian PT-symmetric version of the kicked top is introduced to study the interplay of quantum chaos with balanced loss and gain. The classical dynamics arising from the quantum dynamics of the angular momentum expectation values are derived. It is demonstrated that the presence of PT-symmetry can lead to ''stable'' mixed regular chaotic behaviour without sinks or sources for subcritical values of the gain-loss parameter. For large values of the kicking strength a strange attractor is observed that also persists if PT-symmetry is broken. Classical structures are also identified in the quantum dynamics. Finally, some of the statistics of the eigenvalues of the quantum system are analysed.

I will present three mechanisms for emergence of gravity from 2D conformal field theory (CFT). Two of them are based on entanglement, exemplifying the intricate relation between quantum information and quantum gravity, and one on a deformation of the CFT with an irrelevant operator. While the context of this work is holography, the mechanisms themselves are non-holographic. The resulting gravitational system is of direct relevance to the set-up used in recent papers reporting significant progress on the black hole information paradox

No, we can’t. The time at which the growth in the number of infected individuals halts and starts decreasing cannot be calculated with certainty before the turning point is actually attained. This assertion is illustrated by adding to a standard SIR model a new class for confined individuals. A Bayesian fit to the on-going COVID-19 pandemic in Spain shows that a slow-down in the number of newly infected individuals during the expansion phase allows to infer neither the precise position of the maximum nor whether the measures taken will bring the propagation to the inhibition regime. Our study warns against precise forecasts of the evolution of epidemics based on mean-field, effective models, and supports that only probabilities of different outcomes can be confidently given.

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This talk will describe some current efforts to track, understand, and control the current corona virus pandemic. We will review basic epidemiology and discuss the dynamics of transmissions, as well as monitoring efforts aimed to contain local outbreaks. We will also compare different scenarios of viral evolution.

Quantum systems bring unprecedented advantages to the realization of classical cryptographic tasks. A remarkable example is quantum key distribution, which allows two parties, Alice and Bob, to establish a shared secret key that can be used for unconditionally secure communication. Furthermore, the strong correlations exhibited by quantum systems can be explored to construct cryptographic protocols that are secure even when the parties are completely ignorant about the internal working of their devices. This is the device-independent paradigm. In this talk I will introduce the device-independent scenario and comment on recent results on device-independent quantum key distribution. Then I will move to the scenario of many parties and discuss multipartite cryptographic tasks. Finally, I will present new results on bounding the uncertainty of an eavesdropper in this setting, and discuss their application to multipartite cryptographic protocols.

Since the beginning of Information Theory, people have been interested in the limitations of communication and information. These limitations are completely characterized for two or three parties by Shannon inequalities: Strong subadditivity and monotonicity. However, with four or more parties it turns out there are infinitely many (!) relevant non-Shannon type inequalities, called Zhang-Yeung inequalities. The origin of these inequalities is still poorly understood. In the quantum case, Shannon inequalities are replaced by Von Neumann inequalities, but the question is whether there are indeed also non Von Neumann inequalities. I will talk about such inequalities for specific quantum states and their implications for (quantum) causal structures. These causal structures are a relatively recent development in the area of causal inference. The use of quantum Zhang-Yeung type inequalities might further specify the difference between certain quantum causal structures.

Unsharp quantum measurements provide a resource in scenarios where one faces the trade-off between information gain and disturbance. In this work we propose a scheme for semi-device-independent self-testing of unsharp measurements and show that all two-outcome qubit measurements can be self-tested in a robust way. Along with the main result we introduce a method to bound correlations in sequential measurement scenarios using semi-definite programming. This method can also be applied to refine security analysis of the semi-device-independent one-way quantum key distribution. We also present new information gain – disturbance relations for pairs of dichotomic measurements.

Sampling from quantum Gibbs states has proven to be a task of major importance in quantum computation. This problem is not trivial even for quantum computers since the exponential of a quantum many-body Hamiltonian requires too much computational power. On the one hand, different methods to prepare quantum Gibbs states have already been proposed. On the other hand, as far as we know, no certification protocol has been proposed so far. Here, we propose methods to certify one-dimensional Gibbs state preparations. The main idea behind the methods is to build a fidelity witness for the purification of the target Gibbs state, which is known as the thermofield double state. The procedure to build the parent Hamiltonian relies on a useful locality bound for imaginary-time evolutions, in the same way Lieb-Robinson bounds are used for real-time ones. The method allows a highly efficient estimation of the fidelity lower-bound through block quantum tomography.

Domain walls naturally arise whenever a symmetry is spontaneously broken. They interconnect regions with different realizations of the broken symmetry, promoting structure formation from cosmological length scales to the atomic level. In my talk, I will present 1D spin textures and domain walls with intriguing physical properties, which emerge in spin-spiral multiferroics and chiral magnets and which hold great promise for nanoelectronics and spintronics applications. In particular, I will discuss that a wide variety of new domain walls occurs in the presence of spatially modulated domain states. In contrast to domain walls in conventional ferroics, such domain walls exhibit a well-defined inner structure, which—analogous to cholesteric liquid crystals—consists of topological disclination and dislocation defects. Similar to the magnetic skyrmions, the domain walls can carry a finite topological charge, permitting an efficient coupling to spin currents and contributions to a topological Hall effect. Our studies establish domain walls in chiral magnets as functional nano-objects with non-trivial topology, opening the door to innovative device concepts in information and communication nanotechnology.

The idea to probe, change and control functional materials properties with the help of light has long intrigued researchers in materials science. Using femtosecond laser pulses it is now possible to control the magnetic order or even enhance superconductivity. Femtosecond laser excitation of solid-state systems creates out-of-equilibrium hot electrons that cool down by transferring their energy to other degrees of freedom and ultimately to lattice vibrations of the solid. The understanding of this strongly non-equilibrium dynamics in solids is still very limited, in spite of its emerging importance from a fundamental and applied science viewpoint. The three-temperature model, commonly used for magnetic metals, assumes that the electronic, spin and lattice subsystems are each in separate equilibrium at all times and reach global equilibrium by exchanging heat. The equilibrium concept of “temperature” is so powerful that is is often ignored that it takes time before non-equilibrium dynamics can be described by it. In this lecture I will give an overview how modern ultrafast spectroscopy and scattering techniques allow us to determine in a momentum-resolved way the electron, spin and lattice excitations in the laser-heated state. These results demonstrate that we need to view non-equilibrium dynamics by its associated flow of energy between the various degrees of freedom.

We will first review spectral flow and its application to bulk and boundary properties of the quantum Hall effect. We will then introduce the so-called KO-valued spectral flow, defined in joint work with Alan Carey, Matthias Lesch and Adam Rennie, and how it can be used to recover the classification of topological insulators and superconductors by Kennedy and Zirnbauer. Time permitting, we will then present recent joint work with Johannes Kellendonk and Adam Rennie, which uses the Cayley transform on Hilbert C*-modules to generalise many of these ideas and gives analogous results to recent work by Alldridge, Max and Zirnbauer.

Layered materials host various collective quantum states such as intertwined electronic spatial orders, superconductivity or topological spin liquids. I will present two examples from our recent work, which are concerned with then manipulation of such collective quantum states. The first example is the laser-induced metal-insulator transition of 1T-TaS2, which is currently discussed intensively in relation to a somewhat enigmatic hidden state. Our experiments reveal the microscopic nature of this hidden state and show that the non-thermal metal-insulator transition of 1T-TaS2 corresponds to a transition between two long-range ordered electronic crystals. The second example is alpha-RuCl3 - a candidate material for Kitaev magnetism and the corresponding spin-liquid ground state. However, at ambient pressure and low temperatures this material does not exhibit a spin-liquid phase, but antiferromagnetic long-range order. We discovered a high-symmetry rhombohedral phase of alpha-RuCl3 at non-ambient pressure, which may indeed support a Kitaev spin-liquid ground state.

Fully quantum fluctuation relations incorporate coherence and quantum correlations, but typically require explicit modeling of components that more traditional approaches (both classical and quantum) only treat implicitly. Such components include the system that delivers the work, as well as the system that imposes the external control. In this project, we strive to mimic the general machinery of fully quantum fluctuation relations in a purely classical phase space setting, and within this construction derive classical counterparts to various fully quantum fluctuation relations. One goal is to understand the effects of introducing explicit batteries and external controls in the classical scenario, and how this relates to standard classical approaches. A further aim is to enable a comparison between fully quantum fluctuation relations and classical fluctuation relations on a more equal footing.

Machine learning techniques can boost the exploration of complicated phase diagrams in many-body systems. To date, most studies are tied to situations involving only one phase transition and one order parameter. Systems accommodating multiple phases of coexisting and competing orders remain largely unexplored. Using Support Vector Machines (SVMs) we focus on multipolar orders and their tensorial order parameters, whose identification is difficult with traditional methods. The key property that allows one to make progress is the interpretability of the decision function, from which the physical order parameter can be deduced. Furthermore, we discuss a second intrinsic parameter of the SVM, the bias, which allows one to make an unsupervised graph analysis of the topology of the phase diagram. We illustrate our tool for the classical XXZ model on the frustrated pyrochlore lattice. Unexpectedly, the machine could also learn local constraints hinting at various types of spin liquids resulting in a complete classification of all types of behavior for this model.

In this talk, I will review a recent paper (https://arxiv.org/abs/1912.02207) giving a somewhat interesting perspective on Grover's algorithm in terms of billiard balls.

Mott insulators with strong spin orbit coupling have become a playground to search for exotic quantum phases, spin liquids and emergent Majorana fermions. In this context I will present some of our results for the dynamics of Kitaev (-Heisenberg) models, focusing on magnetic thermal transport, but also on phonon relaxation. Fractionalization of magnetic moments into mobile Majorana matter and a static Z2 gauge field in the pure Kitaev limit, lead to characteristic spectral continua. These continua are strongly affected by thermal flux excitations which act as an emergent disorder. I will discuss the consequences, using results from several approaches comprising a mean-field theory, complete summation over all gauge sectors, exact diagonalization, and quantum typicality calculations.

Quantum information theory combines classical information theory with quantum physics. This combination opens the door to many fascinating applications which either do not have a classical counterpart or outperform them. Prominent examples thereof are quantum communication and quantum computation and simulations. Many of the application are possible due to the subtle properties of quantum many-body systems. In particular entanglement, which is a strong correlation among several quantum systems, plays an essential role. In this talk, I will first present some recent results on multipartite entanglement theory. In particular, I will show that deterministic local transformations among pure n-partite d-level systems are almost never possible. The consequences of these findings in the context of entanglement theory will be discussed. Then I will talk about certain aspects of quantum computations, where entanglement plays an important role. I will consider quantum algorithms which are composed of so-called matchgates. I will show that such an algorithm can always be compressed into an exponentially smaller quantum computation. However, as will be explained, the usage of an additional resource, the so-called magic states, elevates such a computation to universal quantum computation, while maintaining the same gate set. I will present the characterization of these magic states and discuss the consequences of these results in the context of quantum computation.

Twisted bilayer graphene near the magic angle exhibits remarkably rich electron correlation physics, displaying insulating, magnetic, and superconducting phases. Here, using measurements of the local electronic compressibility, we reveal that these phases originate from a high-energy state with an unusual sequence of band populations. As carriers are added to the system, rather than filling all the four spin and valley flavors equally, we find that the population occurs through a sequence of sharp phase transitions, which appear as strong asymmetric jumps of the electronic compressibility near integer fillings of the moire lattice. At each transition, a single spin/valley flavor takes all the carriers from its partially filled peers, "resetting" them back to the vicinity of the charge neutrality point. As a result, the Dirac-like character observed near the charge neutrality reappears after each integer filling. Measurement of the in-plane magnetic field dependence of the chemical potential near filling factor one reveals a large spontaneous magnetization, further substantiating this picture of a cascade of symmetry breakings. The sequence of phase transitions and Dirac revivals is observed at temperatures well above the onset of the superconducting and correlated insulating states. This indicates that the state we reveal here, with its strongly broken electronic flavor symmetry and revived Dirac-like electronic character, is a key player in the physics of magic angle graphene, forming the parent state out of which the more fragile superconducting and correlated insulating ground states emerge.

Topological defects in ferroic materials are low-dimensional systems that may enable promising applications. We study domain wall and vortex-localized vibrations in proper and improper ferroelectrics and their role in dielectric loss, conductivity, acoustic phonon scattering and elastic anomalies. The lowest energy vibrations correspond to sliding modes of these topological defects, are polar and are found at microwave frequencies. An opposite situation of hard localized vibrations is found in cluster compounds, where transition-metal ions form clusters. In these cases vibrations are hard. Particularly, in IrTe2 they give rise to a complex phase diagram with a large number of phases. A simplified model is proposed, parameterized from first principles, and solved for the phase diagram. The influence of spin-orbit coupling, pressure and doping is studied.

Recently, three-dimensional topological spin textures called the magnetic hedgehog lattices (HLs) were discovered in the B20-type compounds MnSi1-xGex [1]. The HLs have periodic arrays of magnetic monopoles and anti-monopoles, which induce interesting transport phenomena, such as the topological Hall effect and the topological thermoelectric transport. However, their stabilization mechanism has not been fully understood thus far. Here we investigate the ground state of an effective spin model with long-range interactions arising from the itinerant nature of electrons by variational calculations and simulated annealing. We find that the HLs are stabilized even at zero field by the synergetic effect of symmetric and antisymmetric interactions from the spin-charge and spin-orbit couplings, respectively. We clarify the full phase diagram in the magnetic field, which includes multiple phase transitions with changes in the number of monopoles and anti-monopoles [2]. We also find that the monopoles and anti-monopoles move, repel, and pair annihilate with topological phase transition by increasing the magnetic field successively [3]. [1] Y. Fujishiro et al., Nat. Commun. 10, 1059 (2019). [2] S. Okumura, S. Hayami, Y. Kato, and Y. Motome, arXiv:1908.05044. [3] S. Okumura, S. Hayami, Y. Kato, and Y. Motome, arXiv:1909.01316.

Graphene is one of the most versatile materials to realize exotic phenomena in condensed matter, as a consequence of the emergent Dirac dispersion at low energies. Introducing one more layer of complexity, we find bilayer graphene, which can be manipulated to achieve an arbitrary twist angle between the two layers. Twisted bilayer graphene (TBG) has recently attracted a lot of attention for the observed superconductivity and correlated electronic behavior at particular twist angles called magic angles. Here, I discuss two novel ways to control the electronic properties of twisted bilayer graphene, in particular: - Artificial gauge fields: We established that the application of a homogeneous electric field perpendicular to the bilayer is mathematically equivalent to a new kind of synthetic gauge field in the small twist angle regime. This identification opens the door for the generation and detection of pseudo-Landau levels in graphene platforms within robust setups, which do not depend on strain engineering. Furthermore, this new artificial gauge field leads to the development of highly localized modes in emergent triangular or Kagome lattices, associated with flat bands close to charge neutrality. - Triple point Fermions: These are elusive electronic excitations that generalize Dirac and Weyl modes beyond the conventional high energy paradigm. Yet, finding real materials naturally hosting these excitations at the Fermi energy has remained a challenge. In this work, we show how TBG can realize robust triple point fermions in two dimensions by the introduction of localized impurities.

Recent experiments in quantum Hall bilayer systems have revealed a new strongly correlated state: the semi-quantized quantum Hall state. This state arises in a bilayer quantum Hall system with strong interlayer-interactions, has a quantized Hall resistance and vanishing longitudinal resistance, while at the same time featuring a gapless sector that allows this state to exist for a continuum of filling factors. In this talk, I will explain what a semi-quantized quantum Hall state is, and use a coupled-wire construction to predict possible future experiments in the already existing experimental platforms.

Large Li2FeSiO4 single crystals featuring the high temperature Pmnb-phase (γ-Li2FeSiO4) were grown for the first time by the optical floating zone method under high Ar-pressure and the single crystal structure was solved. The system exhibits tetrahedrally coordinated Fe2+ ions in a high-spin (i.e.,

Nonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, one can alternatively use localized optical resonances with high quality factors to increase light-matter interaction times at the expense of inelastic losses associated with the nonlinear response. In this context, highly-doped graphene has been identified as a promising material platform for nonlinear optics that combines long-lived, electrically-tunable plasmons with an intensely anharmonic response to light originating from its unique electronic band structure. I will discuss recent progress in the exploration of nonlinear graphene plasmonics, starting with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene and its enhancement by resonant coupling to its supported plasmons of high harmonic and Kerr nonlinearities, strong interactions of nonlinear plasmons with quantum emitters or amongst themselves, and the extraordinary thermo-optical capabilities of this material enabling nonlinear optical switching on the single-photon level.

The study of polynomials that are positive on certain sets has a rich history, going back to Hilbert's seventeenth problem. Here we will look at multivariate polynomials (and more generally, tensor contractions) that have matrices as their variables. We present a family of maps that are positive semi-definite on the positive cone. This extends the well-known concept of positive maps as used in entanglement theory to the multilinear case. We will present connections to polynomial identity rings and central polynomials, concepts that found recent use in quantum information in the context of MPS bond dimension witnesses and remote time manipulation.

The continuum of excitations in RuCl3 have been studied to high temperature—less-well studied are the effects of high magnetic fields. We measured the magnetotropic coefficient—second derivative of the free energy with respect to field-angle—up to 65 tesla. We find a scale-invariant response with applied temperature and magnetic field, contrasting with the large exchange energy apparent in the angle-dependence. This paradox hints at a strongly-correlated magnetic state in the high-field limit of RuCl3.

A unified theory for space-time and matter might be based on finite projective geometries instead of differentiable manifolds and fields. Each point of the world is equipped with a quadratic form over a finite Galois field which define neighbors in the finite set of points. Due to the projective equivalence of all quadratic forms this world is necessarily a 4-dimensional, locally Lorentz-covariant space-time with a gauge symmetry G(3)xG(2)xG(1) for internal points which represent elementary particle degrees of freedom. Thus, matter appears as a geometric distortion of an inhomogeneous field of quadrics and all physical properties (spins, charges) of the standard model seem to follow from its finite geometric structure in a continuum limit. The finiteness inevitably induces a fermionic quantization of all matter fields and a bosonic for gauge fields. The main difference to Einstein's general theory of relativity is the use of finite fields instead of real numbers to parametrize points of events.

Presentation of projects A1-A5, B2, B6-B8

Presentation of projects A6, B1, B3, B4, B5

Divalent silver (Ag2+) is a homologue of its well-known copper sibling, and it comes with Jahn-Teller effect, paramagnetism, and strong influence of electronic correlation on chemical bonding. I will briefly outline our two-decade lasting efforts to show that many analogies exist between parent compounds of oxocuprate superconductors and fluorides of Ag2+. Strong magnetic superexchange and prospect for achieving superconductivity in the latter will be discussed.

The ratchet effect in the colloidal suspension moving in a narrow channel of a periodically varying diameter can be used for particle separation by sizes. We study this phenomenon by mapping the colloid in the channel onto one-dimensional lattice models in which particles stochastically jump between sites with exclusion interaction. Besides allowing multiple occupancy of sites, we consider a mixture of small and big particles as well as complex geometry of the one-dimensional lattice when each site is joined to a lateral pocket. Currents of particles calculated within the mean-field approximation of the master equation approach are compared to Monte Carlo simulations. Phase diagram contains the density domain where the ratchet currents of small and big particles are of opposite directions, thus enabling full size separation

We study the quantum propagation of a skyrmion in chiral magnetic insulators by generalizing the micromagnetic equations of motion. The fluctuations around the skyrmionic configuration give rise to a damping derived microscopically, which in some limit reduces to a skyrmion mass. We demonstrate that a skyrmion in a confined geometry behaves as a massive particle, a discovery with great impact on the technologically important case of linear tracks relevant for magnetic memory devices. In the presence of time-dependent oscillating magnetic field gradients, the unavoidable coupling of the external field to the magnons gives rise to time-dependent dissipation for the skyrmion, with measurable consequences on the skyrmion path. These ac fields act as a net driving force on the skyrmion via its own intrinsic magnetic excitations. We generalize the standard quantum theory of dissipation to include the stochastic effects of the driven bath on the skyrmion dynamics. Our work initiates studies towards the possibility of observing a quantum mechanical behavior at a mesoscopic scale. I will briefly talk about the observability of tunneling events, in particular quantum depinning of a magnetic skyrmion out of a pinning center.

Metal-insulator transitions (MIT), driven entirely by strong electron-electron interactions and/or disorder, continue to underpin many novel phenomena in quantum matter. We study disorder-driven MITs in the "strong disorder" category using a novel semi-analytic cluster adaptation of dynamical mean-field theory (CDMFT) for the simplest, binary-alloy disorder (also known as spinless Falicov-Kimball, or "simplified" Hubbard) model. Such an (oversimplified) model is a caricature for a disordered system where mobile electrons interact strongly with positionally random, localized charges. We uncover anomalous quantum criticality in the full dc conductivity tensor across the MIT which, remarkably, is reminiscent of "Mott quantum criticality" that obtains at the low-T end-point of the line of first-order transitions in the Hubbard model (experimentally best studied for 2D organics). Interestingly, such novel criticality is also found to underpin thermal transport (heat conductivity, Seebeck co-efficient, Lorenz number) across the MIT. Moreover, we also provide an explicit realization of the famed "universal dielectric response" (UDR) of Jonscher in optical responses across the MIT. The associated stretched-exponential relaxation in the long-time dielectric response strongly suggests anomalous diffusion, an emergent "electron glassy" dynamics and weak ergodicity breaking. These findings suggest that a truly novel type of carrier localization obtains in the "strong disorder" limit.

Some of the most well-known examples of adaptive evolution involve single mutations with large beneficial effects. However, predicting the evolutionary impact and fate of a new mutation remains difficult. To simplify this problem, we often bin mutations into categories that are expected to have distinct fitness impacts (e.g. synonymous vs. non-synonymous; coding vs. non-coding). However, there is little empirical evidence for differential fitness effects of mutational types across the genome, because prior work has largely focused on the distribution of fitness effects (DFE) for beneficial mutations in single genes. Thus, the general importance of mutation type relative to other factors such as genetic background and environment remains unclear. In this talk, I will describe our efforts to address this problem using bacterial strains, each with a single mutation in its genome that either occurred under strong selection or under genetic drift. Briefly, I suggest that the power and promise of new mutations depends more on the genetic background on which they occur, and very little on the mutational category to which they belong.

We explore the intriguing spatial patterns that emerge in a two-dimensional spatially inhomogeneous Katz-Lebowitz-Spohn (KLS) driven lattice gas with attractive nearest-neighbor interactions. The domain is split into two regions with hopping rates governed by different temperatures T > T_c and T_c, respectively, where T_c indicates the critical temperature for phase ordering, and with the temperature boundaries oriented perpendicular to the drive. In the hotter region, the system behaves like the (totally) asymmetric exclusion processes (TASEP), and experiences particle blockage in front of the interface to the critical region. To explain this particle density accumulation near the interface, we have measured the steady-state current in the KLS model at T > T_c and found it to decay as 1/T. In analogy with TASEP systems containing "slow" bonds, transport in the high-temperature subsystem is impeded by the lower current in the cooler region, which tends to set the global stationary particle current value. This blockage is induced by the extended particle clusters, growing logarithmically with system size, in the critical region. We observe the density profiles in both high-and low- temperature subsystems to be similar to the well-characterized coexistence and maximal-current phases in (T)ASEP models with open boundary conditions, which are respectively governed by hyperbolic and trigonometric tangent functions. Yet if the lower temperature is set to T_c, we detect marked fluctuation corrections to the mean-field density profiles, e.g., the corresponding critical KLS power law density decay near the interfaces into the cooler region. If the temperature interface is aligned parallel to the drive, we observe the cooler region to act as an absorbing sink for particle transport, with blockages emerging at the subsystem boundaries.

We give a quantum speedup for solving the canonical semidefinite programming relaxation for binary quadratic optimization. The class of relaxations for combinatorial optimization has so far eluded quantum speedups. Our methods combine ideas from quantum Gibbs sampling and matrix exponent updates. A de-quantization of the algorithm also leads to a faster classical solver. For generic instances, our quantum solver gives a nearly quadratic speedup over state-of-the-art algorithms. We also provide an efficient randomized rounding procedure that converts approximately optimal SDP solutions into constant factor approximations of the original quadratic optimization problem.

Raman spectroscopy is a widely used, powerful, and nondestructive tool for studying the vibrational properties of bulk and low-dimensional materials. Raman spectra can be simulated using first-principles methods, but due to the high computational cost calculations are usually limited only to fairly small unit cells, which makes it difficult to carry out simulations for alloys and defects. We recently developed an efficient method for simulating Raman spectra at nearly first-principles accuracy of (certain) systems which commonly require excessively large supercells. In my talk, I will introduce the method and demonstrate its applicability in the case of 2D materials when the parent material is modified through alloying, introduction of defects, or stacking of layers, although the approach is not limited to 2D materials and should be applicable to any crystalline solid with defects and impurities. As a result, we can not only provide simulated spectra that can be directly compared to the experimental one, but also gain atomistic insight on the modifications of the vibrational spectra that are responsible for the observed changes.

Driven-dissipative systems can potentially exhibit non-equilibrium phenomena that are absent in their equilibrium counterparts. However, phase transitions present in these systems generically exhibit an effectively classical, equilibrium behavior in spite of their non-equilibrium origin. In this talk, I investigate an experimentally motivated model where two Ising-like order parameters interact and form a multicritical point. I will discuss how at such a multicritical point, new non-equilibrium criticality can emerge. These non-equilibrium multicritical points exhibit a variety of exotic phenomena with no counterpart in equilibrium, including spiraling phase boundaries, the emergence of discrete scale invariance rather than the more familiar continuous scale invariance, and the violation of the fluctuation-dissipation theorem at all length scales, resulting in a sytem which becomes “hotter” and “hotter” at longer and longer wavelengths. Furthermore, I will discuss how for more complex order parameters with a different form of symmetry, additional non-equilibrium multicritical points can emerge.

The concept of electronic quasiparticles, as introduced by Landau, is one of the cornerstones of the theory of quantum many-body systems. However, a growing number of recent experiments in strongly correlated quantum materials have forced us to confront the existence of quantum matter for which the concept of electronic quasiparticles does not apply. Inspired by the rich phenomenology of the parent states of numerous high-temperature superconductors, I will describe some recent progress in our understanding of metallic states that do not admit a quasiparticle description but that nonetheless have a sharply-defined Fermi surface. I will also present some experimental results on unconventional transport properties of magic-angle twisted bilayer graphene and comment on their possible connections with the rest of my talk.

The postulates of quantum mechanics can be divided into two parts: the dynamical part which defines the pure states, their dynamics and composition rule and the probabilistic part which defines the structure of measurements and the probability rule. In this talk I will show that the dynamical part of quantum theory fully determines the probabilistic part, within the operational framework. First I will show that for single systems there are infinitely many probabilistic structures compatible with the dynamical structure, and that these can be put into correspondence with certain representations of the unitary group. Following this I will outline the operational conditions which arise when considering multiple systems, and how they constrain the possible probabilistic structures. I will then outline how the condition of associativity (of systems) singles out the measurement postulates of quantum mechanics amongst all possible alternatives. In a final part I will discuss how this result relates to existing work, such as Gleason’s theorem and Zurek’s derivation of the Born rule from envariance.

Continuum limit of Dirac solids can be described as a Minkowski spacetime. Certain nonsymmorphic 2D lattices host tilted Dirac fermions. The effective spacetime felt by these electrons is a deformation of Minkowski spacetime where temporal and spatial affairs are mingled. In this talk, first we use irreducible representations of the 8Pmmn borophene to show that not only the resulting spacetime is the tilt deformation of Minkowski spacetime, but also the resulting tilt can be manipulated by a perpendicular electric field. We discuss unusual plasmons and electromagnetic response of 2D tilted-Dirac cone solids.

It is an open question how well tensor network states in the form of an infinite projected entangled-pair states (iPEPS) tensor network can approximate gapless quantum states of matter. In this talk we address this issue for two different physical scenarios: (i) a conformally invariant (2+1)d quantum critical point in the incarnation of the transverse-field Ising model on the square lattice and (ii) spontaneously broken continuous symmetries with gapless Goldstone modes exemplified by the S=1/2 antiferromagnetic Heisenberg and XY models on the square lattice. We find that the energetically best wave functions display finite correlation lengths and we introduce a powerful finite correlation length scaling framework for the analysis of such finite bond dimension (finite-D) iPEPS states. The framework is important (i) to understand the mild limitations of the finite-D iPEPS manifold in representing Lorentz-invariant, gapless many-body quantum states and (ii) to put forward a practical scheme in which the finite correlation length ξ(D) combined with field theory inspired formulas can be used to extrapolate the data to infinite correlation length, i.e., to the thermodynamic limit. The finite correlation length scaling framework opens the way for further exploration of quantum matter with an (expected) Lorentz-invariant, massless low-energy description, with many applications ranging from condensed matter to high-energy physics.

I will discuss two examples of the interplay of distortion and Mott insulator physics. In certain ABO3 perovskite materials including BiNiO3, it is found that the Mott transition is accompanied by a symmetry change and a colossal volume collapse upon entering the metallic phase with pressure or temperature. We show that may be viewed as a new type of “hybridization-switching” Mott transition. In a different setting, the spinel compound NiRh2O4 has been argued to possibly form a S=1 spin liquid state on the diamond lattice. However, we show that the physics in the presence of distortion and spin-orbit coupling is quite distinct, leading to a non-magnetic “atomic” ground state, and we use slave boson theory to discuss its magnetic excitations, make comparison with current experiments, and predict “dark states” which are invisible to neutrons but may be possibly probed using RIXS experiments.

I will present a consistent scheme of quantization of chiral flows (flows with extensive vorticity) in ideal hydrodynamics in two dimensions. Chiral flows occur in rotating superfluid, rotating turbulence and also in electronic systems in the magnetic field in the regime of a fractional Hall effect. The quantization is based on a geometric relation of chiral flows to two-dimensional quantum gravity and is implemented by the gravitational anomaly. The effect of the gravitational anomaly violates the major property of classical hydrodynamics, the Helmholtz law: vortices are no longer frozen into the flow. Effects of quantization could be cast in the form of quantum stress. I show that the quantum stress generates Virasoro algebra, the centrally extended algebra of holomorphic diffeomorphisms.

The Sachdev-Ye-Kitaev (SYK) model describes a system of a large number of randomly interacting Majorana fermions. It stands in the tradition of the random k-body interaction models pioneered in nuclear physics and later applied in condensed matter contexts. The SYK model can be looked at from three interrelated perspectives: a) a system showing many body chaos and random matrix correlations, b) a paradigm of strongly correlated (Majorana) quantum matter, and c) the holographic shadow of a two-dimensional AdS2 gravitational bulk. The interplay of these three has made it a focus of intensive research. Previous analytic work was restricted to the study of quantum correlations at time scales short compared to the inverse of the many body level spacing. In this talk, we address the complementary regime of large times. We will apply a matrix integral formalism to identify a set of collective modes in Fock space describing the relaxation of the system towards an ergodic long time limit. We will discuss universal signatures of these modes in spectral correlations, and compare our results to numerics. Finally, we will discuss the structure of the system’s many body wave functions and point out differences to the wave functions of chaotic single particle systems.

Spin plays a central role in a wide range of optoelectonic processes in organic materials and devices. I will introduce electrical and optical detection schemes for electron spin resonance (ESR), which can be used to probe the Optoelectronic properties of organic electronic devices. I will then discuss results of ESR experiments on materials for singlet fission, electrical and optical detection of ESR in organic light emitting diodes, and correlations between ESR and magnetoluminescence and magnetoconductivity experiments in OLEDs.

In 2016 a localization dichotomy result for periodic Schrödinger operators has been proved. This result shows that the localization properties of the Wannier functions associated to a periodic Schrödinger operator are deeply connected to the topological properties of the quantum system. In this seminar I will start by discussing the generalization of Wannier functions and Chern number to non-periodic systems. After that, I will present some results regarding the generalization of the localization dichotomy to generic gapped quantum systems.

An infinite projected entangled pair state (iPEPS) is a variational tensor network ansatz to represent 2D ground states in the thermodynamic limit where the accuracy can be systematically controlled by the bond dimension D of the tensors. Thanks to several methodological advances in recent years, iPEPS has become a very powerful tool for the study of 2D strongly correlated systems, in particular models where quantum Monte Carlo fails due to the negative sign problem. In the first part of my talk I will report on recent progress in simulating the 2D Hubbard model, focussing on a particularly challenging point in the phase diagram, U/t=8 and 1/8 doping. A very close competition between several low-energy states is found, including a uniform d-wave superconducting state and different types of stripe states. Systematic extrapolations to the exact, infinite D limit show that the ground state is a period 8 stripe, while stripes with periods 5-7 being very close in energy. Consistent results are also obtained with density matrix embedding theory, density matrix renormalization group, and constrained-path auxiliary field quantum Monte Carlo. On the other hand, period 4 stripes - which are typically observed in experiments on the cuprates - are clearly higher in energy. However, they become energetically favored upon adding a realistic next-nearest neighbor hopping term. In the second part I will show how to systematically study 2D quantum critical phenomena with iPEPS using so-called finite correlation length scaling. This approach is very similar to conventional finite size scaling, but instead of performing the scaling analysis using different system sizes, it is done based on an effective correlation length extracted from the iPEPS. We apply this method to interacting spinless fermions on a honeycomb lattice at half filling and obtain a critical coupling and critical exponents which are in agreement with quantum Monte Carlo results.

Over the years, the coherence of superconducting qubits has been significantly improved, so that at present they reach quality factors in excess of 10^6. Further improvements in coherence would help reducing the overhead necessary for implementing quantum error correction algorithms; therefore, despite the past successes, it is still necessary to investigate decoherence processes and to devise ways to suppress them. Since the early years of superconducting qubits, quasiparticles have been recognized as a possible source of decoherence. I will briefly review the history of quasiparticle effects in qubits, both in theory and experiments. Interestingly, the qubit sensitivity to quasiparticles makes it possible to measure their dynamics. I will discuss ways to control this dynamics, so that the detrimental effects of quasiparticles can be minimized. Finally, I will discuss some recent findings hinting at possible origins of quasiparticles.

We study a paradigmatic driven-dissipative model, dubbed interpolating Dicke-Tavis-Cummings (IDTC), where a collection of two-level systems interact with both quadratures of a quantum cavity mode. The closed system exhibits rich physics, including discrete and continuous symmetry-breaking phase transitions meeting at a multicritical point. We show that the closed system's phase diagram is profoundly altered by the presence of dissipation, eliminating the presence of broken continuous symmetry transitions, leading to novel regions of coexistence of phases and the appearance of two tricritical points. Using Keldysh formalism, we explore the dynamical response of this fascinating new phase diagram. Surprisingly, we find that an additional intriguing dynamical phase transition manifests in the frequency response of the system. At the transition, low-energy particle- and hole-like processes change roles due to a soft mode gap closing.

In magnetically ordered systems, fluctuations on top of the ordered state in the form of magnons can induce effective interactions. These magnon-mediated interactions may act to stabilise exotic phases of matter, and also significantly contribute to non-equilibrium responses. I will discuss two recent examples. Numerical results provide evidence for the existence of a partially disordered ground state in the antiferromagnetic stuffed honeycomb lattice, in which collinearly ordered moments coexist with an only short-range correlated subsystem. I show how magnons in the ordered honeycomb system mediate interactions among the central spins and derive an effective model. Fluctuations near the transition between two competing ground states of the model are conjectured to lead to a quantum disordered ground state. I will further discuss how magnon interactions are crucial for the optical generation of coherent magnons in easy-plane antiferromagnets, with a particular focus on the spin-orbit Mott insulator Sr2IrO4. Here, laser fields which directly couple to the local moments induce effective fields which drive a low-energy magnon, giving rise to a finite out-of-plane magnetization. The subsequent dynamics can be probed using a pump-probe protocol.

It has been the believe that generic quantum many-body systems necessarily approach thermal equilibrium after a long time evolution. As a consequence any quantum information encoded in the initial state is lost in the course of the dynamics and the late time dynamics is described by simple hydrodynamics. However, recently exceptions have been found to the rule. For example, disordered and interacting many-body systems realize the non-ergodic, many-body localized phase. In this talk, we show how constraints can significantly alter the quantum dynamics of interacting many-body system. To this end, we will first discuss the far-from-equilibrium dynamics of a two-dimensional quantum dimer model, which leads to a rich dynamical phase diagram with some almost ergodicity breaking regimes [1]. As a second example we show that the combination of charge and dipole conservation, characteristic of fractonic quantum matter, leads to an extensive fragmentation of the Hilbert space, which in turn can lead to a breakdown of thermalization [2]. [1] Dynamical Phase Transitions in the Quantum Dimer Model on a Square Lattice. Johannes Feldmeier, Frank Pollmann, Michael Knap [arXiv:1901.07597] [2] Ergodicity-breaking arising from Hilbert space fragmentation in dipole-conserving Hamiltonians. Pablo Sala, Tibor Rakovszky, Ruben Verresen, Michael Knap, Frank Pollmann [arXiv:1904.04266]

Two-dimensional materials have a large fraction of their atoms, if not all of them, directly "feeling the world" around them. Their properties can thus be strongly influenced by the contact to other materials, for instance a growth substrate, and in return, materials with a large contact-to-volume ratio will be influenced by their interface with the two-dimensional material. Different kinds of interactions and sometimes unconventional properties are found depending on the nature of the two-dimensional material and of the contacted material. In the past few years, we have addressed single-layer graphene and molybdenum disulphide, which have very different electronic properties -- semimetallic and semiconducting respectively. We prepare these materials by epitaxial growth on metallic substrates, and in between the two materials, we intercalate very thin layers of transition or alkali metals. Our first concern is to understand (and hopefully, control) how the process of intercalation occurs, whenever possible with the help of real-time observations. Is it reversible? Is the space between the two-dimensional material and the substrate, where the intercalated material grows, sealed? And how confined growth can occur in such a sealed two-dimensional region? Our interest is then on the effects of the interaction between the two-dimensional material and the intercalated layer. We have discovered that the magnetic properties of cobalt intercalated underneath graphene are very unusual, with a strong tendency to perpendicular-to-the-surface magnetisation that is appealing for spintronics applications. Using cesium intercalation, we observe electron doping in molybdenum disulphide together with a significant electronic disorder.

Almost periodic chains (models for incommensurate phases) and quasiperiodic chains (models for quasicrystals) have a richer topological phase structure than periodic chains, due to the existence of phasons. This effects the bulk boundary correspondence for these models. We investigate in particular quasiperiodic chains where the phason degree of freedom lives in a totally disconnected space. We show how we can nevertheless define a winding number for the boundary resonances and thus obtain an equation which relates the integrated density of states at a gap to a winding number which can be interpreted as the work the phason motion exhibits on the edge states of the system.

Largely unnoticed by theoretical physics, the last 20 years have seen a revolution in nano-particle processing. One example is that powders - although very porous, when freshly produced - can be converted into a dense solid, which still keeps a microstructure at the nanoscale. The processes are new developments related to what traditionally was called "Spark Plasma Sintering", but has nothing to do with sparks and plasmas. Computer simulations predict intermediate steps of these processes and reveal the underlying mechanisms. I will give a review of recent simulations, with a focus on so-called flash-sintering.

Topological insulators (TIs) are fascinating materials due to their protected edge-states and their possibilities for spintronic applications. Spin-orbit coupling and exchange effects at interfaces give us a possibility to tailor the properties of two-dimensional (2D) topological materials. While magnetism is usually detrimental to the topological properties, we show that exchange interactions offer the possibility to realize a variety of new phases depending on the structural symmetry and the direction of the magnetic (exchange) field. Using density functional theory we explore the quantum spin Hall to quantum anomalous Hall state transition in TIs and topological crystalline insulators. We propose the existence of new states like mixed nodal-line semimetals or mixed Weyl semimetals in the phase diagram of these 2D materials, showing interesting magnetoelectric coupling phenomena as function of the magnetization direction.

Atomically thin two-dimensional layered materials receive great interest because of their unique properties. Particularly, monolayers of semiconducting transition metal dichalcogenides (SC-TMDs), such as MoS2, excel due to their strong light-matter interaction that is dominated by exciton phenomena [1-3]. Key to the integration of SC-TDM and related artificial van der Waals solids into circuitries is the possibility to tune and engineer their properties on demand and on-chip e.g. by defects, dielectric environment or doping [4-8]. We apply inelastic light scattering together with emission and absorption to study the manifold coupling mechanism in van der Waal hetero- and hybrid structures. We introduce the influence of the dielectric environment, the charge carrier density as well as defects on the optical properties of these atomically thin materials. Moreover, interlayer excitons (IX) in vdW hetero-bilayers are intriguing systems to explore classical and quantum phases of interacting bosonic ensembles due to their enhanced lifetimes. We observe multiplet IX emission lines for MoSe2/WSe2 and MoS2/WS2 hetero-bilayers that are interpreted in terms of multi-valley excitons [3, 9]. All-2D stark effect devices allows for the manipulation of the excitons by external electric fields [9]. Our results provide fundamental insights into long-lived interlayer states in van der Waals heterostructures with possible bosonic many-body interactions. We acknowledge support by the Deutsche Forschungsgemeinschaft (DFG) via excellence cluster Nanosystems Initiative Munich (NIM), e-conversion as well as DFG project WU 637/4-1. [1] U. Wurstbauer, et al. J. Phys. D: Appl. Phys. 50, 173001 (2017). [2] S. Funke, et al., J. Phys.: Condens. Matter 28, 385301 (2016). [3] B. Miller, et al., Nano Lett. 17(9), 5229–5237 (2017). [4] E. Parzinger, et al., Nature 2D material 1, 40 (2017). [5] S. Diefenbach, et al., J. Phys. Chem. C, 122 (17), 9663–9670 (2018). [6] J. Klein, et al., 2D Materials 5, 011007 (2018), J. Klein et al., arXiv:1901.01042 (2019). [7] B. Miller, et al., Appl. Phys. Lett. 106, 122103 (2015). [8] B. Miller et al. Nature Commun. 10, 807 (2019). [9] J. Kiemle et al. arXiv:1901.01042 (2019).

I will discuss ways in which the quantum world is different from the classical world, and how these differences might be used to design quantum computers which are more powerful than classical computers, at least for certain problems. I shall focus on one approach to quantum computing, called the Quantum Adiabatic Algorithm, which is used to solve optimization problems, which are of great importance in science, engineering and industry. An optimization problem is one in which one has to minimize (or maximize) an energy function in which there is competition between different terms. I will discuss results both from computer modelling of the Quantum Adiabatic Algorithm and real experiments on a device which has of order 1,000 superconducting quantum bits. The difficulty of getting a "quantum speedup" from this device due to the great sensitivity of the ground state to changes in parameters (chaos) will be discussed.

Soon after the discovery of superconductivity in CuxBi2Se3 [Hor2010], this compound has been predicted to be a topological superconductor (TSC) [Fu2010]. Recently, spontaneous symmetry breaking of the SC state in the bulk as well as on the surface of CuxBi2Se3 was demonstrated experimentally [Matano2016, Yonezawa2017, Tao2018]. The observed nematic SC state is in agreement with the proposed odd-parity two-component superconducting order parameter [Fu2010] and confirms the topological nature of the SC state. In the first part of this talk, I will present preliminary results recently obtained in the M-STM lab on the surface of CuxBi2Se3 crystals grown in-house. In particular, scanning tunneling spectroscopy performed at 400 mK shows that the three-fold symmetry of the underlying lattice is broken in the nematic SC state. Specifically, the SC density of states measured as a function of the azimuthal angle between the external magnetic field (|B|=0.5 T ) direction and the sample surface shows a characteristic two-fold symmetry. Taking advantage of the atomic resolution capabilities of the STM we can deduce the nematic axis with respect to the crystal lattice locally on the sample surface. Going beyond pervious work [Tao2018], we were able to find all three symmetry related nematic domains on the same sample thus offering the prospect to study the physics at the boundary of the nematic SC domains in the future. In the second part, I will discuss possible future research to be performed in the M-STM lab. The long-term goal could be to bridge the gap between clean-room based (nano)device-fabrication and characterization of such devices with the STM on the atomic scale. In order to achieve this goal several obstacles have to be overcome. Specifically, I will propose an approach which allows (i) identification (and positioning) of nano-devices on mm sized sample carriers and (ii) realization of ultra-clean surfaces. [Hor2010] Hor et al. Phys. Rev. Lett. 104, 057001 (2010) [Fu2010] Fu and Berg, Phys. Rev. Lett. 105, 097001 (2010) [Yonezawa2017] Yonezawa et al. Nat. Phys. 13, 123 (2017) [Tao2018] Tao et al. Phys. Rev. X 8, 041024 (2018)

Recently, magnon excitations have been explored as a potential route to realizing analogues of the topological band structures that have become ubiquitous in electronic systems. However, magnons are not necessarily stable quasi-particles, with spontaneous magnon decay a generic feature of anisotropic magnets or magnets with non-colinear order. In this talk I will discuss two aspects of this physics: First, I will discuss how sharp topological magnon bands can exist in spite of the presence allowed decays, as realized in a Kitaev model in a large magnetic field. Second, I will discuss an alternative scenario where decay in fact drives the appearance new kinds of intrinsically non-Hermitian topological physics. This can lead to new kinds of spectral features, such as exceptional points and bulk "arcs" connecting them. I will illustrate this physics through a concrete example: a honeycomb ferromagnet with Dzyaloshinskii-Moriya exchange, comparing interacting spin-wave calculations with an effective non-Hermitian model. Finally, I discuss more broadly how the zoo of known topological protected magnon band structures may serve as a nearly ideal platform for realizing non-Hermitian physics in solid-state systems.

Mesoscopic transport with ultracold atoms: particles, spin and heat" Laura Corman, ETH Zürich The quantum behavior of fermionic particles can be revealed when the structure through which they flow is smaller than their coherence length. For electrons, this principle is the foundation of the field mesoscopic physics which has emerged thanks to the tremendous progress in fabrication techniques. Another platform to study mesocopic transport has appeared in atomic physics using ultracold gases of fermionic lithium. Although they are orders of magnitude more massive, more dilute and colder than electron gases in materials, it is possible to access similar regimes in both systems. This was demonstrated by the measurement of quantized conductance for neutral matter at an atomic quantum point contact [1]. Ultracold atom transport experiments allow some advantages compared to their electronics counterpart. First, structures on the order of the Fermi wavelength can be optically imprinted, which is fast and flexible. Second, the interactions between particles can be varied in a broad range, from weakly interacting (attractive or repulsive) to the unitary limit which corresponds to saturated contact interactions. In this talk, I will present how we engineer such transport experiments with atoms as well as some recent results of transport through 1D systems. First, by engineering the structure in which the atoms flow, we were able to explore insulating behaviors from a band insulator to a correlated, Luther-Emery insulator [2]. Second, we extended these structures to control to the effective spin degree of freedom of the atoms, creating local effective magnetic fields with Zeeman shifts on the order the Fermi energy [3]. Last, we can also study the coupling between different transported quantities to understand the nature of a system's elementary excitations, as we did for the unitary Fermi gas by observing the breakdown of the Wiedemann-Franz law [4]. [1] Krinner et al., Nature 517, 64-67 (2015). [2] Lebrat et al., Physical Review X 8, 011053 (2018). [3] Lebrat et al., arXiv:1902.05516 (2019) [4] Husmann et al., PNAS 115, 8563-8568 (2018).

Arkady V. Krasheninnikov 1,2 1 Helmholtz Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Germany 2 Department of Applied Physics, Aalto University School of Science, Finland Following isolation of graphene, many other 2D systems, e.g., single sheets of transition metal dichalcogenides (TMDs) have been manufactured. All these materials contain defects and impurities, which may govern their electronic and optical properties. Moreover, defects can intentionally be introduced using beams of energetic particles – ions and electrons. All of these calls upon the studies on defects and mechanisms of their formation. In my talk, I will present the results [1] of our recent theoretical studies of defects (native and irradiation-induced) in inorganic 2D systems obtained in close collaboration with several experimental microscopy groups. I will further discuss defect- and impurity-mediated engineering of the electronic structure of TMDs. [1] https://users.aalto.fi/~ark/publist.html

Josephson parametric amplification is a tool of paramount importance in circuit quantum electrodynamics (circuit-QED), especially for the quantum-noise-limited single-shot readout of superconducting qubits. I will first present the development and characterization of a flux-pumped Josephson parametric amplifier (JPA) based on a lumped-element LC resonator, in which the inductance L is composed by a geometric inductance and an array of superconducting quantum interference devices (SQUIDs) [1]. In addition, I will present preliminary experiments where the JPA is used as the first stage of amplification for the readout of a superconducting qubit based on a coaxial architecture recently developed in our lab in Oxford [2]. Finally, I will introduce my future scientific direction based on using JPAs for generation and control of non-classical states in microwave photons. [1] M. Esposito et al. EPJ Web of Conferences 198, 00008 (2019) [2] J. Rahamim et al. Applied Physics Letters 110, 222602 (2017)

In condensed matter physics, Raman spectroscopy is an extremely powerful probe of the inelastic and dynamic response of many-body systems and helps to identify the relevant energy scales that drive fundamental physical phenomena. It can also provide information on the symmetry of the relevant order parameter, even those exotic states that remain “hidden” to other spectroscopies, unveiling the rich microscopic and anisotropic nature of the phase in question, as well as its possible (complex) interplay with other ordered states. Over the past few years, I have developed two unique Raman scattering experiments, capable of probing low energy excitations (0.6 meV) in extreme conditions of low temperatures (4.2 K), high pressures (17 GPa) (Materials and Quantum Phenomena Laboratory, Paris) and high magnetic fields (30-37 T) (High Field Magnet Laboratory, Nijmegen). To illustrate the scope and potential of these new spectroscopic tools, studies carried out on heavy fermion systems [1, 2], filled skutterudites [3] and multiferroics [4] will be presented. [1] JB et al. PRL 113 266405 (2014) [2] JB et al. arXiv:1805.11307 (2018) [3] JB et al. In preparation [4] JB et al. PRL 115 267204 (2015)

Electrical and thermal conductances of a quantum Hall bar reflect the topological structure of the incompressible bulk phase. Here we show that noise of electrical current carried through the edge evidences the interplay between these two topological observables. Transport through an edge of particle-hole conjugate states is modeled by a voltage-biased line junction made up of counter-propagating modes. Specifically, we focus on the edge of a \nu = 2/3 fractional quantum Hall state. Noise is generated at a point distinctly separated from the hot spot (where most of the Ohmic dissipation takes place) and reflects the competition between ballistically carried downstream current and diffusively carried heat (which can propagate also upstream). We propose specific setups where our predictions can be measured.

Magnetic impurities in conventional superconductors induce a pair-breaking potential, which leads to bound states inside the superconducting energy gap. These states are called Yu-Shiba-Rusinov states, or short: Shiba states, and can be probed by scanning tunneling spectroscopy at the atomic scale. Here, we explore the Shiba states of individual magnetic atoms on superconducting substrates. The patterns of Shiba states of individual Mn atoms on a Pb(100) surface exhibit the shape of the corresponding spin-carrying d-orbital [1]. When the adatoms are brought into sufficiently close distance, they hybridize by forming symmetric and anti-symmetric linear combinations of the Shiba wave functions [2]. NbSe2 is a van der Waals-layered material with quasi-2D character. In addition to its superconducting properties it hosts a charge-density wave (CDW) at low temperatures. Magnetic atoms and molecules have been shown to give rise to long-range oscillatory Shiba states [3,4]. Here, we show that the CDW affects the energy and symmetry patterns of Shiba states around individual Fe atoms. [1] M. Ruby, Y. Peng, F. von Oppen, B.W. Heinrich, K.J. Franke, Phys. Rev. Lett. 117, 186801 (2016) [2] M. Ruby, B.W. Heinrich, Y. Peng, F. von Oppen, K.J. Franke, Phys. Rev. Lett. 120, 156803 (2018) [3] G. Menard, et al, Nat. Phys. 11, 1013 (2015) [4] S. Kezilebieke, M. Dvorak, T. Ojanen, P. Liljeroth, Nano Lett. 18, 2311 (2018)

Geometric phases in condensed matter play a central role in topological transport phenomena such as the quantum, spin and anomalous Hall effect (AHE). In contrast to the quantum Hall effect - which is characterized by a topological invariant and robust against perturbations - the AHE depends on the Berry curvature of occupied bands at the Fermi level and is therefore highly sensitive to subtle changes in the band structure. We demonstrate that the Berry curvature can be manipulated by interface engineering of the correlated itinerant ferromagnet and antiferromagnets. Using theoretical calculations, we show that the tunability of the AHE at interfaces arises from the competition between two topologically non-trivial bands. D. J. Groenendijk, C. Autieri et al. Berry phase engineering at oxide interfaces, https://arxiv.org/abs/1810.05619. M. Asa, C. Autieri et al. Anomalous Hall effect at the antiferromagnetic/non magnetic interface, in manuscript

In this work, we consider optimal state discrimination for a quantum system that interacts with an environment, i.e., states evolve under a quantum channel. We show the conditions on a quantum channel and an ensemble of states such that a measurement for optimal state discrimination is preserved. In particular, we show that when an ensemble of states with equal a priori probabilities is given, an optimal measurement can be preserved over any quantum channel by applying local operations and classical communication, that is, by manipulating the quantum states before and after the channel application. Examples are provided for illustration. Our results can be readily applied to quantum communication protocols over various types of noise.

The input-output relations of a biological system summarize its functional characteristics. Therefore, answering many fundamental questions on biomolecular machines ultimately reduces to understanding their input-output relations. Enzymes, that catalyze biochemical reactions, are among the simplest machines for which both input and output are chemical in nature. Many enzymatic reactions follow the "Michaelis-Menten" relations between input concentrations and output concentration of the molecular species involved in the reaction. We begin with a graph theoretic analysis of the input-output relation of a single enzyme. Input-output response of a graph can be characterized by the steady-state concentrations of the vertices. We present the conditions that must be satisfied by the structure of the graph (network) of states for the validity of the Michaelis-Menten input-output relation. This analysis sets the stage for understanding, at the next level of complexity, the input-output relation of a molecular machine, that is a macromolecular complex, and then at an even broader context of a group of interacting machines. I'll present an overview of our recent results on these input-output relations at multiple scales.

Reciprocity is a fundamental symmetry in physics; in optics it can be understood as the principle of ‘if I can see you, you can see me’. Breaking this symmetry results in asymmetric information transfer between two systems, i.e., the transmission amplitudes change under the exchange of source and detector. In the optimal situation, information transfer only occurs in one direction, which is a highly valuable feature for quantum information processing, where one aims to read out a quantum system while protecting the signal source. The violation of the symmetry of reciprocity requires rather special conditions, and one may ask the question if there is a general way to break reciprocity between two systems. This is indeed possible, we find that one can construct nonreciprocal interactions by combining the appropriate coherent and dissipative dynamics. Our nonreciprocity concept was experimentally confirmed within an optomechanical array setup in a collaboration with Oscar Painter’s group at Caltech. Furthermore, a number of related experiments can essentially be mapped to our approach as well. Overall, the nonreciprocity protocol is a powerful tool for realize directional interactions between two quantum systems and its full potential has yet to be explored. In this talk I give an introduction of the basic concept on how to engineer nonreciprocal interactions and devices.

Antrittsvorlesung.

I will review recent developments of transport in materials with a very small overlap between the bottom of the conduction band and the top of the valence band. I will focus on hydrodynamic and kinetic manifestations of chiral anomalies. After introducing basic theoretical concepts I will discuss relativistic Weyl particles. Next I will introduce materials, the so-called multi-Weyl semi-metals, which do not have relativistic dispersion yet are anomalous at the quantum level. Finally, I will present recent developments and challenges in measuring hydrodynamic transport of electrons in semi-metals.

Superconducting quantum circuits are artificial structures with a possibility to engineer their key parameters by design and fabrication. Moreover, many of these parameters can be in-situ tuned during experiments. These attractive properties have made superconducting circuits an ideal platform for quantum engineering such as quantum information processing and quantum optics on a chip. In this talk, I will underline the main physical mechanisms of these systems and will focus on benefits of the superconducting devices for waveguide quantum electrodynamics (QED), a regime where one or several quantum emitters are coupled strongly to a continuum of electromagnetic modes in one dimension. I will show that a basic ingredient of waveguide QED, a single qubit in a waveguide, behaves as a mirror with its transparency depending on the frequency and power of the incoming radiation. I will then show that when two qubits are inserted in a waveguide one can observe additional cooperative effects dependent on the distance between the qubits. Finally, I will outline how these non-local cooperative interactions can be used to create non-reciprocal transmission, distribute entanglement and generate new physical regimes of light-matter interaction.

Semidefinite programs have found applications in many fields of physics and mathematics. They are well suited to the computation of upper bounds on the maxima of commutative or noncommutative polynomial problems. They also match nearly one to one the objects studied in quantum information theory. However, SDPs have richer structure, and are computationally more intensive than linear programs. Symmetry reduction is a well-known tool to reduce the computational requirements of solving those problems. However, there is currently no turn-key solution to exploit symmetries coming from representation of arbitrary groups. In this talk, we discuss numerical techniques to decompose finite representations of arbitrary finite groups, and a recent Octave/MATLAB implementation.

Quantum advantage refers to exploiting the two essential elements of quantum physics, non-locality and coherence, in modern technologies such as high-precision sensing or large-scale computing. The actual operation of a quantum device requires a unitary operation, and the corresponding time evolution needs to be engineered dynamically. The desired unitaries are implemented by time-dependent external electromagnetic fields. Quantum optimal control theory provides a means to derive the shape of these fields. I will discuss example applications of optimal control theory from quantum sensing and quantum computing and I will show how suitably shaped pulses reach maximum fidelity in minimum time, despite the presence of losses and imprecision.

Floquet systems offer a versatile toolbox for engineering quantum phases that are not allowed or at least more difficult to obtain in equilibrium systems. A prominent example are time crystals, which spontaneously break discrete time-translation symmetry. Closely related to time crystals are Floquet topological phases, such as those found in the driven Kitaev chain. I will discuss how to perform braiding of non-Abelian degrees of freedom - the elementary operation of topological quantum computation - in such a driven topological superconductor. Unlike the equilibrium case, where braiding can only be performed in two-dimensional systems (or at least 2d networks of 1d systems), braiding here is realized in a strictly one-dimensional system, thus potentially simplifying experimental demonstration.

In spite of being more than 5 decades old, Bell's theorem is still at the center of quantum foundations, also playing a crucial role in the device-independent framework for quantum information processing. In this talk we will discuss some fresh results about Bell non-local correlations. First we introduce a natural and geometrical way to quantify non-locality. Based on that, we will see interesting concentration phenomena taking place in the set of Bell correlations. Finally, we will discuss something we have called quantum voids: regions on the faces of the non-signalling set (those compatible with special relativity) where all non-local correlations are of a post-quantum nature. By the end, and if time permits, we will see how deep learning techniques provide a novel tool to characterize and discover new kinds of non-local correlations.

Quantum computers offer important speed-ups in the solution of a variety of problems. One of them is the simulation of quantum many-body systems, as they appear in atomic, condensed matter or high-energy physics, or quantum chemistry. Other, like SAT, are related to classical optimization problems. In this talk I will first review some of the quantum algorithms to simulate the equilibrium and dynamics of quantum many-body systems. Then I will present a hybrid quantum-classical algorithm to solve SAT problems with (relatively) small quantum devices.

Port-based teleportation (PBT) is a variant of the well-known task of quantum teleportation where the receiver Bob, instead of having to apply a non-trivial correction unitary, merely has to pick up the right quantum system at a “port” specified by the classical message he received from Alice. While much more resource-demanding than standard teleportation, PBT has applications to instantaneous non-local computation and can be used to attack position-based quantum cryptography. A perfect PBT protocol would violate the linearity of quantum theory, so there is a trade-off between error and entanglement consumption (or the number of ports) which can be analyzed using representation theory of the symmetric and unitary groups. In particular the resource state has a “purified" Schur-Weyl duality symmetry. I will give an introduction to the task of PBT and its symmetries. Subsequently, I will present how the asymptotics of existing formulas for the optimal performance for a given number of ports can be derived. To achieve this, a connection between representation theory and the random matrix ensemble GUE_0, and a surprising appearance of the discrete Laplacian are exploited.

The discovery of topological insulators (TIs) has fuelled the search for novel topological phases of matter, by combining theory, computational high-throughput screening and experiments [1, 2]. From the experimental point of view, angle-resolved photoelectron spectroscopy (ARPES) has played a key role due its direct access to the material band structure. The advent of efficient spin detector, and ultrafast light source in the UV energy range, has further extended our capability to map also the vectorial spin texture and the out-of-equilibrium dynamics of the topological states. We have recently exploited these techniques to investigate TIs and several topological semimetal. We have tracked the evolution of the band structure of MoTe2 across the topological phase transition between a trivial semimetal and a type-II Weyl semimetal, as a function of the sample temperature [3]. Our results show that the presence of several bulk and surface states hamper a direct determination of the topological phase. However, the temporal dynamics of the photoexcited electrons reveals clear signatures of the Weyl point formation in the unoccupied band structure [4]. In the second part of my talk, I will show that optical excitation can be also used to alter the material band dispersion, as for the case of the nodal line semimetal ZrSiSe [5], which displays both spin polarized surface states and a bulk hidden spin polarization. [1] M. Z. Hasan and C. L. Kane, Reviews of Modern Physics 82, 3045 (2010). [2] B.-J. Yang and N. Nagaosa, Nature Communications 5, 1 (2014). [3] A. Crepaldi, et al., Phys. Rev. B 95, 041408 (2017). [4] A. Crepaldi, et al., Phys. Rev. B 96, 241408(R) (2017). [5] G. Gatti, et al., in preparation.

Bacterial evolution of resistance to antibiotics is a major public health concern. Once prevalent, resistant strains can transmit among patients and become difficult to eradicate. However, ultimately, the resistant strains must originate from a de novo mutation or horizontal gene transfer event in a single bacterial cell. Using high-replicate experiments with Pseudomonas aeruginosa facing streptomycin as a model system, we quantified the probability that a single resistant cell establishes a large surviving lineage. Strikingly, this probability drops to near zero well below the standardly measured minimum inhibitory concentration (MIC) of the antibiotic. We show that this result can be explained by a simple model in which the antibiotic has partial inhibitory effects below the MIC, and each cell has an independent chance of establishment.

Intensive study of RuCl3 has been motivated recently by signatures of strongly anisotropic and frustrated interactions reminiscent of the Kitaev honeycomb model. Even though RuCl3 orders antiferromagnetically at zero field, it exhibits a broad continuum of magnetic excitations, which coexists with sharp magnons in different regions of k-space. Many works have interpreted the breakdown of magnons at certain wavevectors as a signature of proximity to the Kitaev spin liquid. A similar interpretation has been suggested for the continuum of excitations arising when magnetic order is suppressed either by high temperature, or in-plane magnetic fields. In order to evaluate these proposals, it is important to understand the underlying interactions and energy scales in RuCl3 in the context of recent experiments. We have considered the dynamical response and stability of magnons at zero and finite field with respect to the full range of realistic magnetic interactions suggested by recent ab-initio calculations. Combining extensive exact diagonalization studies with semiclassical analysis, we will address the (i) relevant mechanisms for magnon breakdown, (ii) origin and robustness of the continuum, (iii) evolution of the spectra under applied field, and (iv) possible nature of the field-induced state.

An exact formula for the temperature dependent Hall number of metals is derived. It is valid for nonrelativistic fermions or bosons, with an arbitrary potential and interaction. This dc transport coefficient is proven to (remarkably) depend solely on equilibrium susceptibilities, which are more amenable to numerical algorithms than the conductivity. An application to strongly correlated phases is demonstrated by calculating the Hall sign in the vicinity of Mott phases of lattice bosons.

All animals and plants are associated to a huge number of microbes that live together with them or even within them. These microbes often have fundamental roles in the functioning of their hosts, from metabolic functions to defense against diseases. In many cases, a disease microbiome seems distinct from a healthy one. This multi-stability of microbiomes can be understood in terms of game theoretical models that describe the interactions between microbes, but it is challenging to assess the associated parameters from experimental data. On the other hand, many microbes will randomly vary in their abundance, such that stochastic models are more appropriate to describe them. In the most extreme case of neutral models, it is assumed that all microbes are identical. Such neutral models are widely used in theoretical ecology and they can be directly applied to the microbiomes of a wide variety of host organisms. Neutral models allow to identify key microbial types, which are not necessarily those that are found in particularly high abundance.

Chiral spin liquids are highly entangled phases of interacting spins that break time reversal and inversion symmetries, but lack conventional magnetic order even at zero temperature. While chiral spin liquids have not been unambiguously observed in nature yet, they are expected to exhibit exotic properties following from a spectrum of fractional excitations. Theoretically, chiral spin liquids are often described by strongly coupled gauge theories which are hard to tackle analytically beyond mean-field approximations. In this talk, I will describe controllable approaches that allow us to understand various chiral spin liquids starting from lower-dimensional building blocks, namely arrays or junctions of Heisenberg spin chains. In the case of junctions, I will discuss the connection between chiral spin liquids and chiral fixed points of boundary conformal field theory.

In the past decade we have witnessed enormous progress in experiments that consist of placing magnetic atoms at predetermined positions on substrates and building magnetic nanostructures, one atom at a time. The interactions between magnetic moments are mediated by the conduction electrons through a mechanism understood in terms of a theory developed decades ago by Ruderman, Kittel, Kasuya, and Yosida, dubbed "RKKY theory", which applies when the spins are classical. The quantum nature of the electronic spin introduces another degree of complexity and competition with other quantum phenomena: the Kondo effect. This competition is quite subtle and non-trivial, and can only be studied by numerical means. We explore these phenomena on different lattice geometries in 1,2 and 3 dimensions by introducing an exact mapping onto an effective one-dimensional problem that we can solve with the density matrix renormalization group method (DMRG). We show a clear departure from conventional RKKY theory and important differences that can be attributed to dimensionality and geometry. In particular, for the square and cubic lattices at half filling, Kondo physics dominates even at short distances, while the ferromagnetic RKKY state is energetically unfavorable, translating into a finite range for the RKKY interaction. In the case of larger spin S=1, RKKY correlations can coexist with (partial) screening.

We shall briefly review the present day situation with unexpectedly high-temperature superconductivity in FeSe monolayers on SrTiO_3 (FeSe/STO), the main experimental results, the electronic structure (ARPES data and calculations) and possible mechanisms of Tc enhancement. We shall discuss the origin of unusual shallow conduction bands in FeSe/STO with quite small values of Fermi energy, which we attribute to the increasing electronic correlations (as compared to the bulk FeSe). We shall briefly review the idea of Tc enhancement due to FeSe electrons interactions with high - energy optical phonons of STO, stressing the importance of antiadiabatic effects, which will be discussed within the Eliashberg - McMillan approach.

The intimate connection between topology and quantum physics has been widely explored in solid-state physics, revealing a plethora of remarkable physical phenomena over the years. Building on their universal nature, topological properties are currently studied in an even broader context, ranging from ultracold atomic gases to photonics, where distinct observables and probes offer a novel view on topological quantum matter. In this talk, I will discuss how the geometry of quantum states can be revealed using a universal scheme based on excitation-rate measurements upon periodic driving [1,2,3]. When applied to Chern insulators or Landau levels, this approach leads to a quantized circular dichroism phenomenon [1,2], which can be interpreted as the dissipative counterpart of the quantum Hall effect. Besides, we will present protocols allowing for the experimental detection of the quantum metric tensor [3], which could be applied to detect new forms of monopoles in higher dimensions [4]. Finally, I will report on the first experimental observation of quantized circular dichroism in an ultracold Fermi gas [5]. [1] D. T. Tran, A. Dauphin, A. G. Grushin, P. Zoller and N. Goldman, Science Advances 3, e1701207 (2017). [2] D. T. Tran, N. R. Cooper, and N. Goldman, Phys. Rev. A 97, 061602(R) (2018). [3] T. Ozawa and N. Goldman, Phys. Rev. B 97, 201117(R) (2018). [4] G. Palumbo and N. Goldman, arXiv:1805.01247(2018). [5] L. Asteria, D. T. Tran, T. Ozawa, M. Tarnowski, B. S. Rem, N. Fläschner, K. Sengstock, N. Goldman and C. Weitenberg, arXiv:1805.11077 (2018).

The late time behavior of disordered, wave bearing systems, acoustic, elastic or optical, has been of great interest in recent years. From a practical perspective, late time measurements correspond to large distances in a disordered system, and are thus relevant in many imaging or non-destructive evaluation applications. From a basic science perspective, late time observations can reveal interference behavior resulting from the approach to the Anderson transition, and offer the prospect of observing classical wave systems in a localized phase. While the Anderson transition has been theoretically predicted to occur in such systems for some time, clear evidence of a localized phase has unexpectedly been quite difficult to find. Our theoretical understanding of the current status is still incomplete, with continuing efforts employing direct numerical simulations, self-consistent, multiple scattering models, and field theoretic methods. Of these, direct numerical simulations and field theoretic methods enable a description of the localized phase. Direct numerical simulations, particularly those based on a monopole or dipole scattering approximation, have achieved some promising results but still describe highly idealized systems, and require substantial computational resources to describe systems substantially larger than a mean free path. Field theoretic methods on the other hand, while yielding semi-analytic results, have in the past been restricted to even more idealized systems. We discuss recent results in these areas, particularly with a view of moving towards a more realistic description of disordered, elastic systems. This research was funded by the Office of Naval Research.

Transition-metal compounds have long been a fertile field to explore new materials and novel properties. In this talk, I will present our recent works about unusual magnetism in some transition-metal compounds, using crystal field level diagrams, density functional calculations, and Monte Carlo simulations. We will show how a joint effect of special ligand field, atomic multiplet and spin-orbit coupling produces a record breaking giant magnetic anisotropy energy for Co, Ru, or Os adatoms on MgO (001) surface [1]. We will explain the surprising high-Tc ferromagnetic metallicity in the disordered perovskite SrFe0.5Ni0.5O3 using the unusual high-spin and high valence states [2]. Moreover, we find a quite strong ferromagnetism in the layered insulator LaSrNiRuO4 with a very unique charge-spin-orbital state [3,4]. [1] X. Ou et al., PRL 115, 257201 (2015). [2] F. Fan et al., PRB 94, 214401 (2016). [3] S. Zhu et al., EPL 117, 37005 (2017) (Editor’s choice). [4] K. Yang et al., JPCC, in press (2018).

Topological Weyl magnets are characterized by a magnetic semimetallic state that harbors Weyl fermions near the Fermi energy. As a parent state of such a magnetic Weyl state, we first introduce a new class of semimetals with quadratic band touchings, the “Luttinger semimetal” [1,2]. We demonstrate the case in pyrochlore iridium oxides that exhibit various exotic phenomena, including a spontaneous Hall effect without magnetization in a spin liquid state [3]. Then, we discuss a new type of frustrated antiferromagnets, Mn3X (X = Sn and Ge) as the examples of a topological Weyl magnet [4,5,6]. We show that the cluster multipole order on the kagome lattice of Mn moments can be easily controlled and allows the system to exhibit a variety of new functions at room temperature that have never been seen in antiferromagnetic metals. These include the large anomalous Hall and Nernst effects [4,5,7], large magnetic optical Kerr effect [8] and a novel type of spin Hall effect (magnetic spin Hall effect). Finally, we show that they should be significantly useful for designing antiferromagnetic spintronics, and energy harvesting technology. [1] T. Kondo et al., Nature Communications 6, 10042 (2015). [2] B. Cheng et al., Nature Communications 8, 2097 (2017). [3] Y. Machida, S. Nakatsuji, S. Onoda, T. Tayama, and T. Sakakibara, Nature 463, 210 (2010). [4] S. Nakatsuji, N. Kiyohara and T. Higo, Nature 527, 212 (2015). [5] N. Kiyohara, T. Tomita, S. Nakatsuji, Phys. Rev. Applied 5, 064009 (2016). [6] K. Kuroda, T. Tomita et al., Nature Materials 16, 1090 (2017). [7] M. Ikhlas, T. Tomita et. al., Nature Physics 13, 1085 (2017). [8] T. Higo et al., Nature Photonics, 12, 73 (2018).

Quantum communication allows us to solve tasks that are provably impossible using classical communication. A famous example is quantum key distribution which enables secure communication. Here, security can directly be traced back to fundamental properties of quantum entanglement, and is guaranteed even if an eavesdropper holds a quantum computer now or in the future. Many other applications have already been identified ranging from solving clock synchronization, to exotic applications such as extending the baseline of telescopes to make better observations. In this talk, we will give an overview of the state of the art of quantum communication, and look at how quantum entanglement might help us to solve cryptographic problems. We continue to look ahead to some of the many challenges to be solved in order to realize large scale quantum networks, which may eventually allow us to create entanglement between any two points on earth yielding new insights to physics, and bringing entirely new internet applications.

Dissipative quantum many-body systems are extremely challenging to analyze, as most theoretical tools developed for equilibrium systems cannot be applied. I will present a variational principle that can be applied to obtain a better understanding of the essential physics within these systems, such as dissipative phase transitions [1,2]. Within the context of a dissipative Ising model, I will compare the variational method to recent results obtained using a tensor network approach [3]. Finally, I will demonstrate how a better fundamental understanding of dissipative quantum many-body systems enables the realization of a novel class of dissipative quantum sensors [4]. References [1] H. Weimer, Phys. Rev. Lett. 114, 040402 (2015). [2] V. R. Overbeck, M. F. Maghrebi, A. V. Gorshkov, H. Weimer, Phys. Rev. A 95, 042133 (2017). [3] A. Kshetrimayum, H. Weimer, R. Orus, Nature Commun. 8, 1291 (2017). [4] M. Raghunandan, J. Wrachtrup, H. Weimer, Phys. Rev. Lett. 120, 150501 (2018).

I will discuss recent developments in quantum simulation of quantum many-body systems with atomic platforms from a theory perspective. Systems of interests include atoms in optical lattices, Rydberg atoms in optical tweezer arrays, and trapped ions. Quantum simulation has so far been discussed as analog simulation, where we physically build a system with the desired Hamiltonian; or as digital quantum simulation, where time evolution of a many-body system is represented as a sequence of quantum gates on a quantum computer. I will add to this variation quantum simulation, where a quantum feedback loop between a classical computer and an analog quantum simulator, which acts as a quantum co-processor. As an example I will present results from an ongoing theory - experiment collaboration in Innsbruck: here our quantum resource is a trapped-ion 20-qubit analog quantum simulator, representing a transverse Ising model, and we compute on the quantum device the ground and excited states of the Lattice Schwinger Model as 1D Quantum Electrodynamics. Remarkably, variational quantum simulation allows "self-verification" of quantum results on the quantum machine, as assessment of the error. As a second topic we will discuss novel measurement protocols for Rényi entropies, and for out-of-time-ordered correlation functions (OTOCs), which can be extracted from on statistical correlations between randomized measurements. I will show a recent experiment with an ion chain, demonstrating experimental observation of entanglement entropies in quench dynamics in 10 and 20 qubit devices. I will conclude my talk with a brief outlook on possible future directions, including sub-wavelength optical lattices for atomic Hubbard models, and quantum chemistry.

Graphene can acquire salient properties by the intercalated nano structures. To functionalize the graphene as designed, understanding the growth kinetics of the nano structures is a prerequisite. In that regards, Kr atoms are selectively intercalated just below the surface graphene layer of HOPG(0001) by the incidence of low energy Kr ions. The growth kinetics of the encapsulated Kr nano structures is investigated by both scanning tunneling microscopy, molecular dynamics simulations, and the first principles calculations. The intercalation proceeds via defect sites, such as surface vacancies. At room temperature, the thermal diffusion of intercalated Kr is almost frustrated despite the low diffusion barriers, which remains as a puzzle. The growth of Kr nano structures proceeds via the transient mobility of both the intercalating Kr atoms and previously intercalated Kr atoms that are mobilized by collision with the incident Kr ions. At the elevated temperatures where thermal diffusion becomes effective, some Kr nano structures disappear, releasing pressurized Kr atoms, while others coalesce to form blisters via the delamination of the adjacent graphene. Some of the larger blisters explode to leave craters of varying depths at the surface. In contrast to growth on the substrate, the growth of each encapsulated nano structure depends significantly on extrinsic variables, such as surface vacancies and local topography around the nano structure, that affect the Kr diffusion and limit the maximal Kr pressure.

Quantum Monte Carlo (QMC) methods, when applicable, offer dependable ways to extract the nonperturbative physics of strongly-correlated many-body systems. However, there are some formidable bottlenecks to the applicability of these methods such as the sign problem and algorithmic update inefficiencies. Using the t-V model Hamiltonian, I demonstrate how the fermion bag approach--originally developed in the context of Lagrangian lattice field theories--led to the first sign problem solution for this model. I then show how using fermion bag ideas to develop a new efficient QMC algorithm to study the t-V model allowed us to compute critical exponents for the chiral Ising universality class (involving one flavor of four-component Dirac fermions) that seem to be more reliable than those from previous QMC calculations. Finally, I discuss how the fermion bag approach offers certain advantages to the study of other models involving Dirac fermions and also extends to fermion-spin interactions and Z_2 gauge theories.

We study the relationship between the physics of topology and zero modes in frustrated systems and metamaterials. Zero modes that exist in topological matters are distinct from the ones arising from symmetry breaking. Incidentally, a prominent aspect of frustrated systems and metamaterials also is to harbor such kind of zero modes in form of an accidental degeneracy. Taking cues from these two apparently different phenomena, we ask a simple question: are the robust features of frustration topologically protected and if so can we classify different types of frustration using topology? In answering these questions we invoke the tools of topological mechanics to identify the key agent at play, namely the rigidity matrix, which is a non-Hermitian matrix and decides the topology of spin-wave zero modes in a frustrated magnet or phonon modes in metamaterials. Further developments of the theory rely on combining the recent developments in our understanding of Maxwell constraint counting and generalizing the ten-fold way classification of Hermitian matrices to non-Hermitian matrices. The result is a three-fold way classification for each Maxwell counting index. We illustrate the classification by demonstrating the existence of a new vortex-like invariant for real rigidity matrices using random matrices and through example frustrated spin models. So by classifying all the rigidity matrices, we answer the question of the origin of frustration (i.e. zero modes in the form of accidental degeneracy) in a wide class of frustrated magnets and metamaterials by linking it to topological invariants.

An electronic Ising-nematic order spontaneously breaks the rotation symmetry of the underlying lattice down to two-fold. It has been observed in various systems, in particular the cuprate and iron-based high temperature superconductors. In the vicinity of a nematic quantum critical point — achieved by tuning some external parameter such as pressure or doping — the physics is described by that of low-frequency long-wavelength order parameter fluctuations coupled to a Fermi surface. However, due to the momentum-conserving nature of the induced electron-electron interaction, the temperature dependence of the resistivity near an Ising nematic QCP remains unclear. In this talk, we shed light on the problem by incorporating disorder and Umklapp process into the low-energy theory. Our work can be viewed as solving an extended Boltzmann equation, with a collision integral that accounts for complicated multi-particle scattering processes important near the QCP.

Motivated by supersymmetry, Batalin and Vilkovisky reformulated the equations governing Lagrangian mechanics in the variational calculus as a Maurer-Cartan equation (vanishing of curvature). This allowed them to arrive at a new understanding of symmetries off-shell (i.e. where the Euler-Lagrange equation does not hold). In this talk, I will show how a modification of their Maurer-Cartan equation can handle the action of the diffeomorphism group on the world-sheet (i.e. general covariance of the theory). Our approach involves the introduction of a curvature to Maurer-Cartan equation. This curvature is central (a scalar multiple of the identity matrix) : this is analogous to the Berry phase in the Hamiltonian approach to quantum theory (though this is really nothing more than a formal analogy).

Luminescence is a standard method to study the excited electronic states and their relaxation dynamics in solids, especially in insulators and semiconductors. However, the luminescence from metals and semimetals have rarely been studied. Several years ago, we found femtosecond infrared luminescence from some semimetals, such as graphite [1], bismuth, antimony [2] and also from metallic states at the surface of topological insulators [3]. We successfully obtained versatile information about the relaxation dynamics in these materials. In addition, very recently, we discovered ultrafast infrared luminescence in many ordinary metals, Au, Ag, Cu, Pt, Al etc., when they have surface roughness [4]. In this talk, I would like to propose the "femtosecond infrared luminescence" as a conventional tool for investigating the electron dynamics in metals, providing information similar to that could be obtained from time-resolved photoemission spectroscopy. References [1] T. Suemoto, S. Sakaki, M. Nakajima, Y. Ishida, and S. Shin, Phys. Rev. B87, 224302 (2013). [2] M. Takeda, S. Maezawa, H. Watanabe and T. Suemoto, Spring meeting of Physical Society of Japan (2015). [3] S. Maezawa, H. Watanabe, M. Takeda, K. Kuroda, T. Someya, I. Matsuda and T. Suemoto, Sci. Rep. 5. 16443 (2015). [4] T. Suemoto, N. Sugimoto, K. Nishikawa, K. Yamanaka, S. Inagaki, The XXI International Conference on Ultrafast Phenomena (Hamburg, 2018).

Recent progress in making oxide interfaces and heterostructures has led to an interest in understanding how one might realize topological phases or flat bands in such correlated systems. In particular, this might be accomplished by spin-orbit coupling or strain effects. In this context, I will discuss our theory work on understanding the "atomic limit" of spin-orbit coupled oxides, showing that one can use resonant inelastic X-ray spectroscopy of such systems to extract microscopic couplings. Using such "lego" building blocks, I will discuss mixed oxide 2DEGs comprised of local moments coupled to a "conduction fluid" of electrons with strong spin-orbit coupling, to show that one can realize high temperature quantum anomalous Hall insulators. I will also discuss a different setting in which strain in Dirac materials acts as a pseudo-gauge field. This provides a way to realize Landau-like levels of Bogoliubov quasiparticles in high Tc d-wave superconducting oxides. Spin-orbit coupling and strain can thus pave the way to realizing novel topological quantum phases in solids.

The aim of this talk is two-fold. In the first part I want to explain how the study of interacting quantum gases in the continuum benefits from tools developed for quantum systems on the lattice, and how these tools appeared in the study of classical systems of the type of vertex models. These developments were carried out or were initiated by the many research activities of Prof. J. Zittartz especially in the late 80s and the 90s. In passing I want to point out how matrix-product and tensor-network states evolved in this environment and where the early exact and variational treatments led to. In the second part I will investigate the thermodynamics of the two-component one-dimensional Bose gas with contact interactions in the vicinity of the quantum critical point separating the vacuum and the ferromagnetic liquid regime. The quantum critical region is found to belong to the universality class of the spin-degenerate impenetrable particle gas which, surprisingly, is very different from the single-component case.

We developed general approach to the calculation of power-law infrared asymptotics of spin-spin correlation functions in the Kitaev honeycomb model with different types of perturbations. We have shown that in order to find these correlation functions, one can perform averaging of some bilinear forms composed out of free Majorana fermions, and we presented the method for explicit calculation of these fermionic densities. We demonstrated how to derive an effective Hamiltonian for the Majorana fermions, including the effects of perturbations. For specific application of the general theory, we have studied the effect of the Dzyaloshinskii-Moriya (DM) anisotropic spin-spin interaction; we demonstrated that it leads, already in the second order over its relative magnitude D/K, to a power-law spin correlation functions, and calculated dynamical spin structure factor of the system. We have shown that an external magnetic field h in presence of the DM interaction, opens a gap in the excitation spectrum of magnitude proportional to Dh.

Quantum states in low-dimensional spin systems are characterized by their characteristic spin dynamics. In this seminar, I will talk about terahertz spectroscopy of quantum spin dynamics in a spin-1/2 Heisenberg-Ising antiferromagnetic chain as a function of temperature and magnetic field. At zero field, confined spinon excitations are observed below the Néel temperature [1]. In a longitudinal magnetic field, high-energy string excitations are observed as well as low energy spin excitations [2]. In contrast, in a transverse magnetic field, emergent fermionic excitations are observed, when the confinement is suppressed and an order-disorder phase transition is induced by the transverse field [3]. These experimental results are understood by comparison to precise calculations of dynamic structure factors using Bethe ansatz and infinite time evolving block decimation, respectively, for the longitudinal and transverse fields [2,3,4]. In addition, the results in transverse fields provided strong indication of possible existence of an experimentally well-defined quantum critical point of the transverse-field Ising-chain [5]. The existence of such a quantum critical point indeed has been revealed by our recent thermodynamic measurements [6], the quantum criticality of which is consistent with a recent analytical study of the transverse-field Ising-chain [7]. [1] ZW et al. Phys. Rev. B 91, 140404(R) (2015). [2] ZW et al. Nature 554, 219–223 (2018). [3] ZW et al. Phys. Rev. B 94, 125130 (2016). [4] Wang Yang et al. arXiv:1702.01854 (2017) [5] S. K. Niesen et al. Phys. Rev. B 87, 224413 (2013). [6] ZW et al. Phys. Rev. Lett. 120, 207205 (2018). [7] Jianda Wu et al. Phys. Rev. B 97, 245127 (2018).

The rates at which bacterial populations grow have been studied for many decades, and have led to the formulation of deterministic growth laws that describe “macroscopic” properties of populations. However, this ignores heterogeneities within populations of cells. Current methods allow us to look at individual cells, and they reveal interesting distributions and lineage correlations of growth variables that present new explanatory challenges. Further, when populations of growing bacteria are exposed to antibiotics, cells may die and divide at comparable rates, and it becomes important to understand how cells in an identical environment suffer different fates. We present empirical observations both at the population and single-cell level, and propose stochastic models of intracellular processes to explain the observations.

Transition metal oxides (TMOs) represent the natural playground for the investigation of the exotic phases produced by electronic correlations, which include high-temperature superconductivity, giant magnetoresistance and topological insulators. The peculiar electronic and magnetic properties of TMOs stem from the interplay between the electron-electron correlations, the crystal field and the spin-orbit coupling (SOC) of the TM d valence electrons. The balance of these energy scales significantly depends on TM element considered: moving from the 3d to the 5d row of the periodic table, the electronic correlations decrease due to the larger size of the atomic orbitals, while SOC increases as a result of the increased atomic charge. In the present talk, I present three separate case studies of 3d, 4d and 5d TMOs which highlight the impact of the competing interactions just mentioned on the electronic and magnetic properties of the system. Concerning 5d TMOs, I address the spin-wave spectrum of the electron-doped perovskite iridate (Sr1-xLax)2IrO4, where the strong SOC of 5d electrons gives rise to a spin-orbit entangled Mott state with peculiar exchange interactions. I then examine the intermediate case of the 4d oxide Ca2RuO4: here, electronic correlations, SOC and octahedral distortions act on an equal footing to determine the TM ground state. In particular, I show how the crystal field tuning achieved by La substitution affects the electronic and magnetic properties. Finally, I discuss the magnetic ground state of a family of weak ferromagnets, where the weak SOC is responsible for the appearance of a net magnetization in the main antiferromagnetic order.

Kagome antiferromagnets display a host of exotic magnetic phenomena originating from a massive degeneracy of classical ground states. Presumably, this massive degeneracy is an accident and so the community's focus has been to discover ideal kagome antiferromagnets where it is present and study their properties. I will argue it is not an accident and that locality and, for example, a mirror symmetry are enough to guarantee frustration. In the process, I will uncover a rich theory of distorted kagome antiferromagnets, showing their ground states map onto triangulated surfaces with (origami) or without (kirigami) holes. I will show the exchange constants J_ij behave like a background gauge field whose geometry can determine some of the zero modes. From a different perspective, I will further show there are zero modes in the spin wave spectra determined by a topological classification of Maxwell counting by extending the 10-fold way classification of Hermitian matrices that underpin topological insulators to non-Hermitian matrices. I will conclude by applying the theory to two materials and demonstrate explicitly the existence of either flat bands (Cs_2ZrCu_3F_{12}) and line-degeneracies (Cs_2CeCu_3F_{12}) in the spin wave spectra and how this relates to the curvature and folding modes in the associated triangulated surface that characterize their ground states.

Fluids with an intrinsic gradient, for example in their temperature, charge, or density distribution, are known to produce a directed drift in suspended colloidal particles. This phenomenon is called phoresis. In this talk, I will summarize some recent theoretical work with the main focus on thermophoresis, mostly performed by means of mesoscopic simulations. These studies have also allow us to propose new applications in the design of promising phoretic machines such as microswimmers, turbines, or micropumps.

In recent years, enormous data sets have begun to appear in real-space visualizations (scanning probes) and reciprocal-space visualizations (scattering probes) of electronic quantum matter. Increasing volume and variety of such data present new challenges and opportunities that are ripe for a new approach: machine learning. However, the scientific questions in the field of electronic quantum matter require fundamentally new approaches to data science for two reasons: (1) quantum mechanical imaging of electronic behavior is probabilistic, (2) inference from data should be subject to fundamental laws governing microscopic interactions. In this talk, I will review the aspects of machine learning that are appealing for dealing with quantum complexity and present how we implemented a machine learning approach to analysis of scanning tunneling spectroscopy data.

Formulating consistent theories describing strongly correlated metallic topological phases is an outstanding problem in condensed-matter physics. I will present an explicit construction of a fractionalized analog of the Weyl semimetal state: the fractional chiral metal. Our approach is to construct a 4+1D quantum Hall insulator by stacking 3+1D Weyl semimetals in a magnetic field. In a strong enough field, the low-energy physics is determined by the lowest Landau level of each Weyl semimetal, which is highly degenerate and chiral, motivating us to use a coupled-wire approach. In the presence of electron-electron interactions a gapped phase emerges; its electromagnetic response is given in terms of a Chern-Simons field theory. A boundary of this four dimensional phase remains gapless. The boundary's response to an external electromagnetic field is determined by a chiral anomaly with a fractional coefficient. We suggest that such an anomalous response can be taken as a working definition of a fractionalized strongly correlated analog of the Weyl semimetal state.

The theory of large deviations provides probabilistic and analytical tools for describing the exponential decay rate of very small probabilities in terms of variational formulas. In the development of this theory, driving forces came from various applications in physics, like random path models with interactions having long or short memories, and from static interacting particle systems like the interacting Bose gas. We will explain how to apply the theory to such examples and how much it can contribute to a rigorous understanding of the underlying physics. The audience should be familiar with the most fundamental concepts of measure theory based probability and with the concept of weak convergence of probability measures.

The theory of large deviations provides probabilistic and analytical tools for describing the exponential decay rate of very small probabilities in terms of variational formulas. In the development of this theory, driving forces came from various applications in physics, like random path models with interactions having long or short memories, and from static interacting particle systems like the interacting Bose gas. We will explain how to apply the theory to such examples and how much it can contribute to a rigorous understanding of the underlying physics. The audience should be familiar with the most fundamental concepts of measure theory based probability and with the concept of weak convergence of probability measures.

The time evolution of the probability distribution of a Markov process is given by a generator in a fashion that is formally very similar to the time evolution of the (complex) probability amplitude of a quantum system in imaginary time. This correspondence allows for the import of technical tools and notions from quantum mechanics to the study of stochastic processes (and vice versa) which has proved to be useful particularly in the context of stochastic interacting particles and quantum spin systems. This mini-course begins with an introduction into the formal aspects of this relationship. In the second part we discuss as applications: (a) Symmetry and duality, (b) Large deviations and Non-hermitian quantum mechanics, and, briefly, (c) Large-scale behaviour.

The time evolution of the probability distribution of a Markov process is given by a generator in a fashion that is formally very similar to the time evolution of the (complex) probability amplitude of a quantum system in imaginary time. This correspondence allows for the import of technical tools and notions from quantum mechanics to the study of stochastic processes (and vice versa) which has proved to be useful particularly in the context of stochastic interacting particles and quantum spin systems. This mini-course begins with an introduction into the formal aspects of this relationship. In the second part we discuss as applications: (a) Symmetry and duality, (b) Large deviations and Non-hermitian quantum mechanics, and, briefly, (c) Large-scale behaviour.

Quantum computers hold the promise to allow one to solve important problems that cannot be efficiently treated on classical computers. To date, the construction of a fault-tolerant quantum computer remains a fundamental scientific and technological challenge, due the influence of unavoidable noise which affects the fragile quantum states. In our talk, we first introduce basic concepts of topological quantum error-correcting color codes, which allow one to protect quantum information during storage and processing by distributing logical quantum information over quantum many-body spin systems. We then discuss progress on experimental quantum error correction, in particular the realisation of a minimal topological color code with trapped ions, which for the first time demonstrated basic quantum computations on an encoded logical qubit. In the second part, I will focus on fault-tolerant quantum computing in trapped-ion quantum processors, in which scalability can be achieved by shuttling ions between various trapping regions and by a toolbox of ion-crystal reconfiguration techniques. I will present recent theory work of our group on resource-efficient and fault-tolerant protocols to control single and coupled logical qubits of increasing size and robustness.

A major challenge in the study of strongly correlated electron systems is to establish a firm link between microscopic models and effective field theory. Quite often, this step involves a leap of faith, and/or extensive numerical studies. For fractional quantum Hall model wave functions, there exists - in some cases - a scheme to infer the long distance physics of the state that is both compelling and simple, and leaves very little room for ambiguity. This scheme involves a local parent Hamiltonian for the state, which unambiguously defines a "zero mode space" of elementary excitations, and what’s known as a "generalized Pauli principle", which efficiently organizes the zero mode space through one-dimensional patterns satisfying local rules. Where this works, universal properties of the state unambiguously emerge from counting exercises in terms of these patterns, which efficiently encode degeneracies, quasi-particle types and charges, and which completely determine an edge conformal field theory. There is even a natural scheme to infer braiding statistics directly, for both Abelian and non-Abelian states. Unfortunately, such a framework thus far exists for some quantum Hall states but not for others. In this talk, I will review the state of the art of this formalism, give reasons of why its "plain vanilla form" is insufficient to describe some important fractional quantum Hall states (e.g., Jain states), and explain how to address this deficiency through a new, more general concept, called "entangled Pauli principles". It will turn out that for some interesting quantum Hall states, the efficient description advertised here involves simple matrix-product-type entanglement. [1] arXiv:1803.00975

The theory of strong interactions, QuantumChromoDynamics, exhibits a strongly correlated low energy regime. This regime is governed by two phenomena, strong spontaneous chiral symmetry breaking and confinement. While the former phenomenon is responsible for most of the visible mass in the universe, the emergence of the latter is still not fully understood. Experimental access to QCD at large and small energies, temperatures and densities is obtained via heavy ion collisions within a non-equilibrium evolution. In summary the understanding of QCD requires theoretical approaches that can deal with both, strongly-correlated physics as well as non-equilibrium processes. The past years have seen tremendous progress in the description of QCD at vanishing and finite temperature and density with functional diagrammatic approaches, such as the functional renormalisation group or Dyson-Schwinger equations. Within these approaches QCD correlation functions of quarks, gluon and hadrons are computed non-perturbatively from first principles. In the present talk I will discuss respective results for strongly-correlated vaccum QCD, the phase structure of QCD at finite temperature and density, as well as its dynamics close to equilibrium. The talk concludes with a discussion of the further prospects for our understanding of the phase structure and dynamics of QCD.

The circuit model of quantum computation defines its logical depth or “computational time” in terms of temporal gate sequences, but the measurement-based model could allow totally dif- ferent time ordering and parallelization of logical gates. We introduce a deterministic scheme of universal measurement-based computation, using only Pauli measurements on multi-qubit hyper- graph states generated by the Controlled-Controlled-Z (CCZ) gates. In contrast to the cluster-state scheme where the Clifford gates are parallelizable, our scheme enjoys massive parallelization of CCZ and SWAP gates, so that the computational depth grows with the number of global applications of Hadamard gates, or, in other words, with the number of changing computational bases. An exponentially-short depth implementation of an N-times Controlled-Z gate illustrates a novel trade- off between space and time complexity.

Isolated systems tend to evolve towards thermal equilibrium, a special state that has been a research focus in physics for more than a century. By contrast, most processes studied in biological systems are far from equilibrium. A fundamental overarching hallmark of all these processes is the emergence of structure, order, and information, and we are facing the major challenge to identify the underlying physical principles. Two particular exciting problems are the self-organised formation of spatio-temporal patterns and the robust self-assembly of complex structures. In both fields there are recent advances in understanding the underlying physics that will be reviewed in this talk.

We study the temperature dependence of the electrical resistivity in a system composed of critical spin chains interacting with three dimensional conduction electrons and driven to criticality via an external magnetic field. The relevant experimental system is Yb2Pt2Pb, a metal where itinerant electrons coexist with localized moments of Yb-ions which can be described in terms of effective S=1/2 spins with dominantly one-dimensional exchange interaction. The spin subsystem becomes critical in a relatively weak magnetic field, where it behaves like a Luttinger liquid. We theoretically examine a Kondo lattice with different effective space dimensionalities of the two interacting subsystems. We characterize the corresponding non-Fermi liquid behavior due to the ``local criticality'' from the spins by calculating the electronic relaxation rate and the dc resistivity and establish its quasi linear temperature dependence.

We theoretically study an ultra-cold gas of spin-1 polar bosons in one spatial dimension which are subject to a quadratic Zeeman field and a Raman induced spin-orbit coupling. Concentrating on the regime in which the background fields can be treated perturbatively we analytically solve the model in its low-energy sector, i.e. we characterize the relevant phases and the quantum phase transitions between them. Depending on the sign of the effective quadratic Zeeman field epsilon, two superfluid phases with distinct nematic order appear. In addition, we uncover a spin-disordered superfluid phase at strong coupling. We employ a combination of renormalization group calculations and duality transformations to access the nature of the phase transitions. At epsilon=0, a line of spin-charge separated pairs of Luttinger liquids divides the two nematic phases and the transition to the spin disordered state at strong coupling is of the Berezinskii-Kosterlitz-Thouless type. In contrast, at epsilon not equal to 0, the quantum critical theory separating nematic and strong coupling spin disordered phases contains a Luttinger liquid in the charge sector that is coupled to a Majorana fermion in the spin sector (i.e. the critical theory at finite epsilon maps to a quantum critical Ising model that is coupled to the charge Luttinger liquid). Due to an emergent Lorentz symmetry, both have the same, logarithmically diverging velocity. We discuss the experimental signatures of our findings that are relevant to ongoing experiments in ultra-cold atomic gases of 23Na.

Area laws were first discovered by Bekenstein and Hawking, who found that the entropy of a black hole grows proportional to its surface area, and not its volume. Entropy area laws have since become a fundamental part of modern physics, from the holographic principle in quantum gravity to ground state wavefunctions of quantum matter, where entanglement entropy is generically found to obey area law scaling. As no experiments are currently capable of directly probing the entanglement area law in naturally occurring many-body systems, evidence of its existence is based on studies of simplified theories. Using new exact microscopic path integral ground state Monte Carlo simulations of superfluid 4He, we demonstrate for the first time area law scaling of entanglement entropy in a real quantum liquid in three dimensions. We validate the fundamental principles underlying its physical origin, and present an "entanglement equation of state" showing how it depends on the density of the superfluid.

In functional magnetic nanostructures the acoustic, magnetic and plasmonic excitations can co-exist and interact on the nanometer spatial and ultrafast time scales. Optical spectroscopy with femtosecond laser pulses highlights a variety of nontrivial spatio-temporal dynamics, which are not only used to monitor individual excitations in real time, but also study interaction mechanisms between them, often observed in frequency mixing phenomena. Whereas the second harmonic generation (SHG), sum- and different frequency mixing in the optical frequency range originate from the chi(2)-nonlinearities, in acoustics and magnetism they are often dominated by parametric resonances, where system parameters are modulated at frequencies comparable to the natural oscillation frequencies, typically in the MHz-GHz range. Keeping in mind the intrinsic differences in the physical nature and frequency range of these phenomena, here we discuss, in a comparative manner, two examples of frequency mixing in magneto-plasmonics [1,2] and magneto-acoustics [3,4]. The ability to experimentally tune both systems through Surface Plasmon Resonance (SPR) and Ferromagnetic Resonance (FMR) as well as to theoretically describe these resonant interactions within the framework of phenomenological models based on the Lorentz oscillator, represent the key idea behind this presentation. In nonlinear magneto-plasmonic experiments, the Kretschmann configuration SPR resonances for different optical wavelengths occur at different angles Fig. (a,b), offering the unique possibility to match the fundamental and SHG resonances. In magneto-plasmonic Au/Co/Ag/glass samples the plasmonically assisted SHG also depends on the direction of magnetization M in ferromagnetic cobalt, which can be reversed with a weak external magnetic field. A simple model utilizing the resonant plasmonic enhancement of the chi(2)-susceptibility confirms the experimental observation that magnetic effects are most pronounced between the SHG and fundamental SPR resonances [1,2]. In the second experiment [3,4] the magnetization in a Ni/glass sample is excited by two distinct transient surface acoustic waves (SAW and SSLW). Magnetic tuning of the FMR frequency in resonance to their SHG, sum- and difference frequencies demonstrates the full variety of frequency mixing phenomena Fig. (c). In contrast to nonlinear optics, the frequency mixing is dominated by the parametric effect in the externally driven FMR oscillator. An analytical model based on the resonant enhancement of frequency-mixed signals explains the experimental observations [4]. A practical application of these findings to magneto-elastic switching in bi-stable systems, i.e. nickel nanomagnets, is under way [5]. Whereas the use of phenomenological, non-microscopic modeling may fail to capture fine details, it represents a useful tool for experimentalists measuring macroscopic quantities such as magnetization dynamics and/or optical nonlinearities. The presented methodology can further exploited to design experimental architectures based of coupled resonances of different nature (acoustic, SPR, FMR, excitons in semiconductor quantum dots, nanomagnets) providing a playground for future optical metrology and the physics of nano-devices [5,6]. References: [1] I. Razdolski et al., ACS Photonics 3, 179 (2016) [2] V.V. Temnov et al., J. Opt. 18, 093002 (2016) [3] J. Janusonis et al., Phys. Rev. B 94, 024415(2016) [4] C.L. Chang et al., Phys. Rev. B 95, 060409 (2017) [5] V.S. Vlasov et al., Multifunctional Materials (to appear in 2018) [6] D.A. Kuzmin et al., Nanophotonics 7, 597 (2018)

In recent years, a lot of effort has been devoted to the quest for experimental realizations of Kitaev interactions in spin systems. Recently, many materials have been proposed which seem to realize extended Kitaev models, where the Kitaev interactions are supplemented by Heisenberg and other bond dependent terms. The symmetry and electronic structure of these materials renders the determination of a minimal model a non trivial pursuit. In this talk I will concentrate on one of these particular materials, A-Li2IrO3, for which various minimal models have been proposed. Employing large scale Monte Carlo simulations I will show how the number of prospective models can be reduced. I will show that many of those models do not reproduce the most recent experimental results. Furthermore, I will propose and predict the results of an experiment which could help determine which one of the proposed models, if any, is the minimal model of A-Li2IrO3.

We develop a numerical technique based on the pseudofermion functional renormalization group (PFFRG) to calculate hopping and pairing amplitudes of emergent spinon quasiparticles in spin-1/2 quantum spin liquids. Within this approach, we first formulate a self-consistent Fock-like equation for these amplitudes where instead of the bare propagators and couplings we use the fully renormalized ones from PFFRG. We solve these equations using different ansätze for the hoppings and pairings which we take from a projective symmetry group (PSG) classification. From the overall size of these amplitudes we identify which of the PSGs are preferably realized in the system. We apply this approach to the antiferromagnetic J1-J2 Heisenberg model on the square lattice and to the antiferromagnetic nearest neighbor Heisenberg model on the kagome lattice. For the J1-J2 model, we find that in the regime of maximal frustration (J1~J2/2) the largest amplitudes are obtained for a U(1) pi-flux state with a Dirac cone spinon dispersion. In the case of the kagome model we identify a gapless Z2 pi-flux state where the bands show a Dirac-cone-like structure at finite energies but also feature a small circular Fermi surface at zero energy. We discuss our findings and benchmark them against variational Monte Carlo results.

The scaling behavior near the transition between plateaus of the Integer Quantum Hall Effect (IQHE) has traditionally been interpreted on the basis of a two-parameter renormalization group ow conjectured from Pruisken's non-linear sigma model. Yet, the conformal Field theory (CFT) describing the critical point remained elusive, and only fragments of a quantitative analytical understanding existed up to now. Here we carry out a detailed study of the current-current correlation function for the conductivity tensor, initially in the Chalker- Coddington network model for the IQHE plateau transition and then in its exact reformulation as a supersymmetric vertex model. We take the continuum limit of the non-local conductivity response function at criticality and thus identify a non-Abelian current algebra at level n = 4. By proposing precise lattice expressions for the CFT primary Fields we predict the multifractal scaling exponents of critical wave intensities to be Δ_q = q(1-q)/4.

Why does the wave-function of a quantum system collapse during a measurement? It maybe possible to answer this question, as in the many-worlds interpretation, without modifying the theory. In this case, quantum theory is exact. On the other hand, it maybe that the theory has to be modified, as in the phenomenological model of Spontaneous Collapse. This model proposes that every quantum particle in nature undergoes spontaneous collapse, and that collapse is a fundamental property of nature, along with Schrödinger evolution. The predictions of this model differ from those of quantum theory, and we review the current experimental bounds on these models. We also review work which attempts to provide a theoretical underpinning for this phenomenological model, including the possible role of gravity, and a possible inter-connection between the measurement problem and the problem of time in quantum theory.

The quest for understanding and exploiting many-body phenomena has received new spur in the last decades by the increasing capacity of harnessing quantum particles to tailor more and more sophisticated systems. Among others, cold atomic gases are one of the most flexible platforms for realising such “Synthetic Quantum Matter”. Here, I focus on the interplay of geometrical constraints, tunable interactions of various range and (artificial) gauge fields to access i) interacting topological states of matter and ii) many-body effects in the transport properties of low-dimensional systems. In order to broaden our understanding, besides mappings onto effective models, I exploit quantum-information-inspired numerical techniques, namely Tensor Networks algorithms. In this way, insights about the entanglement structure of correlated states are also gained. In this talk, I will provide an overview of some of my recent results: in particular, I will concentrate on the Creutz-Hubbard ladder, a neat playground to address the above challenges, including the generation of flat bands as well as of non-doubled Dirac dispersion relations. In [1], I will present a theoretical analysis of the competition between correlated topological phases and orbital quantum magnetism, and the prediction of topological quantum phase transitions with different underlying conformal field theories (CFTs). In [2], I will examine the response of an interacting system of Dirac-Weyl fermions confined in a one-dimensional (1D) ring: i will show that the tuning of repulsive interactions leads to a unique many-body system that displays an enhancement of the Drude weight—the zero-frequency peak in the ac conductivity—with respect to the non-interacting value. Finally, I will review our proposal to experimentally realize this model in a synthetic ladder, made of two internal states of ultracold fermionic atoms in a one-dimensional optical lattice. References: [1] J. Jünemann, et al., PRX 7, 031057 (2017) [2] M. Bischoff, et al., PRB 96, 241112(R) (2017)

Last few decades, the oxide heterostructure systems were intensively studied as a bulk crystal and thin film. Therefore, we have a deep understanding of the core physics on these systems. But still we can find or design a new research playground among these systems. Dimensionality, artificial structure, interface engineering, and exploring new device technique, these keywords opened the new research opportunities. In this talk, Firstly, I would like to introduce dimensional study on ultra-thin manganite film with element specific scattering/spectroscopy technique. Secondly, the Topological Hall Effect in SrRuO3 system will be discussed.

Granular materials are characterized by an additional degree of freedom, rotations, which become particularly relevant for spherical particles. They allow for soft modes under shear which do find realizations for instance in tectonic faults. A packing of spheres is called bi-chromatic if every loop formed by contacts is even. In three dimensions, bi-chromatic bearings have many different sliding-free configurations, so called bearing states. If all loops have length four the system exhibits four continuous degrees of freedom and a systematic way of constructing such bearing states can be devised. By considering spheres of different size, packings with bearing states can even be made space-filling. The construction and mechanical properties of such space-filling bearings will be discussed. Their bearing states can be viewed as a realization of solid turbulence exhibiting Kolmogorov scaling and anomalous heat conduction. In three dimensions a continuum of such configurations can be obtained as cuts through four-dimensional space-filling bearing states. Bearings states can be perceived as physical realizations of networks of oscillators with asymmetrically weighted couplings. These networks can exhibit optimal synchronization properties through tuning of the local interaction strength as a function of node degree or the inertia of their constituting rotor disks through a power-law mass-radius relation . Under this condition, the average participation per disk is maximized and the energy dissipation rate is homogeneously distributed among elementary rotors. The synchronization of rotations occurs in avalanches following a broad size distribution.

Part A: "A Fine Discrete FFCA Pedestrian Model and Its Application On Evacuation"; Part B: "How Sychronization Improves Pedestrian Traffic by Sound: from Macroscopic to Microscopic"

Quantum field theories play an important role in many condensed matter systems for their description at low energies and long length scales. In 1+1 dimensional critical systems the energy spectrum and the spectrum of scaling dimensions are intimately related in the presence of conformal symmetry. In higher space-time dimensions this relation is more subtle and not well explored numerically. In this talk we motivate and review our recent effort to characterize 2+1 dimensional quantum field theories using computational techniques 2+targeting the energy spectrum on a spatial torus. We discuss several examples ranging from the O(N) Wilson Fisher theories and Gross-Neveu-Yukawa theories to deconfinement- confinement transitions in the context of topological ordered systems. We advocate a phenomenological picture that provides insight into the operator content of the critical field theories.

Topological superconductors hosting spatially well-separated Majorana bound states offer the possibility for realizing robust qubits protected from local environmental perturbations. Revealing new properties of Majorana bound states as well as establishing an experimentally feasible platform for Majorana-based quantum computing constitute two of the most urgent challenges in the field. In this talk, I will address these issues by discussing two setups of topological superconductors coupled to conventional superconductors: (1) The first setup comprises a Coulomb-blockaded time-reversal invariant topological superconductor island with Majorana Kramers pairs placed in an s-wave superconductor Josephson junction. I will discuss a 2Pi Josephson effect which is mediated by the Majorana Kramers pairs and whose direction is controlled by the joint parity of all four Majorana bound states on the island, a "Parity-controlled 2Pi Josephson effect". (2) The second setup constitutes a Majorana-based qubit in an all-superconducting circuit, a "Majorana Superconducting Qubit". I will demonstrate how universal quantum computation can be achieved in such a device and discuss advantages over conventional superconducting qubits. [1] C. Schrade and L. Fu, Parity-controlled 2Pi Josephson effect mediated by Majorana Kramers pairs, arXiv:1801.03511 [2] C. Schrade and L. Fu, Majorana Superconducting Qubit, arXiv:1803.01002

I will review our recent investigations of semiconducting materials at the focus of optoelectronic research. We measure the surface of CH3NH3PbI3 single crystals by making use of two-photon photoemission spectroscopy. By these means, we explicitly discriminate the initial thermalization of the electrons from slower dynamical processes. The picosecond localization of excited electrons in degraded CH3NH3PbI3 samples is consistent with the progressive reduction of photoconversion efficiency in operating devices. In the second part of the seminar I will discuss the dynamics of hot carriers in InSe. The electrons excited by photons of 3.1 eV experience a manifold relaxation. First, they thermalize to the electronic states degenerate with the M valley within 0.35 ps. Subsequently, the electronic cooling is solely dictated by Froehlich coupling to phonon of small momentum transfer. Ab-initio calculations of the electron-phonon coupling are in excellent agreement with the observed dynamics.

The high reactivity of magnetic substrates toward molecular overlayers has so far inhibited the realization of more sophisticated on-surface reactions, thereby depriving these interfaces of a significant class of chemically tailored organics such as graphene nanoribbons (GNRs), oligonuclear spin-chains, and metal−organic networks. In the first part of this talk, I will present a multi-technique characterization of the polymerization of 4,4″-dibromo-p-terphenyl precursors into ordered poly(p-phenylene)[1] arrays on top of an emerging class of two-dimensional magnetic surface alloys[2,3]. The chemical reaction was followed by temperature-dependent X-ray photoelectron spectroscopy (XPS) and the structural characterizations of supramolecular and polymeric phases were performed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). Taking advantage of the high homogeneity of the samples, the electronic structure of the valence band was determined with angle-resolved photoemission spectroscopy (ARPES). Moreover, ferromagnetic ordering in the two-dimensional GdAu2 alloy was demonstrated for all phases by X-ray absorption spectroscopy (XAS)[4]. In the second part, I will briefly show that GdAu2 is not only a suitable template for the surface-confined Ullmann-like coupling reaction but is also thermally sufficiently stable to sustain a cyclodehydrogenation reaction for the on-surface synthesis of more complex GNRs. Thus, the system is demonstrated to close a longstanding experimental gap in the study of GNRs as it enables spin-polarized STM characterizations[5] of e.g. the spin-polarization of the elusive GNR edge-states, which were theoretically predicted for chiral GNRs[6] and experimentally, albeit lacking spin-sensitivity, observed on Au(111)[7]. However, successive theoretical studies questioned [8] and refined [9], respectively, the experimental prerequisites under which magnetic ordering of edge-states in GNRs may be observed, thereby highlighting the need for direct experimental access of the spin-polarization of these states. References: [1] Vasseur, G.; Abadía, M.; Miccio, L. A.; Brede, J.; Garcia-Lekue, A.; de Oteyza, D. G.; Rogero, C.; Lobo-Checa, J. & Ortega, J. E. Pi Band Dispersion along Conjugated Organic Nanowires Synthesized on a Metal Oxide Semiconductor J. Am. Chem. Soc. 138, 5685-5692 (2016) [2] Corso, M.; Fernández, L.; Schiller, F. & Ortega, J. E. Au(111)-Based Nanotemplates by Gd Alloying ACS Nano 4, 1603-1611 (2004) [3] Ormaza, M.; Fernández, L.; Lafuente, S.; Corso, M.; Schiller, F.; Xu, B.; Diakhate, M.; Verstraete, M. J. & Ortega, J. E. LaAu2 and CeAu2 surface intermetallic compounds grown by high-temperature deposition on Au(111) Phys. Rev. B 88, 125405 (2013) [4] Abadía, M.; Ilyn, M.; Piquero-Zulaica, I.; Gargiani, P.; Rogero, C.; Ortega, J. E. & Brede, J. Polymerization of Well-Aligned Organic Nanowires on a Ferromagnetic Rare-Earth Surface Alloy, ACS Nano 11, 12392-12401 (2017) [5] Brede, J. & Wiesendanger, R. Spin-resolved imaging and spectroscopy of individual molecules with sub-molecular spatial resolution MRS Bulletin 39, 608-613 (2014) [6] Yazyev, O. V.; Capaz, R. B. & Louie, S. G. Theory of magnetic edge states in chiral graphene nanoribbons Phys. Rev. B 84, 115406 (2011) [7] Tao, C.; Jiao, L.; Yazyev, O. V.; Chen, Y.-C.; Feng, J.; Zhang, X.; Capaz, R. B.; Tour, J. M.; Zettl, A.; Louie, S. G.; Dai, H. & Crommie, M. F. Spatially resolving edge states of chiral graphene nanoribbons, Nature Physics 7, 616-620 (2011) [8] Kunstmann, J.; Özdoğan, C.; Quandt, A. & Fehske, H. Stability of edge states and edge magnetism in graphene nanoribbons Phys. Rev. B, 83, 045414 (2011) [9] Golor, M.; Lang, T. C. & Wessel, S. Quantum Monte Carlo studies of edge magnetism in chiral graphene nanoribbons Phys. Rev. B, 87, 155441 (2013)

Certain phase transitions in a graphene sheet are known to be described at criticality by an underlying continuum quantum field theory which is the Gross-Neveu model. While the Ising Gross-Neveu model has been widely studied for many years, perturbative and large N computations in the related chiral Heisenberg Gross-Neveu model have not been carried out to the same level of precision. In the talk we will review recent four loop renormalization of the Ising Gross-Neveu model before developing the large N expansion for the chiral Heisenberg Gross-Neveu case. In the latter case the first three terms of various critical exponents are determined including eta at O(1/N^3). To achieve this accuracy the large N conformal bootstrap method is used. Finally estimates of exponents are given for the values of N relevant to the graphene transitions and compared to Monte Carlo and functional renormalization group results as well as the epsilon expansion of four dimensional perturbation theory.

In der Veranstaltung wird das Lernmanagement-System Ilias, auf das jeder Dozent und Studierender der UzK Zugriff hat, vorgestellt. An Hand vonkonkreten Beispielen werden Einsatzmöglichkeiten und Funktionsumfang des Systems illustriert. Zur Demonstration der Handhabung von Ilias empfiehlt es sich, einen eigenen Laptop mitzubringen! Zielgruppe sind neben den Dozenten auch die Veranwortlichen für den Übungsbetrieb.

In the recent years, a growing interest on pedestrian traffic has led to a better knowledge on the movement of large crowds inside public facilities such as transportation hubs and recreational halls. However, there are still many aspects which are unknown and, especially when psychological aspects arise, managing and controlling large crowds is a challenging task. In this talk, classical methods to analyze and measure pedestrian crowds will be introduced and discussed. We will start with the bidirectional flow (for example people moving in a corridor), which, although rather simple, presents most of the characteristics of pedestrian dynamics, in particular regarding the presence of collision avoidance and emergent phenomena (in the form of lanes). Later the discussion will be extended to more complex cases, with the chaotic motion being the least organized. In the presentation a focus will be set on methods to analyze the level of congestion and the intrinsic risk of human crowds by also discussing methods to simulate pedestrian behavior with the aim of predicting future changes. Novel techniques (for example the use of inertial data from portable devices) will be presented showing advantages and limitations compared to classical approaches typically taken for pedestrian traffic and more generally for transportation systems.

It is a well-known fact that the spreading of information in lattice quantum systems is not instantaneous, but it rather exists a maximum velocity dictated by the Lieb-Robinson bound. The existence of such a lightcone deeply affects equilibrium properties as well as out-of-equilibrium ones, which have been a subject of outstanding interest in the recent years. In this talk, a novel out-of-equilibrium protocol critically affected by the presence of a maximum velocity is proposed and discussed. Specifically, a one dimensional lattice model is considered, where a localized impurity is suddenly created and then dragged at a constant velocity. Focussing on a simple, but far from trivial, free model the response of the system at late times is analyzed, with emphasis on its transport properties. The finite maximum velocity is responsible for a rich phenomenology, for which exact results are provided. Taking into account the experience acquired so far, more general models are discussed and unpublished results presented, with exact predictions in completely generic (non integrable) one dimensional lattice systems. Reference paper: A. Bastianello, A. De Luca, Phys. Rev. Lett. 120, 060602 (2018).

In this blackboard talk, I will first review the old results of Peschel en Emery, who devised a set of interacting spin-1/2 models, for which the degenerate ground states can be written in terms of product states. This model has more interesting properties, such as excited states that can be constructed explicitly, and one can find exact, local operators that swap the ground states. These results can be generalized to three state Clock models, and models with arbitrary spin. If time allows, I will comment on how to adapt the construction to arbitrary lattices.

Antiferromagnets and ferromagnets represent two fundamental forms of magnetism with antiferromagnets being the more abundant of the two. However, it has been notoriously difficult to manipulate and detect antiferromagnetic order by any practical means due to the compensated magnetic moment. This has left antiferromagnets over their hundred-year history poorly explored, in striking contrast to the thousands of years of fascination and utility of ferromagnets. This has changed with the proposal and subsequent discovery of a new relativistic spin-torque phenomenon, the Neél spin-orbit torque, that allow us to efficiently control antiferromagnetic moments in spintronic devices. This allows for antiferromagnets to become active elements in devices. An additional concept that has emerged is that antiferromagnets provide a unifying platform for realizing synergies among three prominent fields of contemporary condensed matter physics: Dirac quasiparticles and topological phases. These recent developments have unlocked a multitude of known and newly identified unique features of this "dormant-giant" class of materials that the community is beginning to explore.

Quantum Hall effect is one of the most interesting examples quantum many-particles systems. It occurs in certain two-dimensional electron systems at low temperatures and in high magnetic fields, which exhibit plateaux with the quantized values of the Hall conductance. The Fractional Quantum Hall effect (FQHE), when the Hall conductance takes on fractional values, is an example of the strongly-interacting quantum many-particles. The standard physics approach to the FQHE is to assign a certain many-body wave function to each plateaux. I will talk about a program as to how one can use a combination of probabilistic, asymptotic and geometric methods to learn more about the physics and mathematics of the FQHE wave functions, in particular, describe the electromagnetic and gravitational responses, asymptotics for a large number particles, novel quantized coefficients for the adiabatic transport.

In recent years, considerable experimental efforts have been devoted to dynamically generate complex states and monitor their evolution. Despite remarkable advances, the theoretical principles behind the non-equilibrium dynamics of strongly correlated quantum matter are still far from being fully understood. In particular, very few studies have sought to clarify the influence of environmental couplings on the propagation of correlations. We attempt here to fill this gap. Considering first an interaction quench in the Bose-Hubbard model under the effect of dephasing, we observe that dissipation effectively speeds up the propagation of single-particle correlations while reducing their coherence. In contrast, for two-point density correlations, the initial ballistic propagation regime gives way to diffusion at intermediate times. In a second time, considering the XXZ spin-1/2 model in contact with a similar environment, we find this system to display aging. A dynamical phenomenon characterized by a breakdown of time-translation invariance, a slow non-exponential relaxation of two-time correlations and the presence of dynamical scaling.

I will discuss several results (also in the mathematical sense) about the dynamics of quantum spin systems. The main theme will be 'localization' which is a term that has been much in vogue in the last years. Usually, it is understood as 'absence of dissipative behaviour'. I will explain what this means and how it comes about. The phenomenon that is probably to be expected to apply more broadly than 'localization' is 'sparsity of resonances', sometimes also called 'asymptotic localization' or 'quasi-localization'. It manifests itself in quantum and classical systems, where transport and thermalization take place only due to effects that are non-perturbative (smaller than any power) in the natural parameters of the system.

Recent developments in diverse areas - ranging from cold atomic gases over light driven semiconductors to microcavity arrays - move systems into the focus, which are located on the interface of quantum optics, many-body physics and statistical mechanics. These "driven open quantum systems" share in common that coherent and driven-dissipative dynamics occur on an equal footing, creating genuine non-equilibrium scenarios without immediate counterpart in equilibrium condensed matter physics. A case in point are so-called exciton-polaritons in two spatial dimensions. We briefly explain the physical basis, their description, and how to detect the violation of thermal equilibrium conditions in the formalism. We then show that a paradigmatic hallmark of low-temperature equilibrium systems -- the presence of quasi-long range order, i.e. the algebraic decay of spatial correlation functions -- must be absent due to non-equilibrium conditions. This conclusion is drawn based on a connection to the problem of surface roughening.

We demonstrate that the exact nonequilibrium steady state of the one-dimensional Heisenberg XXZ spin chain driven by boundary Lindblad operators can be constructed explicitly with a matrix product ansatz for the nonequilibrium density matrix. For spin 1/2 the matrices satisfy the quantum algebra U_q[sl(2)]. For the isotropic Heisenberg chain, coupled at the ends to boundary reservoirs polarized in different directions with twist angle theta, we calculate the exact magnetization profiles and magnetization currents in the nonequilibrium steady state of a chain N sites. For large N the in-plane steady-state magnetization profiles are harmonic functions with a frequency proportional to the twist angle. In-plane steady-state magnetization currents are subdiffusive, while the transverse current saturates when the coupling strength is sufficiently large. For the anisotropic chain we find a current resonance at the specific values of the anisotropic interaction strength where the transverse current is independent of system size, even for non-integrable higher-spin chains.

In standard formulations of the uncertainty principle, two fundamental features are typically cast as impossibility statements: two noncommuting observables cannot in general both be sharply defined (for the same state), nor can they be measured jointly. The pioneers of quantum mechanics were acutely aware and puzzled by this fact, and it motivated Heisenberg to seek a mitigation, which he formulated in his seminal paper of 1927. He provided intuitive arguments to show that the values of, say, the position and momentum of a particle can at least be unsharply defined, and they can be measured together provided some approximation errors are allowed. Only now, nine decades later, a working theory of approximate joint measurements is taking shape, leading to rigorous and experimentally testable formulations of associated error tradeoff relations. Here we briefly review this new development, explaining the concepts and steps taken in the construction of optimal joint approximations of pairs of incompatible observables. As a case study, we deduce measurement uncertainty relations for qubit observables using two distinct error measures. We provide an operational interpretation of the error bounds and discuss some of the first experimental tests of such relations. The talk is based on the review paper arXiv:1512.00104v3.

In physics, there is the prevailing intuition that we are part of a unique external world, and that the goal of physics is to understand and describe this world. This assumption of the fundamentality of objective reality is often seen as a major prerequisite of any kind of scientific reasoning, delineating science from pseudoscience, and explaining why successful empirical science is possible in the first place. However, here I argue that we should consider relaxing this assumption in a specific way in some contexts. Namely, there is a collection of open questions in and around physics that can arguably be addressed in a substantially more consistent and rigorous way if we consider the possibility that the first-person perspective is ultimately more fundamental than our usual notion of external world. These are questions like: which probabilities should an observer assign to future experiences if she is told that she will be simulated on a computer? How should we think of cosmology’s Boltzmann brain problem or assign probabilities to properties of ‘possible worlds’? What can we learn from the fact that measurements in quantum theory seem to do more than just reveal preexisting properties? Why are there simple computable laws of physics in the first place? In the talk, I sketch a mathematically rigorous approach along these lines, suggesting a simple and unified framework (rooted in algorithmic information theory) to address questions like those above. It is not meant as a ‘theory of everything’ (in fact, it predicts its own limitations), but it shows how a notion of objective external world, looking very much like our own, can provably emerge from a starting point in which (only) the first-person perspective is primary. Based on arXiv:1712.01826 (full version) and arXiv:1712.01816 (short version).

Starting from examples of experimental systems which exhibit growth and pinning I will give an introduction into the physics of elastic systems with quenched disorder in non-equilibrium situations. We review developments in the description of depinning and avalanches, as well as in the study of the Kardar-Parisi-Zhang (KPZ) class of stochastic growth. We introduce some of the analytical methods which lead to predictions testable (and sometimes tested) in experiments. Our last example is the calculation of memory effects in the KPZ class in expanding geometries which illustrates the remarkable connections between growth and disordered systems.

Strong correlations between spin, charge and orbital degrees of freedom play an important role in materials and a recent development of ultrafast spectroscopies enabled to disentangle these relevant degrees of freedom by their temporal evolution. I will start with a summary of the charge carrier relaxation after the photo-excitation in Mott insulators described within the dynamical mean field theory (DMFT) and continue how this formalism can be extended to including the role of dynamical screening and non-local fluctuations (GW+EDFMT)[1,2]. Then I will open the question how to use the laser pulse to manipulate screening in Mott insulators. As an extreme example I will present a self-trapping of the system in the negative temperature state by a proper manipulation of the screening environment, which leads to the enhanced subgap response in the charge susceptibility. This population inversion leads to the low-energy anti-screening and I will comment on its experimental relevance. In the second part I will shed light on the role of spin fluctuations in the relaxation dynamics, which can be analysed by an extension of DMFT[3]. I will exemplify how optical pump probe techniques can be used to detect some basic theoretical ideas in higher dimensional doped antiferromagnets, like string states, Trugman paths and the lack of spin-charge separation. At the end I will provide an outlook how to extend these tools to an ab-initio description of strongly correlated materials out of equilibrium. [1] D. Golez, M. Eckstein, and P. Werner. Phys. Rev. B, 92:195123, Nov (2015). [2] D. Golez, L. Boehnke, H. U. R. Strand, M. Eckstein, and P. Werner. Phys. Rev. Lett. 118:246402 (2017). [3] N. Bittner, D. Golez, M. Eckstein, P. Werner, in preparation.

Multiferroics permit the magnetic control of the electric polarization and the electric control of the magnetization. These static magnetoelectric (ME) effects are of enormous interest: The ability to read and write a magnetic state current-free by an electric voltage would provide a huge technological advantage. Dynamic or optical ME effects are equally interesting, because they give rise to unidirectional light propagation as recently observed in several multiferroic compounds [1]. In conventional media light propagation is reciprocal, that is counter-propagating beams experience the same refractive index. However, reciprocity can be violated in multiferroic materials, where the refractive index depends not only on the polarization of light but also on the +- k direction of the propagation [1]. Such unidirectional transmission is the consequence of the dynamic magnetoelectric effect emerging in materials with simultaneously broken time reversal and spatial inversion symmetries. This phenomenon, exclusively observed in multiferroic and magnetoelectric materials [2-5], may allow the development of optical diodes transmitting unpolarized light in one, but not in the opposite, direction [5]. Recently, the emergence such unidirectional light propagation, governed the dynamic magnetoelectric effect, has also been demonstrated in multi-antiferroics [6], i.e. in materials with coexisting purely antiferroelectric and antiferromagnetic orders. References [1] D. Szaller et al., Phys. Rev. B 87, 014421 (2014) [2] I. Kezsmarki et al., Phys. Rev. Lett. 106, 057403 (2011) [3] S. Bordacs et al., Nat. Phys. 8, 734 (2012) [4] I. Kezsmarki et al., Nat. Commun. 5, 3203 (2014) [5] I. Kezsmarki et al., Phys. Rev. Lett. 115, 127203 (2015) [6] V. Kocsis et al., arXiv:1711.08124 (2017)

In recent years there have been important advances in understanding isolated quantum systems far from equilibrium. Prominent examples include the (pre-)heating process in the early universe after inflation, the initial stages in collisions of relativistic nuclei at giant laboratory facilities, as well as table-top experiments with ultracold quantum gases. Even though the typical energy scales of these systems vastly differ, they can show very similar dynamical properties. Certain characteristic numbers can even be quantitatively the same, defining nonthermal universality classes. One may use this universality to learn from experiments with cold atoms aspects about the dynamics during the early stages of our universe.

Exactly solvable nonlinear wave equations -- colloquially known as the soliton systems -- are widely regarded as one of the greatest achievements of mathematical physics. But somehow, aside of several mathematical frameworks and other formal aspects, not a lot seems to be known about statistical properties of classical integrable field theories and, in particular, classical transport properties at finite temperature. In this talk, we present a kinetic equation to deal with dense soliton gases, expressed in terms of a linear integral dressing equation for the soliton spectral function which accounts for renormalization of the soliton velocities due to the interactions with a soliton many-body state. This is accomplished in the framework of the algebro-geometric integration technique which permits to classify all quasi-periodic solutions of an integrable equation of motion in terms of the moduli of finite-genus Riemann surfaces. By identifying soliton excitations, applying Born-Sommerfeld quantization for soliton orbits, extracting the two-body S-matrix, and finally taking the thermodynamic finite-density, the free energy functional is expressed as the saddle point of the action-space path-integral. Our hydrodynamic equations can understood as the thermodynamic analogue of the celebrated Whitham's modulation equations. The equations are universal, and even apply to the quantum theories of solitons. If time permits, we show how to obtain a closed compact formula for the Drude weight in the quantum Heisenberg spin chain, and discuss peculiarities related to it.

We discuss significant events in the recent Renaissance triggered by the enigmatic and elusive, but rich stochastic nonlinear PDE of Kardar, Parisi & Zhang, a celebrated equation whose reach far exceeds its grasp, touching such diverse phenomena as non-equilibrium stochastic growth, extremal paths through disordered landscapes, driven lattice gases & dissipative condensates, as well as the statistics of random matrices & permutations.

Starting from an amorphous partitioning of space into cells, we iteratively optimize the `centrality' of the cells, minimizing the so-called Quantizer energy. The energy landscape is replete with local minima to which the system converges despite the existence of lower-energy crystalline configurations. Irrespective of the level and type of disorder in the initial configurations, the tessellations converge to the same amorphous state, as measured by the same structure factor and energy distributions. The final disordered configurations exhibit an anomalous suppression of long-wavelength density fluctuations, known as hyperuniformity. For systems related to the Quantizer problem, such as selfassembled copolymeric phases, our findings suggest the possibility of stable disordered hyperuniform phases.

Tensor network states, and in particular projected entangled pair states, play an important role in the description of strongly correlated quantum lattice systems. They do not only serve as variational states in numerical simulation methods, but also provide a framework for classifying phases of quantum matter and capture notions of topological order in a stringent and rigorous language. In this talk I will present how virtual symmetries in tensor networks, summarized by the framework of matrix product operator (MPO) injectivity, are substantial to the classification of not only bosonic but also fermionic topological order in two dimensional systems. I will briefly discuss how this fact relates to Levin-Wen string net models and Tuarev-Viro state sum constructions. For the sake of concreteness, two examples of fermionic topological order, the fermionic toric code and the Majorana dimer model, are discussed using the language of fermionic tensor networks.

We develop a novel real-time approach to computing the entanglement between spatial regions for Gaussian states in quantum field theory. The entanglement entropy is characterized in terms of local correlation functions on space-like Cauchy hypersurfaces. The framework is applied to explore an expanding light cone geometry in the particular case of the Schwinger model for quantum electrodynamics in 1+1 space-time dimensions. We observe that the entanglement entropy becomes extensive in rapidity at early times and that the corresponding local reduced density matrix is a thermal density matrix for excitations around a coherent field with a time dependent temperature. Since the Schwinger model successfully describes many features of multiparticle production in electron-positron collisions, our results provide an attractive explanation in this framework for the apparent thermal nature of multiparticle production even in the absence of significant final state scattering.

A metal is a solid with a Fermi surface. It is known how to reconstruct the shape of the Fermi surface – by immersing the metal in a magnetic field and measuring the period of field-induced oscillations of the magnetization/resistivity. I will show how to extract information about the quantum-mechanical wavefunction of the Fermi surface – from measuring the phase offset of these same oscillations. In some metals, this information is robust against deformations of the Hamiltonian (describing the metal), and may therefore be viewed as a topological invariant.

The paradigmatic condensed-matter models where zero-energy localized Majorana fermions have been studied so far have the distinguishing feature of not conserving the number of fermions. Moreover, the accepted definition of "Majorana fermion" naturally belongs to this scenario. Is it possible to discuss Majorana fermions in "canonical" particle-conserving settings? In this seminar I will present several exact and numerical results on Majorana fermions in particle-conserving scenarios. I will start from the discussion of a model for bosons and fermions where, in a proper limit, the physics of the celebrated Kitaev's chain appears. I will continue by presenting exact results on Majorana fermions in ladder models where the two legs of the system can only exchange pair of particles. Finally, I will comment on the possibility of making experiments with Majorana fermions in particle-conserving settings using cold atoms.

A revolutionary new direction in the field of superconductivity emerged recently with the synthesis of superconductors with strong inherent spin-orbit coupling of electrons, such as the half-Heusler compounds YPtBi or LuPdBi. Due to band inversion, the low-energy degrees of freedom are electrons at a three-dimensional quadratic band touching point with an effective spin 3/2, which allows for Cooper pairs with spins ranging from 0 to 3. I will illuminate some of the unconventional superconducting properties that arise from this band structure and attractive short-range interaction: (i) At strong coupling, the system features an s-wave superconducting quantum critical point with non-Fermi liquid scaling of fermions and several other unusual scaling properties. (ii) The system may further undergo a transition into a phase with complex tensor order, which is a superconducting state captured by a complex-valued matrix order parameter describing Cooper pairs having spin 2. Here the interplay of both tensorial and complex nature results in a rich and intriguing phenomenology. I will discuss the mean-field phase structure as a function of doping and temperature, and relate our finding to experiments in YPtBi. Further, the critical properties of this new paradigm for superconductivity will be addressed.

One of the most interesting properties of topologically ordered models with long ranged entanglement, such as Kitaev's toric code, is that they have anyonic excitations. The properties of these anyons can be recovered from first principles using techniques from mathematical physics, making it possible to put conjectures about such systems in a precise mathematical language. After explaining the main ideas behind this construction, I will discuss recent results on the stability of the excitation structure and on applications to quantum information theory.

In this talk I will given an overview of pure state quantum statistical mechanics, which is a new way of understanding issues at the foundation of statistical mechanics and thermodynamics. In particular, I will explain recent developments concerning equilibration and thermalization in closed quantum many-body systems. We will see how equilibration and thermalization can be defined in unitarily evolving and finite dimensional quantum systems, under which conditions they can be proved to happen, and what we know about the apparent equilibrium states. I will further speak about general results on structural properties both short- and long-range interacting many-body systems, with a focus on the decay of correlations in their thermal states. Finally, I will provide a glimpse of ongoing work concerning the use of machine learning tools form many-body physics and point out opportunities for future work.

Driving systems out of equilibrium can provide new ways to control properties and extract new functions. Recent development of intense laser in a wide frequency range has led to intriguing observations such as light-induced superconductivity-like behaviors above the transition temperature [1], enhancement of the excitonic condensation [2] and high-harmonic generation in solid states [3]. Motivated by this intriguing situation, we have been developing nonequilibrium methods based on the dynamical mean-field theory (DMFT). Here we would like to present recent two applications of the nonequilibrium DMFT for the time-periodic steady states (Floquet DMFT). In the first part, we discuss the high-harmonic generation (HHG) in the Mott insulator under periodic AC driving [4]. We show that qualitative behavior of the HHG spectrum is different between weak and strong field regimes, which originates from qualitative difference in doublon/holon dynamics under the driving. We also discuss the similarity and the difference of the HHG intensity in the Mott insulator compared to that of semiconductors and disordered systems. In the second part, we discuss the effects of the parametric-phonon driving on conventional SC, which has been considered to enhance SC[5][6]. By studying transient dynamics and steady states, we demonstrate that even though the attractive interaction can be enhanced by the driving, SC is always suppressed, in particular, at the parametric resonance. Our systematic analysis shows that, in a wide parameter range, the heating of the system is the dominant effect and the parametric phonon driving has a negative effect on SC. [1] M. Mitrano et al., Nature 530, 461 (2016). [2] S. Mor et al., PRL 119, 086401 (2017). [3] S. Ghimire et al., Nat. Phys. 7, 138 (2011). [4] Y. Murakami, M. Eckstein, P. Werner to be published. [5] M. Knap et al., PRB 94, 214504 (2016); M. Babadi et al., PRB 96, 014512 (2017). [6] Y. Murakami et al, PRB 96, 045125 (2017).

Fracton phases are gapped quantum phases in three dimensions that go beyond the standard paradigm of topological order; they have fractionalized excitations that are immobile or only mobile along lower-dimensional subsystems, such as lines or planes. While there are several exactly solvable models of such fracton phases, these models are far from realistic because they involve interactions between many spins at the same time. By generalizing the fermionic parton construction, a standard phenomenological description of fractionalization in quantum phases, we provide simple variational states capturing fracton phases. Moreover, by showing that each variational state is the asymptotic strong-coupling ground state of a corresponding coupled-spin-chain model, we demonstrate that a large class of fracton phases can be realized in more realistic models involving only two-spin interactions.

State-of-the-art nano-fabrication process allow to control the spatial confinement of electronic carriers, with close to atomic precision. Structures are thus designed to separate spatially oppositely charged electrons and holes, notably bilayer heterostructures. When electrons and hole are separated in such adjacent layers they experience a strong Coulomb attraction pairing them into spatially indirect excitons, i.e. boson-like particles, which exhibit a giant electric dipole of about 500 Debye. Excitons then experience a repulsive dipolar potential stabilizing cold gases against collapse while providing access to a rich variety of collective quantum phases, possibly ranging from superfluidity to supersolidity [1]. Here, first signatures for the superfluid crossover of such dipolar excitons are reported [2]. When confined in a microscopic trap we show that they realize a four-component superfluid at sub-Kelvin temperatures, distributed between two optically active and two optically inactive spin states. By imaging the condensate bright part, we study in-situ the profiles of the exciton density and the phase coherence in the trap. We thus reveal quantum spatial coherence, in a sub-Kelvin regime bound to very dilute densities and probably limited by the strength of dipolar interactions. Also, we evidence quantized vortices, efficiently trapped in the slight electrostatic disorder of our trapping potential [2]. Analyzing the interplay between quasi long-range order, vortex formation, and density profiles across the range of explored parameters it is finally shown that our experimental findings provide a direct evidence for a Berezinskii-Kosterlitz-Thouless crossover [3]. The work presented here results from contributions of S.Dang, R.Anankine, M.Beian, M.Alloing, E.Cambril, A.Lemaitre and M.Holzmann.

The celebrated KPZ equation (Kardar, Parisi, Zhang, 1986) is an important milestone in statistical physics, originally introduced to describe the late time dynamics in two dimensional growth models. Over the last 30 years, the KPZ story has evolved in various interesting directions, making links on the way to different areas of physics and mathematics. This includes in particular the link to the famous Tracy-Widom distribution in random matrix theory. The story of KPZ is a very successful one, involving theoretical physics, mathematics and experiments--a fertile playground for interdisciplinary science. In this talk, I will review the evolution of the KPZ story, pointing out the important landmarks as I go along. At the very end, I will discuss some recent developments establishing a nice link between the KPZ height fluctuations and the edge physics in cold atom systems.

Topological semimetals have recently been one of the most active topics in solid state physics, and provide an exceptionally fruitful meeting point for fields as diverse as chemistry, band structure calculations and high-energy field theory. This talk will discuss the observation of quantum anomalies in Weyl semimetals. These anomalies are rooted in an incompatibility of quantum mechanics and classical conservation laws. In particular, I will focus on recent measurements demonstrating the presence of the mixed axial-gravitational anomaly in Weyl semimetals. In contrast to earlier expectations, which would for example search for this phenomenon in the hydrodynamics of neutron stars, we used a conceptually simple thermoelectric transport experiment to find signatures of the mixed axial-gravitational anomaly in the Weyl semimetal NbP.

A question that attracted a lot of attention in the past two decades is whether biologically relevant search strategies can be identified by statistical data analysis and mathematical modeling. A famous paradigm in this field is the Levy Flight Foraging Hypothesis. It states that under certain mathematical conditions Levy dynamics, which defines a key concept in the theory of anomalous stochastic processes, leads to an optimal search strategy for foraging organisms. This hypothesis is discussed very controversially in the current literature. I will review examples and counterexamples of experimental data and their analyses confirming and refuting it. Related to this debate is own work about biophysical modeling of bumblebee flights under predation thread and biological cell migration, both based on experimental data analysis, which I briefly outline.

Within the field of solid-state physics, the discovery of remarkable phases and transitions is often tightly coupled to the design, growth and characterization of novel materials. Therefore, the past several decades of work in the field of correlated electron physics can be described by a list of materials that have defined new states of matter at extreme conditions, e.g. low temperatures, high magnetic field or pressure. In most cases, a thorough understanding of the underlying physical mechanisms is accessible only, if high-quality single crystals with sufficient sizes are available. In this lecture, I shall give an overview about my own contributions to this research area in recent years focusing on three very different material classes (i) quantum criticality and superconductivity in heavy-fermion metals [1-3], (ii) crystal growth of LnFeAsO iron-pnictide superconductors [4,5], and (iii) spin-liquid phases in Cu-based frustrated spin systems [6]. References [1] A. Steppke et al., Science 339, 933 (2013). [2] E. Schuberth et al., Science 351, 6272 (2016). [3] H. Pfau et al., Phys. Rev. Lett. 119, 126402 (2017). [4] A. Jesche et al., Phys. Rev. B 86, 020501(R) (2012). [5] A. Adamski et al., Phys. Rev. B 96, 100503(R) (2017). [6] P. Puphal et al., J. Mater. Chem. C 5, 2629 (2017).

Higher-order topological insulators have been recently proposed as unprecedented quantum states of matter. In two spatial dimensions, they incorporate quantized electric quadrupole insulators, which feature zero-dimensional topological corner midgap states. These states are protected by the bulk gap, reflection symmetries, and a spectral symmetry, while their edges are gapped. Although such novel topological phases are often hard to come by in real materials, there are various analogue platforms that enable us to realize the corresponding band theory concepts in classical systems. I will talk about how we developed and measured a topolectrical circuit design for a quadrupole insulator, the midgap states of which manifest themselves as topological boundary resonances in the corner impedance profile of the circuit. This establishes an instance where topolectrical circuitry is employed to bridge the gap between quantum theoretical modeling and the experimental realization of topological band structures.

We study the contact process in the regime of small infection rates on scale-free networks evolving by stationary dynamics. A parameter allows us to interpolate between slow (static) and fast (mean-field) network dynamics. For two paradigmatic classes of networks we investigate transitions between phases of fast and slow extinction and in the latter case we analyse the density of infected vertices in the metastable state. This is joint work with Emmanuel Jacob (ENS Lyon) and Amitai Linker (Universidad de Chile).

Scanning tunneling microscopy is an ideal technique to explore the structural and electronic properties of low-dimensional electron system. In this talk I will focus on the study of physical properties self-organizing metallic nanowires on semiconductor surfaces and transport through single molecules. The metallic nanowires show unique physical properties owing to their one-dimensional nature. Many of these properties are intimately related to electron-electron interactions, which play a much more prominent role in one dimension than in two or three dimensions. I will present our work on Pt, Au and Ir atom chains on Ge(001) surfaces and provide a few examples of their unique properties, such as Peierls instability, quantum confinement, suppression of the density of density states near the Fermi level and nanowire length quantization. If time allows I will also elaborate on the transport through single molecules.

In the first part of the talk a brief overview of the research activities in area of fabrication and characterization of hybrid device structures based on amorphous silicon and organic semiconductors will be presented. The second part of the talk will be devoted to the discussion of the dynamic grating technique based on non-steady-state photo-EMF effect and its application for characterization of the photo-physical properties of organic, inorganic and hybrid semiconductors.

Single-file motion in quasi one-dimensional geometry appears in wide-ranging systems, ranging from ion transport in biological channels, fluids absorbed in materials with nanopores, colloidal particles moving along circular grooves to pedestrian dynamics regulated along a line. In this talk, we focus on the steady state density profiles in directed single-file motions of a collection of particles along closed heterogeneous landscapes. Modeling these as asymmetric exclusion processes with heterogeneous hopping rates along a closed ring, we explore generic localised (LDW) and delocalised (DDW) domain walls and delocalisation of LDWs in the steady states. We further show how the interplay between heterogeneity and particle non-conserving Langmuir kinetics leads to a phase transition between a two-phase coexistence and a three-phase coexistence state. We eventually discuss the more generic problem of heterogeneities with arbitrary spatial variation and extract a notion of universality, hitherto unknown, for the steady state density profiles and the associated phase diagrams.

One hundred and fifty years ago, Maxwell's demon was first posed as a fundamental challenge to the newly developed field of statistical physics. Just two months later, Maxwell's paper "On governors" gave the first analysis of a feedback system. These two foundational works reflect the fundamental and practical aspects of control. Here, I will present an experiment that unites the two: using feedback to create `impossible' dynamics, we make a Maxwell demon that can reach the fundamental limits to control set by thermodynamics. We test - and then extend - Rolf Landauer's 1961 prediction that information erasure requires at least as much work as can be extracted from a system by virtue of information. These fundamental thermodynamic limits are benchmarks for evaluating the performance of practical information engines, such as those active within cells and other complex systems.

The computational superiority of a computer that operates according to the laws of quantum mechanics relies crucially on quantum parallelism: different sets of data stored as a superposition of quantum states can be processed in parallel. To play out this advantage, however, one has to be able to prepare such superpositions in the first place --- using only a minimum of computational resources. We present a framework to efficiently prepare superpositions of many-body spin configurations with programmable squared amplitudes. The spin configurations are encoded in the degenerate ground states of the lattice-gauge representation of an all-to-all connected Ising spin glass. The ground state manifold is invariant under variations of the gauge degrees of freedom, which take the form of four-body parity constraints. Our framework makes use of these degrees of freedom by individually tuning them to dynamically prepare programmable superpositions. The dynamics combines an adiabatic protocol with controlled diabatic transitions.

We discuss the perturbative control of low-energy effective theory of strongly-interacting systems which cannot be treated within the Landau Fermi liquid framework. These are generically called non-Fermi liquids where quasiparticles do not exist. In particular, we focus on critical Fermi surface states where there is a well-defined Fermi surface, but no quasiparticle resulting from the strong interaction between the Fermi surface and a massless boson. We will show that for Fermi surface having a dimension m>1, the Fermi momentum k_F enters the expressions for physical quantities as a dimensionful parameter leading to UV/IR mixing, thus modifying the naive scaling arguments, whereas for a one-dimensional Fermi surface there is an emergent locality with no such k_F dependence.

Magnetic nanoparticles reveal unique magnetic properties and relaxation phenomena which make them relevant for data storage, electronic and mechanical engineering, and biomedical applications1,2. Whereas the implementation of nanomagnetic properties into technological applications is progressing rapidly, understanding the microscopic origin of phenomena such as magnetization enhancement or decrease, magnetic anisotropy and the related magnetization distribution in individual nanoparticles, and interparticle interactions leading to aggregation or even ordered assemblies of nanoparticles is fundamentally challenging and needs intensive research. Polarized neutrons are an excellent, microscopic probe for spatial and time-resolved studies of magnetism. In this contribution, I will present examples of our recent studies of magnetic nanoparticles on different scales, ranging from atomic magnetic structures to the mesoscale intraparticle magnetization, and interparticle interactions in well-ordered arrays of magnetic nanocubes. References: [1] S. D. Bader, Rev. Mod. Phys. 78, 1 (2006); DOI: 10.1103/RevModPhys.78.1 [2] Q. A. Pankhurst et al., J. Phys. D: Appl. Phys. 36, R167 (2003); DOI: 10.1088/0022-3727/36/13/201

The conceptual basis for a classical nonlocal generalization of Einstein's theory of gravitation is presented. The framework of general relativity is enlarged by the introduction of a preferred frame field; then, history dependence is introduced. Nonlocality --- in the sense of an influence ("memory") from the past that endures --- could be a natural feature of the universal gravitational interaction. Nonlocal gravity is formally analogous to the nonlocal electrodynamics of media. The nonlocal aspect of gravity can simulate dark matter. The implications of nonlocal gravity theory for the problem of dark matter in galaxies, clusters of galaxies, and cosmology are discussed. Special Colloquium honoring Friedrich Hehl's 80th birthday.

Based on the recent conceptual and technical developments of quantum Monte Carlo simulations in correlated electron systems, I will present and discuss fresh results of itinerant quantum critical points, i.e., the critical phenomena arising from the strong coupling between Fermi surface and bosonic fluctuations, which are made possible by the self-learning Monte Carlo scheme. After which, I will also discuss the lately proposed duality relations between interaction-driven topological phase transitions and deconfined quantum critical points, which is verified via unbiased large-scale quantum Monte Carlo simulations.

The properties of thin films and heterostructures of complex and strongly correlated oxides are one of the most fascinating and demanding topics in today´s solid state physics research. Oxide heterostructures give rise to novel and unexpected physical phenomena such as metallicity in nominally non-metallic materials, magnetism in nominally non-magnetic materials, ferroelectricity in nominally non-ferroelectric materials, or so far unobserved topological effects. Many of these phenomena are affected or even more driven by the nanoscale defect structure established during material synthesis and processing, ultimately limited by the thermodynamics of the system. Tuning the thermodynamic processes by engineering dedicated heterostructures may be used to tailor their defect structure in a desired way, hence, to deplete or accumulate defects at interfaces, to spatially separate electronic and ionic charge carriers, or to confine defects to dedicated regions. In this talk, I discuss the low-dimensional electron transport observed along complex oxide heterointerfaces, such as the one in LaAlO3/SrTiO3 heterostructures. While the formation of these 2-dimensional electron gases is attributed to electronic charge transfer triggered by a built-in electric field, the ionic defect structure at these interfaces is still being discussed controversially. I address the thermodynamic processes associated with built-in electric fields and derive ionic defect concentration profiles established at polar/non-polar oxide interfaces. [1] The specific ionic-electronic defect structure stabilized within such interfacial space charge layers strongly depends on ambient oxygen partial pressure applied during sample fabrication and on the strength of the built-in electric field. As will be discussed, the low-temperature behavior of these novel electron system is unambiguously correlated to the adjacent ionic structure typically set at high temperature, making thermodynamic considerations indispensable in order to understand novel phenomena occurring at cryostatic temperatures. Therefore, a comprehensive study of low temperature physics on the one hand and high temperature thermodynamics on the other hand is essential for a unified understanding of the manifold (and sometimes contradictory) phenomena observed in these low-dimensional electron systems. Here, we explicitly compare the thermodynamic ground states obtained for various oxide heterostructure systems [2] and discuss resulting implications for important measures characterizing the electron gas, such as electron mobility [3] as well as its magnetic signature [4], both controllable by thermodynamic means. The thermodynamic model obtained for oxide heterointerfaces is furthermore linked to kinetic space charge formation occurring at complex oxide surfaces [5, 6]. [1] F. Gunkel et al., “Defect concentration profiles at complex oxide interfaces”, Physical Review B 93, 245431 (2016) [2] F. Gunkel et al., “Thermodynamic ground states of complex oxide heterointerfaces”, ACS Applied Materials & interfaces, ACS Appl. Mater. Interfaces, 9 (1), 1086 (2017) [3] C. Xu et al., “Disentanglement of growth dynamic and thermodynamic effects in LaAlO3/SrTiO3 heterostructures”, Scientific Reports 6, 22410 (2016) [4] F. Gunkel, et al., “Defect-control of anomalous and conventional electron transport in NdGaO3/SrTiO3 heterostructures”, Physical Review X, 6, 031035 (2016) [5] R. Meyer et al., “Dynamics of the metal-insulator transition of donor-doped SrTiO3”, Physical Review B 94, 115408 (2016) [6] M. Andrae et al., “Oxygen partial pressure dependence of surface space charge formation in donor-doped SrTiO3”, APL Materials 5, 056106 (2017)

Non-relativistic quantum matter, as realized in ultracold atomic gases, continues to be a remarkably versatile playground for many-body physics. Experimentalists have exquisite control over temperature, density, coupling, and shape of the trapping potential. Additionally, a wide range of properties can be measured: from simple ones like equations of state to more involved ones like the bulk viscosity and entanglement. The latter has received much attention due to its connection to quantum phase transitions, but it has proven extremely difficult to compute: stochastic methods display exponential signal-to-noise issues of a very similar nature as those due to the infamous sign problem affecting finite-density QCD. In this talk, I will present an algorithm that solves the signal-to-noise issue for entanglement, and I will show results for strongly interacting systems in three spatial dimensions that are the first of their kind. I will also present a few recent explorations of the thermodynamics of polarized matter and other cases that usually have a sign problem, using complexified stochastic quantization.

A promising approach to realize quantum spin liquids is to enhance the spin-space symmetry from usual SU(2) to SU(N). While the SU(N) symmetry with a general N is proposed in ultracold atoms using nuclear spin degrees of freedom, its realization in magnetic materials is challenging. Here we propose a new mechanism by which the SU(4) symmetry emerges in the strong spin-orbit coupling limit. The spin-orbit coupling in d^1 transition metal compounds with edge-sharing anion octahedra leads to strongly bond-dependent hopping, which is apparently not SU(4)-symmetric. However, in the honeycomb structure, a gauge transformation maps the system to an SU(4)-symmetric Hubbard model. In the strong repulsion limit at quarter filling, the low-energy effective model is the SU(4) Heisenberg model on the honeycomb lattice, which cannot have a trivial gapped ground state and is expected to host a gapless spin-orbital liquid. By generalizing this model to other three-dimensional lattices, we also propose crystalline spin-orbital liquids protected by this emergent SU(4) symmetry and space group symmetries.

Spin-polarized scanning tunneling microscopy at low temperature (8 K) and in high magnetic fields (6 T) is a powerful technique to investigate magnetic properties of individual nanoscale objects ranging in size form single atoms to several thousand atoms. I focus on the magnetization reversal and the spin-dependent electronic properties of bilayer Co, Fe-decorated Co and Fe islands on Cu(111). We find a novel noncollinear, helical magnetic order in the Fe islands, which is identified by a magnetic stripe contrast with a period of 1.28 nm. The high spatial resolution of the scanning tunneling spectroscopy reveals the significance of structural and electronic relaxation for the magnetic anisotropy and the spin-dependent transport properties of single islands. In an outlook I present first results from our new STM, which operates at 0.3 K in a vector magnetic field, on superconductivity of Pb monolayers and on the proximity effect.

Flows of large numbers of macroscopic particles (sand grains, cereals, ball bearings, etc.) occur in natural phenomena like land slides or avalanches and are an important part of many industrial processes. The flow behavior of granular fluids displays a rich rheology including Newtonian, shear thinning, and shear thickening regimes as well as the famous Bagnold scaling. The non-equilibrium nature of granular flows and the high densities and shear rates involved make a theoretical description challenging. I will begin with a qualitative discussion of what determines the flow behavior. Based on this discussion I will develop a theory from first principles that makes it quantitative and allows the prediction of flow curves and transport coefficients.

The identification of the electron-boson interaction is believed to be the key for rationalizing the Cooper pairing mechanism in a superconductor. In strong- coupling conventional superconductors, the fingerprints of the electronic band renormalization due to the electron-boson interaction are found in the fine structure of tunneling spectra, where the identification of this fine structure with the phononic density of states represents one of the hallmarks of modern condensed matters physics. In many unconventional superconductors, the unknown nature of the pairing boson, absence of momentum resolution, as well as possible inelastic tunneling effects render the interpretation of the tunneling spectra often cumbersome and ambiguous. Here we exploit a so far unnoticed boson-assisted resonant enhancement of the impurity scattering potential and thus the quasiparticle interference (QPI) amplitude in order to identify the nature of a possible pairing boson in the unconventional superconductor LiFeAs. In high-resolution Fourier transform scanning tunneling spectroscopy (FT-STS) measurements we indeed observe a resonance-like enhancement of the QPI amplitude in both the normal and the superconducting state, which implies an important coupling of electronic states to corresponding bosonic states at q~0 and energy at about 8meV. We show that these bosonic states seen in the QPI are in perfect agreement with a strong inelastic contribution on the tunneling dI/dU spectra recorded away from impurities which renders the observed small-q modes promising candidates for providing the pairing interaction in LiFeAs.

We consider a simplified model of a future European electricity network with a high share of renewable generation. In a cost optimal design of such a system, most of the renewable generation capacity is placed at locations with favorable weather conditions, that is for instance onshore wind in countries bordering the North Sea and solar PV in South European countries. Countries with less favorable renewable generation conditions benefit from this capacity by importing the respective electricity as power flows through the transmission grid. Using flow tracing techniques, which are related to directed diffusion processes on networks, we disentangle the emerging pattern of imports and exports and assign shares of the distributed generation capacity in the European system to the countries which actually make use of them. This procedures yields nodal levelized costs, which incorporate both internal and external generation as well as transmission costs associated with the electricity consumption in a country. Compared to a scenario without transmission, these nodal levelized costs are reduced by about 15% and represent the benefit of cooperation.

In the first part of the talk, I will propose a transport experiment of excitons in 2d transition-metal dichalcogenides. Due to their interactions with unbound electrons in the Fermi sea, excitons are dressed by a cloud of particle-hole excitations forming Fermi polarons. An in-plane electric field cannot directly accelerate neutral excitons, however, due to the polaronic dressing, the unbound electrons will drag the excitons along. This phenomenon is similar to Coulomb drag in bilayer semiconductors. We determine the drag conductivity for a low density of excitons using simple arguments as well as a diagrammatic calculation based on a self-consistent T matrix approximation. Due to the strong nonperturbative coupling, the exciton drag mobility is expected to be of the same order as the electron mobility even at zero temperature. Time permitting, I will discuss a novel probe of quantum Hall ferromagnetism in graphene based on thermal transport in the second part of the talk. Strong interactions lift the high degeneracy of the zero-energy Landau level in graphene mono- and bilayers, giving rise to a rich phase diagram of competing spin and valley order. Here we show that thermal transport at low temperatures can give compelling evidence for spontaneous symmetry breaking and exhibits many other signatures distinguishing the various phases.

Since the isolation of monolayer graphene, the library of two dimensional (2d) materials has been continuously growing. Together with recent developments in on-demand stacking of these atomically thin crystals, a whole new class of hybrid materials is coming into reach, which holds promises from the realization of exotic electronic quantum phases at elevated temperatures to novel optoelectronic applications. Interactions between electrons are central to all these kinds of promises. In my talk, I will discuss why Coulomb interactions in 2d materials are very different from their 3d counterparts, how they influence material properties and how interactions can be manipulated in 2d materials. As example systems I will consider graphene as well as transition metal dichalcogenides like MoS2. In these systems interactions renormalize electronic quasiparticles through pronounced band gap shifts, lead to the emergence of many-particle modes such as excitons with unusually high binding energies and can finally drive the electron system towards different kinds of instabilities including superconductivity and charge density wave states.

Peter Schilke (Köln): Das unsichtbare Universum mit ALMA sichtbar gemacht Norbert Wex (Bonn): 50 Jahre Pulsare, 50 Jahre kosmische Uhren für Einstein Andreas Schulz (Köln, Bonn): Die Bewohnbarkeit unseres Planeten Alexander Küpper (Köln): Erfahrungen mit einer inklusiven astron. Lernumgebung Martin Quast (Bonn): Wo sind die Aliens? – Exoplaneten und ihre Bewohnbarkeit

Magnetism of molecules is in the focus of intense research, because it could lead to new spin-based electronic devices. In this context, two molecules adsorbed on a metal surface and carrying magnetic moments constitute an important model system which helps to understand the fundamental properties of molecular magnetism – in particular the interaction between their magnetic moments. Typically the interaction of spins on a metal surface is dominated and tuned by the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction – an indirect exchange interaction mediated by the conduction electrons of the metal. Here we show that the non-magnetic, chemical interaction between the molecules can become the decisive effect if the spin-moment-carrying molecular orbitals extend in space and therefore lead to the formation of bonding and antibonding orbitals due to wave function overlap. We demonstrate that this splitting has a crucial influence on the magnetic properties exemplified by two neighbouring metal-molecule (Au-PTCDA) complexes, i.e. dimers, on the Au(111) surface. Interestingly, in this particular case, competition between the binding energy gain, due to the chemical interaction between the moment-carrying orbitals, and the gain of additional hybridization energy, due to the strong entanglement between the local moment and the conduction band of the substrate, is the driving force of a quantum phase transition (QPT) in the dimer. Small changes in the wave function overlap, involving slightly different dimer configurations, drive the system through a quantum critical point (QCP) from a partially Kondo-screened triplet to a singlet ground state, with one dimer configuration being located extremely close to a quantum critical point.

If you are given a black box unitary U, can you build the inverse U^\dagger out of it? In this talk we are going to discuss this problem, and more generally which transformations on unitary channels can be done according to the rules of quantum mechanics

The previous 11 years have seen several breakthroughs in our understanding of topological states of matter. New Insulators and semimetals, exhibiting new properties imposed by a variety of symmetries (the most well known of which is time reversal) have been theoretically predicted and experimentally discovered. These materials are described by topological indices, a set of "serial numbers" that characterize the state of matter. In this talk, I will review the progress made in these years with a focus on the prediction of new topological indices and on material discovery. I will then show a surprising link between chemistry bonding and topological states of matter which leads to the prediction of many new classes of materials as topological.

The previous 11 years have seen several breakthroughs in our understanding of topological states of matter. New Insulators and semimetals, exhibiting new properties imposed by a variety of symmetries (the most well known of which is time reversal) have been theoretically predicted and experimentally discovered. These materials are described by topological indices, a set of "serial numbers" that characterize the state of matter. In this talk, I will review the progress made in these years with a focus on the prediction of new topological indices and on material discovery. I will then show a surprising link between chemistry bonding and topological states of matter which leads to the prediction of many new classes of materials as topological.

An interesting topic in the theory of quantum phase transitions is that of the transition between a classically ordered state, such as a Néel state, and a quantum ordered state, such as a valence bond crystal. On the classically ordered side of the transition, we have a well developed understanding based on the spontaneous breaking of a continuous symmetry and the occurrence of associated Goldstone modes (spin waves). On the quantum ordered side, things are rather less clear. Even the description of the quantum ordered state itself is not straightforward, since it is not a saddle-point of the usual spin-coherent-state path integral. In this talk, I shall present an approach designed to address this problem by constructing the path integral over matrix product states rather than spin coherent states. This allows both classically ordered states (which are matrix product states of bond dimension 1) and quantum ordered states (which are matrix product states of higher bond dimension) to be captured on an equal footing. I shall use this approach to show that, for at least one example of such a transition, a Landau-Ginzburg-Wilson-type description can be given, where the ‘order parameter’ is a field representing the nearest-neighbour entanglement of the spins. The talk is designed to be entirely self-contained: in particular, no prior knowledge of matrix product states or quantum ordered phases is assumed. The work about which I shall speak was undertaken in collaboration with Andrew Green (University College London), Jonathan Keeling (St Andrews), and Steve Simon (Oxford), under the auspices of the TOPNES programme. Most of the details can be found at https://arxiv.org/abs/1607.01778.

We numerically explore the many body localization transition through the lens of the entanglement spectrum, i.e. the spectrum of the reduced density matrix. The level statistics of the entanglement spectrum as obtained through numerical diagonalization, unveil structures beyond that revealed by more limited measures such as entanglement entropy. We will also show that a simple artificial neural network trained on entanglement spectra of individual states of a many-body quantum system can be used to determine the transition between a many-body localized and a thermalizing regime.