Department of Physics • University of Cologne

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.

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.

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).

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.

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.

Antrittsvorlesung.