Department of Physics • University of Cologne

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)

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

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.

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).

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.