### Poster Sessions at MCQST 2020

Officially there are two Poster Sessions at MCQST2020. During this time, conference participants can visit the poster galleries, discover the diverse projects presented and directly interact with the presenters via video chat. Networking has never been easier.

• Poster Session I - posters with odd numbers (1, 3, 5, etc.) within each topic are presenting.
Monday, 6 July 2020 | 13:10 - 14:30
• Poster Session II - posters with even numbers (2, 4, 6, etc.) within each topic are presenting.
Wednesday, 8 July 2020 | 13:10 - 14:30

You are welcome to visit the poster room at any time - posters are on display and accesible during the entire conference. However, during the official poster session times you can be sure the presenters are available for questions.

On this page you can find an overview of the participanting posters, as well as their abstracts.
The names of the posters presenters are underlined and the numbers within each topic correspond to the poster numbers in the virtual rooms.

### A: Quantum Information Theory Posters

Overview of posters covering Quantum Information Theory projects.
You can find them in the room with the same name.

Message Transmission over Classical Quantum Channels with a Jammer with Side Information: Correlation as Resource, Common Randomness Generation

Authors: Holger Boche, Minglai Cai (TUM), Ning Cai

In this paper we analyze the capacity of a special model for arbitrarily varying classical-quantum channels when the sender and the receiver use a weak resource. In this model a jammer has side information about the channel input. We determine the correlation assisted capacity. As an application, we determine the correlation assisted common randomness capacity with informed jammer. We also analyze these both capacities when only a small amount of correlation is available.

Pseudo-dimension of quantum circuits

Authors: Matthias C. Caro (TUM) and Ishaun Datta

We characterize the expressive power of quantum circuits with the pseudo-dimension, a measure of complexity for probabilistic concept classes. We prove pseudo-dimension bounds on the output probability distributions of quantum circuits; the upper bounds are polynomial in circuit depth and number of gates. Using these bounds, we exhibit a class of circuit output states out of which at least one has exponential state complexity, and moreover demonstrate that quantum circuits of known polynomial size and depth are PAC-learnable.

Correlations for computation and computation for correlations

Authors: Bülent Demirel, Weikai Weng, Christopher Thalacker, Matty Hoban, and Stefanie Barz

Quantum correlations are central to the foundations of quantum physics and form the basis of quantum technologies. Here, our goal is to connect quantum correlations and computation: using quantum correlations as a resource for computation - and vice versa, using computation to test quantum correlations. We derive Bell-type inequalities that test the capacity of quantum states for computing Boolean functions and experimentally investigate them using 4-photon Greenberger-Horne-Zeilinger (GHZ) states. Further, we show how the generated states can be used to specifically compute Boolean functions which can be used to test and verify the non-classicality of the underlying quantum states. The connection between quantum correlation and computability shown here has applications in quantum technologies, and is important for networked computing being performed by measurements on distributed multipartite quantum states.

Semantic Security for Quantum Wiretap Channels

Authors: Holger Boche, Minglai Cai, Christian Deppe, Roberto Ferrara, Moritz Wiese

We investigate the transmission of messages from a sending to a receiving party through a wiretap channel. In this model, there is a third party called an eaves-dropper who must not be allowed to know the information sent from the sender to the intended receiver. The wiretap channel was first introduced by Wyner in [57]. A classical-quantum channel with an eavesdropper is called a classical-quantum wiretap channel. The secrecy capacity of the classical-quantum wiretap channel subject to the strong security criterion has been determined. Strong security means that given a uniformly distributed message sent through the channel, the eavesdropper shall obtain no information about it. This criterion is the most common secrecy criterion in classical and quantum information theory. In the present poster, however, a stronger security requirement will be applied, called semantic security. With this, the eaves-dropper gains no information regardless of the message distribution. This criterion was introduced to information theory from cryptography, motivated by the analogous security criterion of the same name. It is equivalent to message indistinguishability, where the eavesdropper cannot distinguish whether the given cipher text is an encryption of any two messages (which can even be chosen by the eavesdropper). Aside from being the minimum security requirement in practical applications, semantic security is also necessary in the security of identification codes. We determine the semantic security capacity for quantum wiretap channels. We extend methods for classical channels to quantum channels to demonstrate that a strongly secure code guarantees a semantically secure code with the same secrecy rate. Furthermore, we show how to transform a non-secure code into a semantically secure code by means of biregular irreducible functions (BRI functions). We analyze semantic security for classical-quantum channels and for quantum channels."

Wigner process tomography of Unknown quantum propagators

Authors: Amit Devra (TUM) and Steffen J. Glaser

We study the tomography of unknown propagators for the spin system in the context of finite-dimensional Wigner representations, which completely characterize and visualize operators using shapes assembled from linear combinations of spherical harmonics. These shapes can be experimentally recovered by measuring the expectation values of the rotated axial tensor operator. Recent works show the general methodology to experimentally recover the shapes for density matrices (ρ) and known quantum propagators (U). This work extends the tomography approach for the unknown propagators. The approach is experimentally demonstrated for one-qubit quantum gates using NMR spectroscopy.

Quantifying Physical Fermionic Site/Orbital Entanglement

Authors: Lexin Ding (1), Zoltán Zimborás (2), Christian Schilling
1 LMU Munich, Munich Centre for Quantum Science and Technology
2 Zoltán Zimborás, Wigner Research Centre for Physics

Entanglement is one of the most fascinating concepts of modern physics. In striking contrast to its abstract, mathematical foundation, its practical side is remarkably underdeveloped: Even for the simplest scenario of just two orbitals or sites no formula is known for the entanglement in generic mixed quantum states. By exploiting the spin symmetries of realistic electronic systems and implementing the fundamental superselection rule we succeed in deriving a compact formula for the relative entropy of entanglement between any two electronic orbitals. Application to different molecular systems reveals that most of the correlation in molecules is classical. This raises questions about the role of entanglement in chemical bonding and quantum chemistry in general.

Symmetry resolved entanglement: Exact results in 1D and beyond

Authors: Shachar Fraenkel, Moshe Goldstein (Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University, Israel)

Symmetry and the resulting conservation laws have long been known to play a unique role in quantum many body states in condensed matter and high energy physics; more recently, the central importance of entanglement entropy has also been recognized. In our work we study the interplay of these two fundamental concepts: Given a conserved quantity in the system, the entanglement entropy of a subsystem can be resolved into a sum of contributions, each of which arising from states where the subsystem possesses a specific possible value of this quantity out of the total conserved value. Studying these symmetry-resolved contributions provides insight into the internal structure of the entanglement, beyond what can be learned from the value of the entanglement entropy itself. We present exact analytical results for the symmetry-resolved entanglement entropy in the Kitaev chain model, and demonstrate that in the topological phase there is a degeneracy between the even and odd fermion parity sectors of the entanglement spectrum, due to virtual Majoranas at the entanglement cut. These virtual Majoranas mirror the Majorana zero-modes hosted by the physical edges of the open Kitaev chain. Therefore, this result rigorously attests to the strong connection between entanglement and topological quantum phase transitions. We also obtain the leading behavior of the symmetry-resolved entanglement entropy for an ungapped free Fermi gas in a general dimension, and present exact results for the special case of the gapless 1D fermionic tight binding chain, which have a natural interpretation from a conformal field theory perspective.

Renormalized Lindblad-Driven NonEquilibrium Steady-State (RL-NESS) - A Numerically Exact Nonequilibrium Quantum Impurity Solver

Authors: Matan Lotem [1], Andreas Weichselbaum [2,3], Jan von Delft [2], and Moshe Goldstein [1]
[1] Tel Aviv University, Tel Aviv, Israel
[2] Ludwig Maximilian University, Munich, Germany
[3] Brookhaven National Laboratory, Upton, NY, USA

Phase Diagrams of Hierarchical Quantum Spin Glasses

Authors: Chokri Manai, Simone Warzel
Zentrum Mathematik, Technische Universität München

Spin glasses form a major area of research in statistical physics and probability theory with various applications to other ?elds, for instance mathematical biology and quantum computing. In the 1990s theoretical physicists started investigating quantum spin glasses where new quantum phenomena are expected to arise. However, most quantum spin glass models are not well understood and only very few predictions have been established rigorously so far. We present novel Parisi-type formula for the free energy of hierarchical quantum spin glass models. The structure of the phase diagrams is discussed.

Cavity QED with a donor nuclear spin coupled to a Si DQD

Authors: Jonas Mielke and Guido Burkard
Department of Physics, University of Konstanz, D-78457, Germany

Nuclear spins are promising for quantum information applications due to their long coherence times. Here, we derive an effective Hamiltonian describing the nuclear spin dynamics of a system consisting of a cavity coupled to a double quantum dot (DQD) in silicon subject to a magnetic field gradient and filled with a single electron, which, in turn, is coupled to a donor nuclear spin via the hyperfine interaction.
We show that the system can potentially be used for nuclear spin readout even without reaching the strong coupling regime. Using Input-Output theory we predict signatures of the nuclear spin state in the cavity transmission that we expect to be experimentally accessible, and thus allow for nuclear spin readout.

Entanglement-Assisted Data Transmission as an Enabling Technology: A Link-Layer Perspective

Emmy Noether Group Theoretical Quantum System Design, Institute of Theoretical Information Technology, Technical University of Munich

We propose a communication network development approach where subsequently parts of existing infrastructure receive updates with the latest quantum technology. Each update therefore needs to be compatible with existing infrastructure.

In particular we propose here a novel quantum link layer protocol, study its performance, compare to its classical version and highlight its potential impact on the network layer arising from phase transitions in the key metrics of the network link.

Authors: Uzi Pereg (TUM), Christian Deppe, and Holger Boche

Communication over a quantum channel that depends on a quantum state is considered, when the encoder has channel side information (CSI) and is required to mask information on the quantum channel state from the decoder. Full characterization is established for the entanglement-assisted masking equivocation region, and a regularized formula is given for the quantum capacity-leakage function without assistance. For Hadamard channels without assistance, we derive single-letter inner and outer bounds, which coincide in the standard case of a channel that does not depend on a state.

Adiabatic freezing of entanglement with insertion of defects in one-dimensional Hubbard model

Author: Sreetama Das (LMU), Sudipto Singha Roy, Himadri Shekhar Dhar, Debraj Rakshit, Aditi Sen (De), Ujjwal Sen

We report on ground state phases of a doped one-dimensional Hubbard model, which for large onsite interactions is governed by the t-J Hamiltonian, where the extant entanglement is immutable under perturbative or sudden changes of system parameters, a phenomenon termed as adiabatic freezing. We observe that in the metallic Luttinger liquid phase of the model bipartite entanglement decays polynomially and is adiabatically frozen, in contrast to the variable, exponential decay in the phase-separation and superconducting spin-gap phases. Significantly, at low fixed electron densities, the multipartite entanglement remains frozen across all parameter space. We note that entanglement, in general, is sensitive to external perturbation, as observed in several systems, and hitherto, no such invariance or freezing behavior has been reported.

Spectral Gaps and Incompressibility in a ν=1/3 Fractional Quantum Hall System

Authors: Bruno Nachtergaele, Simone Warzel and Amanda Young

We study an effective Hamiltonian for the standard ν=1/3 fractional quantum Hall system in the thin cylinder regime. We give a complete description of its ground state space in terms of what we call Fragmented Matrix Product States, which are labeled by a certain family of tilings of the one-dimensional lattice. We prove that the model has a spectral gap above the ground states for a range of coupling constants that includes physical values. As a consequence of the gap we establish the incompressibility of the fractional quantum Hall states. We also show that all the ground states labeled by a tiling have a finite correlation length, for which we give an upper bound. We demonstrate by example, though, that not all superpositions of tiling states have exponential decay of correlations. Low-energy excitations and an extensive number of many-body scars at positive energy density, but nevertheless low complexity, are also identified using the concept of tilings.

Nonlinear Intersubband Polaritonic Metasurfaces for Frquency Mixing

Authors: J.H. Krakofsky (TUM), G. Böhm, A. Mekkawy, S. Mann, A. Alù and M. A. Belkin

Ultrathin nonlinear intersubband polaritonic metasurfaces based on the coupling of quantum-engineered nonlinearities in semiconductor heterostructures with optical modes of plasmonic nanoantennas have recently achieved record-high (≈0.1%) power conversion efficiencies for second harmonic generation (SHG) using low-intensity (≈10 kW cm−2) illumination. These metasurfaces provide orders of magnitude higher second-order nonlinear optical response compared to nonlinear metasurfaces based on other design principles, such as those employing the nonlinearities of metal nanoresonators, bulk nonlinear materials, or 2D materials. In this poster we present the setup of a state of the art metasurfaces, showing the quantum engineered heterostructure and the nanoresonators design at the example of an SHG nonlinearity. Further we present our most recent approaches based on new materials that promise again a magnitude higher conversion efficiency and therefore a new step in the development of semiconductor-based nonlinearities.

### B: Quantum Simulation Posters

Overview of posters covering Quantum Simulation projects.
You can find them in the room with the same name.

Clock-line photoassociation of strongly bound dimers in a magic-wavelength lattice

Authors: Oscar Bettermann (LMU, Munich, Germany; MPQ, Garching, Germany)
Nelson Darkwah Oppong (LMU, Munich, Germany; MPQ, Garching, Germany)
Giulio Pasqualetti (LMU, Munich, Germany; MPQ, Garching, Germany)
Luis Riegger (LMU, Munich, Germany; MPQ, Garching, Germany)
Immanuel Bloch (LMU, Munich, Germany; MPQ, Garching, Germany)
Simon Fölling (LMU, Munich, Germany; MPQ, Garching, Germany)

An outstanding feature of alkaline-earth-(like) atoms is the existence of a metastable excited electronic state connected to the ground state via an ultranarrow "clock" transition. The interactions between atoms in the different electronic states are governed by the molecular interaction potentials between the atoms and the bound states formed inside these potentials.

Here, we report on the direct production and spectroscopy of the least bound state in Ytterbium-171 and characterization of the interactions between atoms in different electronic states. The dimers are produced by direct single-photon photoassociation via the clock line, in a deep three-dimensional optical lattice. In strong contrast to the shallow bound state present in Ytterbium-173, we find a much larger binding energy, with a much smaller molecular wavefunction therefore largely independent of the external potentials. We also show that the free-to-bound transition can be made insensitive to the depth of the trapping potential, an important aspect in the realization of optical molecular clocks.

Probing the doped Fermi-Hubbard model in and out-of-equilibrium

Authors: Annabelle Bohrdt 1,2,3 , Fabian Grusdt1,2,3 , Christie Chiu3, Geoffrey Ji3, Muqing Xu3, Lev Haldar Kendrick3, Martin Lebrat3, Daniel Greif3, Markus Greiner3, Eugene Demler3, Michael Knap1,2, Jayadev Vijayan2,4, Pimonpan Sompet2,4, Guillaume Salomon2,4, Joannis Koepsell2,4, Sarah Hirthe2,4, Immanuel Bloch2,4,5, Christian Gross 2,4
(1) Department of Physics and Institute for Advanced Study, Technical University of Munich, GER
(2) Munich Center for Quantum Science and Technology, GER
(3) Dept. of Physics, Harvard University, Cambridge MA, USA
(4) Max-Planck-Institut für Quantenoptik, GER
(5) Fakultät für Physik, Ludwigs-Maximilians-Universität, GER

The phase diagram of the Fermi-Hubbard model and its connection to high-temperature superconductivity have been the subject of a vast amount of theoretical and experimental studies in the past decades. Here, we present recent results motivated by the new perspective quantum gas microscopes provide. Our theoretical approach, the geometric string theory, describes doped holes moving in an AFM environment as meson-like bound states of spinons and chargons. Matrix product state based simulations of the ground state show convincing evidence for this scenario. We study the dynamics of a single hole created in one and two dimensions and are able to explain our findings in 2D in the framework of geometric string theory.

We furthermore compare geometric string theory predictions for spin correlation functions as well as string patterns at finite temperature and finite doping to experimental data of a cold atom experiment and find remarkable agreement. For an unbiased comparison of theories and experiment, we apply machine learning to classify experimental data at finite doping into different theoretical categories in order to determine which theory describes the system best on the microscopic level.

Tune-out and magic dipole traps for 23Na40K molecules

Authors: Xingyan Chen (MPQ), Roman Bause, Ming Li, Andreas Schindewolf, Marcel Duda, Svetlana Kotochigova, Immanuel Bloch, Xin-Yu Luo

A versatile, rotational-state dependent optical dipole trap for NaK molecules is created by utilizing a narrow-line transition. By tunning the trapping-laser frequency we can switch between the magic condition where both states experience equal polarizability and the tune-out condition where the polarizability of one state vanishes while the other remain finite. This new tool can be used to achieve long coherence times in superpositions of rotational states and to realize novel cooling schemes in optical lattices.

Anomalous Diffusion in Dipole- and Higher-Moment Conserving Systems

Authors: Johannes Feldmeier 1,2, Pablo Sala 1,2, Giuseppe De Tomasi 3, Frank Pollmann 1,2, Michael Knap 1,2
1 Technical University of Munich, James-Franck-Str. 1, 85748 Garching
2 Munich Center for Quantum Science and Technology (MCQST)
3 T.C.M. Group, Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK

The presence of global conserved quantities in interacting systems generically leads to diffusive transport at late times. Here, we show that systems conserving the dipole moment of an associated global charge, or even higher moment generalizations thereof, escape this scenario, displaying subdiffusive decay instead. Modelling the time evolution as cellular automata for specific cases of dipole- and quadrupole-conservation, we numerically find distinct anomalous exponents of the late time relaxation. We explain these findings by analytically constructing a general hydrodynamic model that results in a series of exponents depending on the number of conserved moments, yielding an accurate description of the scaling form of charge correlation functions. We analyze the spatial profile of the correlations and discuss potential experimentally relevant signatures of higher moment conservation.

Using a quantum simulator to benchmark an approximate theory for localized 1D Fermi-Hubbard systems

Authors: Bharath Hebbe Madhusudhana [1,2,3] , Sebastian Scherg[1,2,3], Thomas Kohlert[1,2,3], Immanuel Bloch[1,2,3], and Monika Aidelsburger[1,3]
[1] Ludwig-Maximilians-Universität, Schellingstr. 4, 80799 München, Germany
[2] Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany
[3] Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, 80799 München, Germany

The dynamics of most quantum many body systems cannot be computed efficiently. Therefore, a quantum simulator that simulates such dynamics is naturally performing a complex computation. While state-of-the-art quantum simulators can simulate quantum dynamics that are well outside classical computational limits, they are limited by noise and the variability of the initial state and observables, making them not an effective computational device. Despite these limitations, here we use a quantum simulator to in effect solve the dynamics of a localized 1D Fermi hubbard systems by using them to benchmark an efficient approximation ansatz. We consider a Wannier-Stark ladder, consisting of a nearest neighbor hopping and a linear on-site potential resulting in a tilt . In the non-interacting case, this system is analytically tractable. However, in the presence of Hubbard interactions, due to an exponential scaling of the dimension of the Hilbert space, theoretical and numerical computations are limited to either small system sizes or short evolution times. We use an analog quantum simulator made of trapped neutral atoms to experimentally study the localization dynamics of this system. Using the experimental results, we develop and benchmark an approximate theoretical model for our system and show that the experimental results are well approximated by the theory. The computational complexity of this theory is at-most linear in the system size and therefore, the system dynamics can be computed efficiently using this theory. We also apply this theory to the Aubry-André model, which is another Hamiltonian that features localization, and shows a good agreement with experimental data.

Fast Long-distance Transport of Caesium Atoms

Authors: Till Klostermann (LMU), Hendrik von Raven, Cesar Cabrera, Jingjing Chen, Julian Wienand, Christian Schweizer, Immanuel Bloch and Monika Aidelsburger

We present optical transport of Caesium atoms over a distance of 43 cm in 33 ms with an efficiency of 80 % and subsequent evaporation to degeneracy in a total cycle time of 12s.

Orbital Differentiation in Hund Metals

Author: Fabian B. Kugler [1], Seung-Sup B. Lee [1], Manuel Zingl [2], Hugo U. R. Strand [2], Andreas Weichselbaum [1,3], Gabriel Kotliar [3], Antoine Georges [2], Jan von Delft [1]
[1] LMU Munich
[2] Flatiron Institute
[3] Brookhaven National Laboratory

Orbital differentiation is a common theme in multi-orbital systems, yet a complete understanding of it is still missing. Here, we focus on three-orbital Hubbard models to describe the t2g bands of materials such as ruthenates and iron-based superconductors, and provide results of unprecedented accuracy by using the numerical renormalization group as real-frequency impurity solver for dynamical mean-field theory. First, we consider a minimal model for orbital differentiation, where a crystal field shifts one orbital in energy, and describe the various phases with dynamic correlation functions. Upon approaching the orbital-selective Mott transition, we find a strongly suppressed spin-coherence scale and uncover the emergence of a singular Fermi liquid and interband doublon-holon excitations. Then, we apply our method to the paradigmatic material Sr2RuO4 in a real-materials framework. We illustrate distinctive Hund-metal features and provide theoretical evidence for a Fermi-liquid scale of about 25 Kelvin.

Floquet prethermalization in a Bose-Hubbard system

Authors: A. Rubio-Abadal(1,2), M. Ippoliti(3), S. Hollerith(1,2), J. Rui(1,2), D. Wei(1,2), K. Srakaew(1,2), V. Khemani(3), S. L. Sondhi(4), C.Gross(1,2,5) and I. Bloch(1,2,6).
1 Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany
2 Munich Center for Quantum Science and Technology (MCQST), 80799 Munich, Germany
3 Department of Physics, Stanford University, Stanford, CA 94305, USA
4 Department of Physics, Princeton University, Princeton, NJ 08540, USA
5 Physikalisches Institut, Eberhard Karls Universität Tübingen, 72076 Tübingen, Germany
6 Fakultät für Physik, Ludwig-Maximilians-Universität, Schellingstraße 4, 80799 München, Germany

Novel exotic quantum many-body phases can be achieved through the periodic drive of a system, commonly dubbed Floquet engineering. However, many-body systems are generically expected to absorb energy from the drive, leading to a featureless “infinite-temperature” state. Few strategies have emerged to avoid this thermalization process. The first one is the introduction of strong disorder in the system, which can lead to a Floquet many-body-localized phase. A second one consists on driving the system at large frequencies compared to the local energy scales of the system, which has been shown to exponentially suppress the heating rates, so-called Floquet prethermalization.
In this poster, we describe experiments using a bosonic cloud of ultracold atoms in a driven optical lattice and identify a prethermal regime in the Bose-Hubbard model. By measuring the temperature increase of the cloud as the driving frequency is increased, an exponential-in-frequency reduction of the heating rate is revealed, which persists over more than 2 orders of magnitude. By tuning the lattice potentials, we explore a range of different interacting regimes and both one- and two-dimensional systems. In addition to the exponential suppression, the spectrum of the heating rates displays features characteristic of the phase diagram of the Bose-Hubbard model, whose understanding is additionally supported by 1D numerics. These results show experimental evidence of Floquet prethermalization and provide insight into the heating processes in driven bosonic systems.

Ergodicity breaking in tilted optical lattices

Authors: Sebastian Scherg [1,2,3], Thomas Kohlert [1,2,3], Bharath Hebbe Madhusudhana [1,2,3], Immanuel Bloch [1,2,3], and Monika Aidelsburger [1,3]
1 Ludwig-Maximilians-Universität, Schellingstr. 4, 80799 München, Germany
2 Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany
3 Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, 80799 München, Germany

Using ultracold fermionic 40K atoms in an optical lattice we study the dynamics of the one-dimensional Fermi-Hubbard model subject to an external linear potential (“tilt”), which has recently attracted considerable theoretical and experimental interest in the context of ergodicity-breaking and constrained dynamics. Starting from a charge-density wave initial state (quarter filling) we measure the spin-resolved time evolution of the occupation imbalance between even and odd lattice sites as a local probe of localization. We identify two fundamentally different regimes: At short times we measure parity-projected real-space Bloch oscillations which, depending on the strength of the tilt, exhibit interaction induced damping and frequency modulation. At long times the dynamics reveal a robust steady state imbalance up to about 700 tunneling times, whose value depends on the interaction strength. We compare our experimental results to numerical calculations employing tDMRG on short time scales and exact diagonalization on long timescales and find excellent agreement throughout. Finally, we give an interpretation of our results in the context of Hilbert space fragmentation.

Probing eigenstate thermalization with the emergence of fluctuation-dissipation relations in quantum simulators

Authors: Alexander Schuckert and Michael Knap
Technical University of Munich

The eigenstate thermalization hypothesis (ETH) offers a universal mechanism for the approach to equilibrium of closed quantum systems. So far, however, only the diagonal part of ETH has been probed in experiment, leaving many of its assumption untested while often needing substantial numerical input to conclude thermalization in experiment. Here, we propose a theory-independent route to probing the entirety of ETH in quantum simulators by observing the emergence of fluctuation-dissipation relations. We present protocols to independently measure fluctuations and dissipations as well as higher order time ordered correlation functions. We show that the emergence of FDRs from a nonequilibrium initial state can be observed in a 2D Bose Hubbard model in superconducting qubits or quantum gas microscopes. In the long range transverse field Ising model (LTFI) implemented by trapped ion experiments, we show that FDRs indicate prethermalization to an Hamiltonian with an approximately conserved quantity and thermalization to the generalized Gibbs ensemble in an integrable region of the LTFI. Confined excitations in the LTFI lead to non-thermal features including a violation of the FDR up to long times, offering a route to identify non-thermal eigenstates in experiment. Our work presents a theory-independent way to characterize thermalization in quantum simulators and paves the way to quantum simulate condensed matter pump-probe experiments."

Quantum gas microscopy of Rydberg macrodimers

Authors: K. Srakaew(1) S. Hollerith(1), J. Rui(1), J Zeiher(1,2), A. Rubio-Abadal(1), D. Wei(1), S. Evered(1,3), I. Bloch(1,4) & C. Gross(1,5)
1.Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany
2.Department of Physics, University of California, CA, 94720-7300, Berkeley, USA
3.Department of Physics, Stanford University, CA 94305-4060, USA
4.Ludwig-Maximilians-Universität, Fakultät für Physik, Schellingstraße 4, 80799 München, Germany
5.Physikalisches Institut, Eberhard Karls Universität Tübingen, 72076 T übingen, Germany

Rydberg macrodimers - molecules consisting of two bound highly excited Rydberg atoms - provide huge bond lengths even resolvable with optical wavelengths. Here we report on a microscopic study of macrodimers with different molecular symmetries in a gas of ultracold atoms in an optical lattice. The size of about 0.7 micrometers matches the diagonal distance of two atoms in the lattice. The geometry of the two-dimensional lattice initially unity filled with ground state atoms allows to control the relative orientation of the molecular axis to an ambient magnetic field and the polarization of the photoassociation light. Using our spatially resolved detection, we observe the associated molecules by correlated atom loss and find characteristic state-dependent features in the photoassociation mechanism.
Furthermore, we present how the molecular excitation rate can be significantly enhanced by the use of two color photoassociation. In a Ramsey sequence, we show that off-resonant coupling to molecular resonances can be used to engineer interaction potentials peaking at the molecular bond length for ground state atoms. Finally, we present the observation of a macrodimer-macrodimer blockade by evaluating our images with respect to g(4) correlators after a two microseconds excitation pulse at maximum coupling rate. Our results highlights the potential of quantum gas microscopy for molecular physics and show how macrodimers could be used to study many body physics.

Matter-Wave Based Studies of Waveguide-QED

Authors: Michael Stewart, Joonhyuk Kwon, Alfonso Lanuza, Youngshin Kim, and Dominik Schneble
Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA

Our group has recently implemented a novel platform for the study of waveguide-QED phenomena in the context of atomic matter waves [1,2]. The platform consists of an array of microscopic atom traps in an optical lattice that, driven by tunable microwave radiation, emit single atoms, rather than single photons, into the surrounding vacuum. This provides a matter-wave analog of photon emission in a photonic-bandgap material. Here, we introduce our platform and its features in detail, and we describe our recent and ongoing work, including emission into a structured vacuum [3] and the formation of bound-state polaritons and their effects on motional dynamics in the matter-wave quantum emitter array. Our system provides a flexible experimental platform for quantum simulations of exotic radiative effects for strong vacuum coupling and for studies of dissipative many-body physics with ultracold atoms.
[1] L. Krinner et al., Nature 559, 589 (2018);
[2] M. Stewart et al., PRA 95, 013626 (2017)
[3] M. Stewart et al., arXiv:2003.02816 (2020)
Work supported by NSF PHY-1607633/1912546 and the SUNY Ctr for QIS on LI.

Toward suppressing photo-induced losses of 23Na40K in an optical box trap

Authors: Renhao Tao
1. Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, 85748 Garching, Germany
2. Ludwig-Maximilians-Universität, Schellingstraße 4, 80799 München, Germany

It’s been postulated that when two bialkali molecules collide, they form “sticky” collisional complexes, the laser excitation of which leads to effectively universal two-body losses widely observed in ultracold-molecule experiments. Recent experiments on KRb and RbCs have confirmed this loss mechanism. We plan to use a repulsive optical box trap to study this complex losses in NaK. Our narrow-linewidth trapping transition enable us to build a dark trap with small single-photon scattering but large polarizability. Simulation using rate equations have shown that molecules lifetime in this box trap can be extended to as large as 15 times the value one gets with universal two-body loss, with gravity fully compensated.

Realization of an anomalous Floquet topological system with ultracold atoms

Authors: Karen Wintersperger (1,2), Christoph Braun (1,2,3), F. Nur Ünal (4,5), André Eckardt (4,6), Marco Di Liberto (7), Nathan Goldman (7), Immanuel Bloch (1,2,3) and Monika Aidelsburger (1,2).
(1) Fakultät für Physik, Ludwig-Maximilians-Universität München, Munich, Germany.
(2) Munich Center for Quantum Science and Technology (MCQST), Munich, Germany.
(3) Max-Planck-Institut für Quantenoptik, Garching, Germany.
(4) Max-Planck-Institut für Physik komplexer Systeme, Dresden, Germany.
(5) T.C.M. Group, Cavendish Laboratory, Cambridge, UK.
(6) Institut für Theoretische Physik, Technische Universität Berlin, Berlin, Germany.
(7) Center for Nonlinear Phenomena and Complex Systems, Université Libre de Bruxelles, Brussels, Belgium.

Ultracold atoms in periodically-driven optical lattices can be used to simulate systems with nontrivial topological properties. Due to the periodic driving, energy conservation is relaxed, which makes it possible to realize systems with properties that go beyond those of conventional static systems. For instance, chiral edge modes can exist even if the bulk is topologically trivial [1].
We study such anomalous Floquet phases experimentally using a BEC of K39 in an optical honeycomb lattice with periodically modulated tunnel couplings. By monitoring the closing and reopening of energy gaps in the band structure we are able to track the transitions between different Floquet phases. Moreover, we probe the topological properties of the bulk by measuring the Hall deflection induced by local changes in the Berry curvature. Combining these measurements enables us to extract the topological invariants of the bulk bands and the energy gaps, which are both required to accurately classify the topological phases of Floquet systems [2, 3, 4].
[1] T. Kitagawa et al., Phys. Rev. B 82, 235114 (2010)
[2] M. Rudner et al., PRX 3, 031005 (2013)
[3] N. Ünal et al., PRL 122, 253601 (2019)
[4] K. Wintersperger et al., arXiv:2002.09840 (2020), Nature Phys. (in press)"

Many Body Localization and beyond in experimentally verifiable scenarios

Authors: Ruixiao Yao (Peking University), Titas Chanda, Jakub Zakrzewski

Motivated by recent experiments on non-ergodic phenomenon in quasi-one-dimensional optical lattice, we analyse theoretically properties of the Many-body Localization driven by disorder and local curvature. For disorder-driven localization transition, we studied the crossover between localized and delocalized regimes. We defined new observables, the mean speed of transport and edge fluctuations at long times. They give direct probe for mobility edge in energy spectrum. For MBL by local curvature, we firstly studied stark localization and revealed its multifractal character as in disorder-driven case. Then we proposed and analyzed a scenario of coexistence of extended and localized phases by simply harmonic envelope with no disorder. Finally we show it may also lead to spectacular trapping of particles in a reversed harmonic field which naively might be considered as an unstable configuration.

### C: Quantum Computing Posters

Overview of posters covering Quantum Computing projects.
You can find them in the room with the same name.

Quantum Fourier Transform in Oscillating Modes

Authors: Q. Chen, F. Deppe, Y. Nojiri, S. Pogorzalek, M. Renger, M. Partanen, K. G. Fedorov, A. Marx, R. Gross (WMI, TUM, MCQST, Germany);
R. Wu, L. Sun, Y. Liu (Tsinghua University, China)

Quantum Fourier transform (QFT) is a key ingredient of many quantum algorithms. In typical applications such as phase estimation, a considerable number of ancilla qubits and gates are used to form a Hilbert space large enough for high-precision results. Qubit recycling reduces the number of ancilla qubits to one, but it is only applicable to semi-classical QFT and requires repeated measurements and feedforward within the coherence time of the qubits. In this work, we explore a novel approach based on harmonic resonators that forms a high-dimensional Hilbert space for the realization of fully-quantum QFT with current superconducting quantum circuits. By employing the perfect state-transfer method, we propose a method to transfer an unknown multi-qubit state to a single resonator, and generate the QFT state in the second oscillator through cross-Kerr interaction and a special projective measurement. This study paves the way for implementing various QFT related quantum algorithms.

Towards the certification of random numbers from ground state preparation of 2D local Hamiltonians

Authors: Esther Cruz, Flavio Baccari, Jordi Tura, Norbert Schuch and Ignacio Cirac
Max Planck Institute for Quantum Optics; MCQST

We study protocols for the generation of certifiable randomness in NISQ [1] (Noisy Intermediate-Scale Quantum) devices. These protocols work as a game in which a quantum, untrusted, prover and a classical, limited, verifier communicate over a classical channel. The verifier sends challenges that the prover must answer correctly in order to convince the verifier that it is following an honest behavior. The quantum nature of the prover allows for generation of intrinsic randomness that is later extracted or amplified.
Protocols of this type have already been proposed [2], [3] but either they require fault-tolerance or the classical verification steps take exponential time. In this work we consider a way to overcome these limitations by making use of adiabatic quantum computation, an algorithm which NISQ devices have already succeeded to implement. Roughly speaking, the verifier generates a 2D local, gapped Hamiltonian. The quantum computer is asked to approximate its ground state adiabatically and to generate samples by measuring this state in random basis that the verifier chooses. These samples serve as a proof of the prover’s honest or dishonest behaviour, since the verifier can check whether they come from the correct distribution.

Generation of motion non-Gaussian entanglement via mediating superconducting qubit

Authors: Najmeh Es’haqi-Sani (1,2), Mehdi Khazaei Nezhad (1), and Mehdi Abdi (3)
(1) Department of Physics, Ferdowsi University of Mashhad, Mashhad, PO Box 91775-1436, Iran
(2) International Centre for Theoretical Physics ICTP, Strada Costiera, 11, I-34151 Trieste, Italy
(3) Department of Physics, Isfahan University of Technology, Isfahan 84156-83111, Iran

Generally, setups in which motion is coupled to a nonlinear quantum object, such as superconducting qubits, can open up the possibility of generating, manipulating, and storing non-Gaussian states in mechanical degrees of freedom. Here, we propose a scheme for entangling the motion of two massive objects in a hybrid electromechanical device and we show that one can overcome the difficulty of creating the non-Gaussian non-classical states in massive mechanical resonators by interposing the superconducting qubits and appropriately driving the qubit. We analytically elaborate on the possibility of creating such states and also numerically verify the performance of our scheme. The results of simulations demonstrate the possibility of generation of non-Gaussian entangled states whose lifetime is limited by coherence time of the qubit. The entanglement is attainable in a wide range of parameters with appropriate control of the qubit. The effect of imperfections, such as asymmetries in the coupling rates as well as mechanical thermal noise are studied and shown how they affect the amount and lifetime of the entangled state. Due to the nonlinear nature of the qubit, the initial Gaussian state of mechanical resonators evolves into a quasi-stationary non-Gaussian state, which is essential for universal quantum information processing in continuous variable systems. This work, therefore, provides the first step towards a universal continuous variable quantum network.

[1] N. Es’haqi-Sani, M. Khazaei Nezhad, and M. Abdi, Phys, Rev. A, 100,023845 (2019).
[2] M, Abdi, M. Pernpeintner, R. Gross, H. Huebl, and M. J. Hartmann, Phys. Rev. Lett. 114, 173602 (2015).
[3] M. J. Woolley et al, Phys. Rev. A, 89, 063805 (2014).
[4] J. Li et al. New J. Phys. 17, 103037 (2015).

Long-Range Interacting Quantum Systems via Rydberg Dressing in Microtraps

Authors: Lorenzo Festa (MPQ)

Neutral atoms in microtrap arrays brought to interaction by Rydberg coupling offer a novel platform to study quantum magnetism. We have constructed a new experiment with potassium atoms, which aims to induce the magnetic interactions via near-resonant Rydberg coupling, so called Rydberg dressing. Here we report on coherent Rydberg coupling in a two dimensional array of single atoms. We observe fast coherent Rabi oscillations of single atoms as well as of small Rydberg superatoms. Finally we discuss first experiments towards Rydberg dressing induced interactions among atomic ground states.

Capacity regions of compound quantum broadcast channels with confidential messages

Authors: Holger Boche, Gisbert Janßen, Sajad Saeedinaeeni (TUM)

We derive universal codes for transmission of broadcast and confidential messages over classical-quantum-quantum and fully quantum channels. These codes are robust to channel uncertainties considered in the compound model. To construct these codes we generalize random codes for transmission of public messages, to derive a universal superposition coding for the compound quantum broadcast channel. As an application, we give a multi-letter characterization of regions corresponding to the capacity of the compound quantum broadcast channel for transmitting broadcast and confidential messages simultaneously. This is done for two types of broadcast messages, one called public and the other common.

Control of Valley Splitting in Ideal and Disordered Si Quantum Dot Spin Qubits

Authors: Amin Hosseinkhani, Guido Burkard
University of Konstanz

In silicon spin qubits, the valley degree of freedom couples to the spin and can limit the qubit lifetime. In this work, we first study in detail how the valley splitting depends on the magnetic and electric fields as well as the quantum dot geometry for both ideal and disordered Si/SiGe interfaces. We model a realistic electrostatically defined quantum dot and find the exact ground and excited states for the out-of-plane electron motion. For a quantum dot with an ideal interface, we show that the valley splitting is slightly increased by applying an in-plane magnetic field. Noticeably, our modeling makes it possible to analyze the effect of any arbitrary configuration of interface disorders. In agreement with previous studies, we show that steps at the Si/SiGe interface can significantly reduce the valley splitting. Interestingly, depending on where the interface steps are located, an in-plane magnetic field can increase or further suppress the valley splitting [1].
We then extend our theory to include valley-dependent envelope functions for a silicon quantum dot with a disordered interface. This enables us to develop a parameter-free theory for the spin relaxation induced by the valley-coupling. We particularly investigate the contribution of intra-valley transitions in the total relaxation rate.

[1] Amin Hosseinkhani and Guido Burkard, Electromagnetic control of valley splitting in ideal and disordered Si quantum dots, in preparation.

Experimental Test of Quantum Advantage 3-player Noisy Dilemma Game

Authors: Pranav Kairon (Delhi Technological University), Kishore Thapliyal, R. Srikanth, Anirban Pathak

Quantum games have been shown to outperform classical games in general. Here we study the extent of validity of that argument under input noise. Specifically by performing an experimental implementation of 3 person Dilemma Game on IBM Q Experience where tainted qubits are provided by the referee. Also we check how payoff depends on varying payoff parameters. It is further noted that quantum advantage is lost if input corruption is greater than 50%.

Quantum Memory with optimal control

Authors: Julia Lamprich1,2, Stephan Trattnig1,2, Michael Renger1,2,3, Qiming Chen1,2,3 , Yuki Nojiri1,2,3, Stefan Pogorzalek1,2, Matti Partanen1,3, Kirill G. Fedorov1,2,3, Frank Deppe1,3, Achim Marx1,2,3,and Rudolf Gross1,2,3
1Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching
2Physik-Department, TU München, 85748 Garching
3Munich Center for Quantum Science and Technology (MCQST), 80799 Munich

Quantum memories are of high relevance in the context of quantum computing and quantum communication. In view of the tremendous publicly-funded and commercial efforts to build scalable architectures based on superconducting quantum circuits, 3D cavities are promising candidates for a quantum memory. Recently, a compact layout exploiting the multimode structure of a rectangular 3D cavity has been demonstrated [1]. As an alternative to an improved operation mode of this device with optimal control strategies [2], we discuss an optimization of the cavity geometry here. Our results are promising with respect to key properties such as storage time and scalability.
We acknowledge support by the Germany’s Excellence Strategy EXC-2111-390814868, Elite Network of Bavaria through the program ExQM, and the European Union via the Quantum Flagship project QMiCS (Grant No. 820505).
[1] E. Xie et al., Appl. Phys. Lett. 112, 202601 (2018).
[2] Shai Machnes et al., Phys. Rev. Lett. 120, 150401 (2018)

Towards a Quantum game of Life

Authors: Marian Lingsch
Ludwig-Maximilians-Universität München, Lehrstuhl für Mobile und Verteilte Systeme; Thomas Gabor, Ludwig-Maximilians-Universität München, Lehrstuhl für Mobile und Verteilte Systeme

We introduce an approach to a quantum game of life using quantum annealing and show some ways of simulating and finding periodic structures in Conway's Game of Life.

A Quantum Interior-Point Predictor-Corrector Algorithm for Linear Programming

Authors: Pablo Antonio Moreno Casares & Miguel Ángel Martín-Delgado

We introduce a new quantum optimization algorithm for dense Linear Programming problems, which can be seen as the quantization of the Interior Point Predictor-Corrector algorithm using a Quantum Linear System Algorithm.
The (worst case) work complexity of our method is, up to polylogarithmic factors, $O(L\sqrt{n}(n+m)\overline{||M||_F}\bar{\kappa}\epsilon^{-2})$ for $n$ the number of variables in the cost function, $m$ the number of constraints, $\epsilon^{-1}$ the target precision, $L$ the bit length of the input data, $\overline{||M||_F}$ an upper bound to the Frobenius norm of the linear systems of equations that appear, $||M||_F$, and $\bar{\kappa}$ an upper bound to the condition number $\kappa$ of those systems of equations.
This represents a quantum speed-up in the number $n$ of variables in the cost function with respect to the comparable classical Interior Point algorithms when the initial matrix of the problem $A$ is dense: if we substitute the quantum part of the algorithm by classical algorithms such as Conjugate Gradient Descent, that would mean the whole algorithm has complexity $O(L\sqrt{n}(n+m)^2\bar{\kappa} \log(\epsilon^{-1}))$, or with exact methods, at least $O(L\sqrt{n}(n+m)^{2.373})$.
Also, in contrast with any Quantum Linear System Algorithm, the algorithm described in this article outputs a classical description of the solution vector, and the value of the optimal solution.

Cavity Control over Heavy-Hole Spin Qubits in Inversion-Symmetric Crystals

Authors: Philipp M. Mutter and Guido Burkard
Department of Physics, University of Konstanz, Germany

The pseudospin of heavy-holes (HHs) confined in a semiconductor quantum dot (QD) represents a promising candidate for a fast and robust qubit. While hole spin manipulation by a classical electric field utilizing the Dresselhaus spin-orbit interaction (SOI) has been demonstrated, our work explores cavity-based qubit manipulation and coupling schemes for inversion-symmetric crystals forming a planar HH QD. Choosing the exemplary material Germanium (Ge), we derive an effective cavity-mediated ground state spin coupling that harnesses the cubic Rashba SOI. In addition, we propose an optimal set of parameters which allows for Rabi frequencies in the MHz range.

Realizing a Deterministic Source of Multipartite-Entangled Photonic Qubits

Authors: Kevin Reuer[1], Jean-Claude Besse[1], Michele C. Collodo[1], Arne Wulff[1], Lucien Wernli[1], Adrian Copetudo[1], Daniel Malz[2,3], Paul Magnard[1], Abdulkadir Akin[1], Mihai Gabureac[1], Graham J. Norris[1], J. Ignacio Cirac[2,3], Andreas Wallraff[1] and Christopher Eichler[1]
[1] Department of Physics, ETH Zurich, CH-8093 Zurich, Switzerland
[2] Max-Planck-Institute of Quantum Optics, Hans-Kopfermann-Str. 1, 85748 Garching, Germany
[3] Munich Center for Quantum Science and Technology, Schellingstr. 4, 80799 München, Germany

Sources of entangled electromagnetic radiation offer unique opportunities for the study of quantum many-body physics in a controlled experimental setting and are a cornerstone in quantum information processing. Previous experimental sources of multi-mode entangled states of radiation are either based on probabilistic processes or restricted to generate specific types of states with a moderate entanglement length. Here, we demonstrate the fully deterministic generation of purely photonic entangled states such as the cluster, GHZ, and W state by sequentially emitting microwave photons from a controlled auxiliary transmon system into a waveguide. We tomographically reconstruct the entire quantum many-body state for up to N=4 photonic modes and infer the quantum state for even larger N from process tomography. We estimate that localizable entanglement persists over a distance of approximately ten photonic qubits, outperforming any previous deterministic scheme.

Optimizing State Tomography Schemes

Authors: Violeta Nikolaeva Ivanova-Rohling (Department of Physics, University of Konstanz; Zukunftskolleg, University of Konstanz; Department of Mathematical Foundations of Computer Sciences, Institute of Mathematics and Informatics, Bulgarian Academy of Sciences) and Niklas Rohling (Department of Physics, University of Konstanz)"

Optimizing State Tomography Schemes
Quantum state tomography (QST) is the gold standard for verifying the functionality of a quantum computer and for eventually debugging it. We search the minimum set (quorum) of measurement operators that provides the most time efficient QST scheme. For non-degenerate observables such an optimal set is formed by measurement operators whose eigenbases are a complete set of mutually unbiased bases (MUBs) [1]. However, each implementation of a quantum computer has specifications, e.g. non-degenerate measurements might not be available while QST is very time-consuming. This motivated our research into customized QST aiming at the most efficient access to the stored quantum state for the given specifications. In [2] we considered the maximal-degeneracy case, where the measurement operators are rank-1 projectors on pure quantum states. For low dimensional Hilbert spaces we used standard numerical approaches to improve the solution given by individually projecting on MUB states.
We proceed with other specifications: if one out of n qubit is measured, MUBs allow the construction of the optimal quorum [3]. We investigate a qubit-qutrit system where only the qubit is measured. This case is not covered by the theoretical results of [3]. The search for the optimal quorum yields an optimization problem with a high number of parameters. Exploratory analysis shows that simple numerical methods do not perform well in this situation. In recent times, machine learning has been widely used to solve problems in physics, including quantum state tomography. We intend to use appropriate state-of-the-art machine learning approaches to improve the results obtained by the simple numerical approaches, such as intelligent parallel search to improve the search space exploration and deep neural networks.

[1] Wootters, Fields, Ann. Phys. 191, 363 (1989)
[2] Ivanova-Rohling, Rohling, PRA 100, 032332 (2019)
[3] Bodmann, Haas, Proc. Amer. Mat

Fast adiabatic ground state preparation with few measurements

Authors: Benjamin Schiffer (1,2), Jordi Tura (1,2), J. Ignacio Cirac (1,2)
(1) Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, D-85748 Garching, Germany
(2) Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, D-80799 München, Germany

Preparing the ground state of a Hamiltonian is a problem of great significance in physics with deep implications in combinatorial optimization. While the quantum adiabatic algorithm (QAA) is known to return the ground state for sufficient long preparation times, the decoherence times of noisy-intermediate scale quantum (NISQ) devices renders this approach infeasible. In recent years, variational approaches such as the quantum approximate optimization algorithm (QAOA) attracted strong research interest as they rely on repeated measurements instead of long preparation times. However, in order to optimize a many parameter landscape of the cost function, this can easily require billions of measurements for a moderate number of qubits preventing any quantum advantage.
In our work, we seek to combine the strengths of the adiabatic and the variational approach for fast and high fidelity ground state preparation.

Entanglement entropy and mutual information of a Matrix Product State inspired 4-qubit GHZ state generated on an IBM quantum computer

Authors: Hisham Ashraf Amer
Undergraduate Student at the University of Science and Technology at Zewail City

It is well established that the use of Matrix Product States (MPS’s), given their bond dimensions (χ) are bounded, reduces the computational cost of running numerical operations on otherwise exponentially large multi-body quantum state vectors. Recently, it was observed that there is a one-to-one mapping between certain tensor networks and quantum circuits. This mapping perceives quantum circuits basically as quantum computer networks which manipulate multi-qubit product states using local unitary gates. These local unitaries translate to Matrix Product Operators (MPO’s), transforming the initial product states into MPS’s. Accordingly, the same computational utility demonstrated with MPS’s extends to quantum circuit design, and since quantum computers also have memory requirements and computational runtimes, the cost of operation becomes a determining factor in the success of a circuit. Here we demonstrated this mapping, by running a trivial MPS inspired quantum circuit on the “ibmq_london” quantum computer to reproduce the, MPS representable, GHZ state. 8192 runs were used to reproduce a 4-qubit GHZ state distribution. Density matrices were calculated for the theorized and experimental cases. To compare the performance of the quantum circuit to the ideal case, correlation quantizers were calculated, for which a bipartition was set, and their reduced density matrices derived. For entanglement entropy between the first qubit and the others, we used the von Neumann entropy for the ideal pure case. For the experimental results, we used a more general quantifier of correlations, the mutual information, given there was an unavoidable degree of decoherence and in turn some mixedness, with a purity at ∼0.268641. The mutual information of the quantum computer results was 1.335, which is less than the ideal case of 2. In conclusion, the trivial MPS inspired circuit did reproduce the 4- qubit GHZ state within some decoherence with a 33% reduction in total corre.

### D: Quantum Communication Posters

Overview of posters covering Quantum Communication projects.
You can find them in the room with the same name.

Optically Addressable and Electrically Tunable S-T Hole Spin Qubits in QD-Molecules

Authors: Frederik Bopp (1),Jonathan Rojas (1), Arash Ahmadi (1), Christian Dangel (1), Arne Ludwig (2), Dirk Reuter (3), Stefan Reitzenstein (4), Arthur Zrenner (3), Kai Müller (5,6), Jon Finley (1,6)
(1) Walter Schottky Institut and Physik Department, TU München, Am Coulombwall 4, 85748 Garching, Germany
(2) Ruhr-Universität Bochum, Universitätsstraße 150, 44801 Bochum, Germany
(4) Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
(5) Walter Schottky Institut and Department of Electrical and Computer Engineering, TU München, Am Coulombwall 4, 85748 Garching, Germany
(6) Munich Center of Quantum Science and Technology (MCQST), Schellingstr. 4, 80799 Munich, Germany

Coherence, ease of control and scalability lie at the heart of all hardware for distributed quantum information technologies. This is particularly true for spin-photon interfaces based on III-V semiconductor quantum dots (QDs) since they combine properties such as strong interaction with light, robust spin-photon selection rules, nearly pure transform limited emission into the zero-phonon line and ease of integration into opto-electronic devices. However, the comparably short spin coherence times of single electrons and holes in QDs (T2* ~ 10-100ns) [1], could limit their applicability for distributed quantum technologies.
Unlike single electron and hole spins which are sensitive to the fluctuating nuclear spin environment in III-V materials, singlet-triplet (S-T) qubits in pairs of coupled dots – quantum dot molecules (QDMs) – have extended spin coherence times when operated at a sweet spot for which the S-T splitting is independent of electric and magnetic field fluctuations. Such optically addressable S-T spin qubits promise to extend the obtainable T2* times by several orders of magnitude whilst retaining the advantages outlined above. Previously, experiments using Schottky gated samples have provided important insights into orbital structure, exchange couplings, phonon couplings and spin-dephasing [2, 3].
We present an approach where the charge status of the QDM is controlled optically, whilst the coupling between the two spins can be tuned to the sweet spot electrically. To achieve this, an AlGaAs tunneling barrier is inserted immediately adjacent to the QDM layer, allowing for sequential optical control of the charge status via tunneling ionization [2] while the tunnel coupling between the two dots can be electrically controlled via a gate voltage. We will present first studies of the dynamics of the optical charging of QD molecules as well as first results on the electric field dependent coupling control.

[1] L. Huthmacher, Phys. Rev. B 97, 24141

Tunable atom-light interaction using subwavelength arrays

Authors: David Castells-Graells [1,2,3], Daniel Malz [1,3], Cosimo Rusconi [1,3], J. Ignacio Cirac [1,3]
1. Max-Planck-Institut für Quantenoptik, D-85748 Garching, Germany
2. Technische Universität München, 85748 Garching, Germany
3. Munich Center for Quantum Science and Technology, München, Germany

A central challenge in quantum optics is the realization of controlled efficient interactions between atoms and photons. One promising approach consists on coupling one or more atoms to an optical medium such as photonic crystal waveguides [1]. The use of these structures not only improves the free-space approaches, but their tailored dispersion relations offer prospects of new paradigms for atom-light interactions. Imperfections and optical losses inside the medium can, however, hinder the observation and use of some its features. In this project we investigate as an alternative subwavelength arrays of atoms, which are known to contain collective states with suppressed - compared to single emitters - emission to free space [2]. These states can be understood as guided modes of the atomic chain in the 1D case. To describe the dynamics of the system, we use a quantization scheme based on the classical electromagnetic Green's tensor, and the master equation that results of tracing out the electromagnetic modes. We, then, engineer the “impurity” atoms that interact with the subwavelength array to achieve an efficient coupling to the subradiant states only. In the Markovian regime, we obtain effective expressions for the dynamics of the impurity atoms, which show many of the interesting features predicted with photonic crystal waveguides.

[1] D. E. Chang, et al., Rev. Mod. Phys. 90.3 (2018): 031002
[2] A. Asenjo-Garcia, et al., Phys Rev. X 7.3 (2017): 031024

QuNetSim: A Software Framework for Quantum Networks

Authors: Stephen DiAdamo (TUM), Janis Nötzel, Benjamin Zanger, Mehmet Mert Beşe

As quantum internet technologies develop, the need for simulation software and education for quantum internet rises. QuNetSim aims to fill this need. QuNetSim is a Python software framework that can be used to simulate quantum networks up to the network layer. The goal of QuNetSim is to make it easier to investigate and test quantum networking protocols over various quantum network configurations and parameters. The framework incorporates many known quantum network protocols so that users can quickly build simulations and beginners can easily learn to implement their own quantum networking protocols.

Quantum Key Distribution with displaced squeezed microwave states

Authors: Florian Fesquet (1,2), Kirill G. Fedorov (1,2) , Stefan Pogorzalek (1,2), Michael Renger (1,2), Qi-Ming-Chen (1,2), Yuki Noriji (1,2) , Matti Partanen (1), Achim Marx (1), Frank Deppe (1,2,3), and Rudolf Gross (1,2,3)
1Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, Germany
2Physik-Department, TU München, 85748 Garching, Germany
3Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, 80799 Munich, Germany"

Quantum key distribution (QKD) is a technique to secretly communicate a key string between two parties. Unconditional security can be achieved by exploiting the laws of quantum physics. We investigate a prepareand-measure continuous-variable QKD protocol based on squeezed microwave states to communicate a Gaussian-modulated key. We theoretically investigate the secrecy and secret key rate of the protocol with an eavesdropper. We show that depending on the additional noise and losses induced by the eavesdropper, the protocol stays secure at the cost of an increased squeezing level. Additionally, we discuss experimental implementation of the protocol in the microwave regime.

QKD on Mobile and Space Platforms

Authors: Peter Freiwang², Lukas Knips¹², Jannik Luhn¹, Leonhard Mayr¹, Gwenaelle Vest¹, Tobias Vogl¹, Markus Rau¹, Wenjamin Rosenfeld¹, QUBE-consortium³⁴⁵⁶ and Harald Weinfurter¹²
1) Max Planck Institute of Quantum Optics (MPQ), Garching, Germany
2) Ludwig-Maximilian-University (LMU), Munich, Germany
3) Center for Telematics (ZfT), Würzburg, Germany
4) German Aerospace Center (DLR) IKN, Oberpfaffenhofen, Germany
5) Max Planck Institute for the Science of Light (MPL), Erlangen, Germany
6) OHB System AG, Oberpfaffenhofen, Germany

Quantum Key Distribution (QKD) over long distances becomes possible using optical links between satellites and ground stations on earth enabling a secure key exchange on a global scale. Smaller and more robust optics enabled QKD over a hand-held freespace link and is well suited as compact nano-satellite QKD-payload. A new level of integration was achieved by using micro optical components and a waveguide circuit resulting in a robust and stable optical unit with a size, which is only a little bigger than a match. This optical part, hermetically packaged, together with the control electronics will be combined on a 9 x 9 cm² FPGA driven printed circuit board to fit into the satellite. The satellite to be employed is a so called cube-satellite as small as 10 x 10 x 30 cm³. Because of their size and weight, cube-satellites represent an economical platform for testing of technologies in space in general. Thanks to the relatively low costs, they have the potential to form flotillas to become the backbone for a global QKD network.

Coupling erbium ions to silicon nanophotonic structures

Authors: L. Weiss, A. Gritsch, F. Burger, J. Frueh, L. Zarraoa Sardon, A. Reiserer
All authors: Quantum Networks Group, MPI of Quantum Optics"

"Erbium dopants are promising candidates for the implementation of large-scale quantum networks since they can combine second-long ground state coherence with coherent optical transitions at telecommunication wavelengths. Unfortunately, the long lifetime of the excited state (several ms) makes it difficult to spectrally resolve and control individual ions in order to harness them for quantum networks.
To overcome this challenge, we use silicon nanophotonic waveguides and resonators implanted with erbium. At optimized implantation parameters, we find narrow (2-3 GHz) fluorescence lines that originate from erbium ensembles which are well integrated into the silicon lattice. We measured the optical lifetime, coherence and magnetic field splitting of these lines. We then fabricated photonic crystal cavities that should allow us to reduce the lifetime by several orders of magnitude and thus to optically observe and control individual dopants.
This would open unique prospects for the realization of entanglement between spins over distances exceeding 100 km using a novel frequency-multiplexed quantum network architecture."

The Origin of Antibunching in Resonance Fluorescence

Authors: Lukas Hanschke(1,2), Lucas Schweickert(3), Juan Camilo López Carreño(4), Eva Schöll(3), Katharina D. Zeuner(3), Thomas Lettner(3), Eduardo Zubizarreta Casalengua(4), Marcus Reindl(5), Saimon Filipe Covre da Silva(5), Rinaldo Trotta(6), Jonathan J. Finley(1,2), Armando Rastelli(5), Elene del Valle(4,7), Fabrice P. Laussy(4,8), Val Zwiller(3),Klaus D. Jöns(3), and Kai Müller(1,2)
(1)Walter Schottky Institut, Technische Universität München, 85748 Garching, Germany
(2)Munich Center for Quantum Science and Technology (MCQST), 80799 Munich, Germany
(3)Department of Applied Physics, Royal Institute of Technology, Albanova University Centre, Roslagstullbacken 21, 106 91 Stockholm, Sweden
(4)Faculty of Science and Engineering, University of Wolverhampton, Wulfruna St, Wolverhampton WV1 1LY, United Kingdom
(5)Institute of Semiconductor and Solid State Physics, Johannes Kepler University Linz, 4040, Austria
(6)Dipartimento di Fisica, Sapienza Università di Roma, Piazzale A. Moro 1, I-00185 Roma, Italy
(8)Russian Quantum Center, Novaya 100, 143025 Skolkovo, Moscow Region, Russia

Epitaxial quantum dots have emerged as one of the best single-photon sources, not only for applications in photonic quantum technologies but also for testing fundamental properties of quantum optics. One intriguing observation in this area is the scattering of photons with subnatural linewidth from a two-level system under resonant continuous wave excitation. In particular, an open question is whether these subnatural linewidth photons exhibit simultaneously antibunching as an evidence of single-photon emission. Here, we demonstrate that this simultaneous observation of subnatural linewidth and antibunching is not possible with simple resonant excitation. First, we independently confirm single-photon character and subnatural linewidth by demonstrating antibunching in a Hanbury Brown and Twiss type setup and using high-resolution spectroscopy, respectively. However, when filtering the coherently scattered photons with filter bandwidths on the order of the homogeneous linewidth of the excited state of the two-level system, the antibunching dip vanishes in the correlation measurement. Our experimental work is consistent with recent theoretical findings, that explain antibunching from photon-interferences between the coherent scattering and a weak incoherent signal in a skewed squeezed state.

Single-photon distillation via a photonic parity measurement using cavity QED

Authors: Lukas Hartung (MPQ), Severin Daiss, Stephan Welte, Bastian Hacker, Lin Li, Emanuele Distante, Gerhard Rempe

Pure single photons are a key to many future quantum-communication technologies and especially to quantum networks. Despite their importance, they are difficult to produce with the required quality. To date, various light sources have been able to strongly suppress contributions of two and more photons. However, the elimination of the electromagnetic vacuum remains a challenging task, albeit a lot of effort has been put into the removal of limiting perturbations. Here, we follow a different approach and distill single photons out of incoming light pulses. We use the reflection of the light from an atom-cavity system and employ a suitable measurement of the atom to herald the success of the operation. We create single-photons with custom-made temporal shapes and a fidelity of 66% out of initial vacuum-dominated coherent pulses. Our scheme could further be used to boost the fidelity of single-photon sources without any fundamental limit. As it only requires an emitter coupled to a resonator, our protocol can be a valuable tool to distill single photons in a wide range of different experimental platforms.

Toward a Photon-Photon Quantum Gate Using Rydberg- Interactions in an Optical Resonator

Authors: Ya-Fen Hsiao, Thomas Stolz, Bianca Röhr, Lukas Husel, Gerhard Rempe, and Stephan Dürr
Max- Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, 85748 Garching, Germany

We recently realized a photon-photon CNOT gate based on free-space Rydberg EIT in an ultracold atomic ensemble [1]. The performance in terms of efficiency and postselected fidelity is limited by dephasing resulting from the interaction between the Rydberg electron and surrounding ground-state atoms. The dephasing rate can be much reduced by working at lower atomic density [2]. To keep the gate operational in this regime, we plan to place the ensemble inside a moderate-finesse optical resonator [3,4]. We report on experimental progress toward this goal.

[1] D. Tiarks et al. Nat. Phys. 15, 124 (2019).
[2] S. Schmidt- Eberle et al. PRA101, 013421(2020).
[3] Y. M. Hao et al. Sci. Rep. 5, 10005 (2015).
[4] S. Das et al. PRA 93, 040303 (2016).

Beyond the Standard Quantum Limit of Parametric Amplification

Authors: Michael Renger (1,2), Kirill G. Fedorov (1,2) , Stefan Pogorzalek (1,2), Qi-Ming Chen (1,2), Yuki Nojiri (1,2), Matti Partanen (1), Achim Marx (1), Frank Deppe (1,2,3), and Rudolf Gross (1,2,3)
1 Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching
2 Physik-Department, TU München, 85748 Garching
3 Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, 80799 Munich, Germany

The low noise amplification of weak microwave signals is crucial for many protocols in quantum information processing. Quantum mechanics sets an ultimate lower limit of half a photon to the added input noise for the phase-preserving amplification of narrowband signals, also known as the standard quantum limit (SQL). This limit, however, can be violated by employing parametric amplification of broadband signals. We show that, in theory, an ideal quantum efficiency of 1 can be reached. Experimentally, we demonstrate quantum efficiency of 0.7 beyond the SQL of 0.5 by employing a flux-driven Josephson parametric amplifier and broadband thermal signals.

Hole spin echo envelope modulations revealing dynamics of a mesoscopic spin bath

Authors: F. Sbresny [2], T. Simmet [1], W. Rauhaus [2], M. Kremser [1], F. Li [3], N. Sinitsyn [3], Ł. Cywiński [4], K. Müller [2], J. J. Finley [1]
[1] Walter Schottky Institut, Physik Department and MCQST, Technische Universität München,
Am Coulombwall 4, 85748 Garching, Germany
[2] Walter Schottky Institut, Department of Electrical and Computer Engineering and MCQST, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany
[3] Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, 87545 USA
[4] Institute of Physics Polish Academy of Sciences, al. Lotników 32/46, PL-02-668 Warsaw, Poland

Self-assembled InGaAs quantum dots are amongst the most performant solid-state quantum emitters. At the same time, they can host electron or hole spin qubits which can be manipulated using ultrafast laser fields. The electron spin coherence time has been shown to be limited by strong contact hyperfine coupling to the noisy nuclear spin environment. A straightforward approach to mitigate the influence of nuclear spin noise involves encoding the qubit on the spin of a valence band hole which has much weaker, albeit anisotropic hyperfine coupling. Here, we probe the inhomogeneous dephasing time of a single hole spin qubit (T2*) using ultrafast pump-probe techniques and obtain T2*>200ns at zero B-field, a value two orders of magnitude longer than for electrons. Ramsey interference measurements performed for various transverse magnetic fields reveal hole spin coherence times in the nanosecond regime and a nonlinear decrease of the coherence time with increasing magnetic field. This observation is attributed to the influence of electrical noise on the spin via the electric field dependent hole g-factor.
Nuclear Zeeman coupling and quadrupolar coupling induce collective dynamics of the underlying nuclear spin bath which are expected to accelerate dephasing. In spin echo experiments, these dynamics lead to strong spin echo envelope modulation. A fast oscillation of the spin echo signal (4.8 MHz/T) is observed in our experiments and identified as being due to the effect of quadrupolar coupling on the longitudinal nuclear field component [1,2]. Uniquely to the hole spin, we observe an additional slow oscillation (0.8 MHz/T) that is identified as stemming from a beating of the Zeeman frequencies of the In and Ga nuclear species and can be used as a sensitive probe of the QD composition [3].

[1] A. Bechtold et al., Nature Physics 11, 1005–1008 (2015).
[2] R. Stockill et al., Nature Communications 7, 12745 (2016).
[3] H. Bluhm et al., Nature Physics 7, 109–113 (2010)

Towards Electrically Pumped Parametric Down-Conversion in Bragg-Reflection Waveguides

Authors: A. Schlager (UIBK), M. Götsch, H. Thiel, R. Chapman, S. Frick, H. Suchomel, M. Kamp, S. Höfling, C. Schneider and G. Weihs

Photon pairs emitted by Bragg-reflection waveguides (BRWs) via parametric down-conversion (PDC) can be utilized for the creation of highly entangled two-qubit states. In this work, we present our efforts towards an on-chip source of entangled photon pairs by integrating the pump laser into the waveguide structure.

Controlling single Erbium dopants in a Fabry-Perot resonator

Authors: Benjamin Merkel (Max-Planck-Institute of Quantum Optics), Alexander Ulanowski (Max-Planck-Institute of Quantum Optics), Andreas Reiserer (Max-Planck-Institute of Quantum Optics, Munich Center for Quantum Sciene and Technology)

Erbium dopants exhibit unique optical and spin coherence lifetimes and show great promise for long-distance quantum networks, as their emission lies in the minimal-loss window of optical fibers. To achieve an efficient spin-photon interface for single dopants we integrate thin host crystals into cryogenic Fabry-Perot resonators. With a Finesse of 110000 we can demonstrate up to 58(6)-fold Purcell enhancement of the emission rate, corresponding to a two-level cooperativity of 530(50). Our approach avoids interfaces in the proximity of the dopants and therefore preserves the optical coherence up to the lifetime limit. Using this system, we plan to resolve and control individual Erbium dopants with frequency-multiplexed spin-qubit readout, opening unique perspectives for the implementation of quantum repeater nodes.

A subradiant optical mirror formed by a 2D atomic array

Authors: David Wei (1,2), Jun Rui (1,2), Antonio Rubio-Abadal (1,2), Simon Hollerith (1,2), Johannes Zeiher (3), Dan M. Stamper-Kurn (3), Christian Gross (1,2,4), Immanuel Bloch (1,2,5)
(1) Max-Planck-Institut für QUantenoptik, 85748 Garching, Germany
(2) Munich Center for Quantum Science and Technology (MCQST), 80799 München, Germany
(3) Department of Physics, University of California, Berkeley, CA 94720, USA
(4) Physikalisches Institut, Eberhard Karls Universität Tübingen, 72076 Tübingen, Germany
(5) Fakultät für Physik, Ludwig-Maximilians-Universität, 80799 München, Germany

When quantum emitters are positionally structured on sub-wavelength scales, photon-mediated dipole-dipole interactions can strongly alter the spectral and directional radiative response. Tightly trapped atoms in optical lattices, only coupled to the electromagnetic vacuum, constitute ideal dipolar emitters to study such cooperative behaviour.
In our experiment, we probe the collective properties of a two-dimensional square array of atomic dipoles by performing spectroscopic absorption and reflection measurements. We directly observe considerably subradiant response and demonstrate that the array acts as a reflective mirror formed by a single mono-layer of a few hundred atoms. By varying the atom density within the array, we are able to control the influence of the dipolar interactions. By introducing positional disorder in the atomic ensemble, we analyze the role of the array structure. Its importance is emphasized by dynamically breaking and restoring the order using atomic Bloch oscillations to control the reflectivity of the atomic mirror.

Non-Classical States of Light from an Optical Cavity

Authors: Stephan Welte, Bastian Hacker, Severin Daiss, Lin Li, Lukas Hartung, Emanuele Distante, Gerhard Rempe
Max-Planck-Institut für Quantenoptik

Superpositions of coherent states (CSS) promise to be useful tools in communication via quantum channels. They offer the possibility to encode qubits in continuous-variable states and error correcting codes on coherent state superpositions in superconducting circuits have already been implemented. Here, we show the creation of CSS in the optical regime where we have full control over all degrees of freedom, such as the optical phase between in the creation process. As a first application we implemented a universal quantum-logic gate between such continuous variables states and the spin of a single atom trapped in an optical cavity by the use of a reflection mechanism, showing the usefulness of CSS in the context of interfacing flying and stationary qubits in framework of quantum networks.

### E: Quantum Metrology & Sensing Posters

Overview of posters covering Quantum Metrology & Sensing projects.
You can find them in the room with the same name.

2D & 3D Imaging of NV spins in a diamond

Authors: M.T. Amawi (1), D. Kwiatkowski (1,2), G. Braunbeck (1), A. M. Waeber (1), M. Kaindl (1), F. Reinhard (1)
1.Walter Schottky Institut and Physik-Department, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany
2.Institute of Physics, Polish Academy of Sciences, al. Lotnikow 32/46, 02-668 Warsaw, Poland"

Measuring magnetic fields has gone a long way from using induction coil to using state of the art superconducting quantum interference devices, SQUIDs. However, the operation of SQUIDs requires cryogenic temperatures causing the devices to be bulky and expensive to run. Another platform for measuring magnetic fields which has been gaining momentum is nitrogen-vacancy (NV) color centers in diamond, a much compact system which can also be run at ambient temperatures. We present our progress, utilizing this magnetic field sensitivity, towards super-resolution imaging of dense ensembles of NV centers in strong magnetic field gradients. This is a necessary step for Fourier imaging of magnetic fields [1] and readout of NV spin qubits in a densely integrated quantum register [2].
My poster will discuss the key challenges we have addressed on the way towards this goal. This, in particular, includes precise control of currents used in driving the magnetic gradient field. To mitigate current fluctuations our group has introduced “Feed Forward Decoupling”. This novel decoupling technique removes decoherence due to classical noise sources by recording the noise in parallel with the quantum manipulation and adjusting subsequent steps of the sequence, such as the readout observable. I will discuss the working principle of FFD and show its implementation in a Hahn echo experiment. I will equally discuss our approach to targeted nanofabrication of gradient magnets around a specific NV center. Finally, I will present our latest progress on 3D imaging of NV centers, as well as results on Fourier imaging to increase the efficiency of the imaging process.
The poster will end with an outlook on the limitations and possible applications of our experiment. Some examples on that are to use this such a system to imaging electron spins and running a compressed sensing scheme to boost the imaging acquisition rate.

[1] Arai, Keigo, et al. "Fourier magnetic imaging with nanoscale re"

On-chip and Free-space Optomechanics

Aurhors: David Hoch (TU Munich), Pedro Soubelet, Xiong Yao, Kevin-Jeremi Haas, Menno Poot

Our group focuses on Quantum Technologies. We make chips using state-of-the-art nanofabrication techniques to study quantum effects in a variety of integrated systems. Here we present our current projects on optomechanics. We have two vacuum chambers available to perform the measurements. In one vacuum chamber we send light via grating couplers into an integrated Mach-Zehnder interferometer design to sense the dynamics of released devices in the vicinity of one of the arms of the interferometer. The other stetup sends light from above onto our devices which form an optomechanical cavity. The back-reflected light is then collected by a photodetector. Most devices are integrated in chips of 330 nm of SiN on a 3300 nm cladding layer of SiO2 on Si substrate. Network analyzers drive the structures via a piezo and process the subsequent signal measured with photodetectors.

Resonator electromechanics on the single- photon level based on a nonlinear Josephson inductance

Authors: T. Luschmann(WMI), P. Schmidt, M.T. Amawi, S. Pogorzalek, F. Deppe, A. Marx, R. Gross and H. Huebl

Light-matter interaction in optomechanical systems is the foundation for ultra-sensitive detection schemes as well as the generation of phononic and photonic quantum states. Electromechanical systems realize this optomechanical interaction in the microwave regime. In this context, capacitive coupling arrangements demonstrated interaction rates of up to 280 Hz [1]. Complementary, early proposals [2] and experiments [3] suggest that inductive coupling schemes are tunable and have the potential to reach the vacuum strong-coupling regime. In the presented work, we follow the latter approach by integrating a partly suspended superconducting quantum interference device (SQUID) into a microwave resonator. The mechanical displacement translates into a time varying flux in the SQUID loop, thereby providing an inductive electromechanical coupling. We demonstrate a sideband-resolved electromechanical system with a tunable vacuum coupling rate of up to 17.8 kHz.

[1] A. P. Reed, et al. Nat. Phys. 13, 1163 (2017)
[2] M. P. Blencowe and E. Buks, Phys. Rev. B 76, 014511 (2007)
[3] I. C. Rodrigues, et al. Nat. Commun. 10, 5359 (2019)"

Quantum metrology in waveguide/cavity QED

Author: Daniel Malz , Martí Perarnau-Llobet, J. Ignacio Cirac.
1. Max Planck Institute of Quantum Optics, Garching, Germany
2. MCQST, Munich

The research shown here bulds on the key principle that atoms coupled to photons offer unique possibilities in photon counting, building on decades of progress of controlling and manipulating atoms. The poster discusses recent work on (i) number-resolving (and non-destructive) photon detection with atomic arrays coupled to waveguides, and (ii) quantum advantages in weakly-invasive quantum metrology.

Interferometric Josephson mixer for quantum microwave circuits

Authors: M. Partanen (1), S. Gandorfer (1,2) , K. G. Fedorov (1,2), S. Pogorzalek (1,2), M. Renger (1,2), Q. Chen (1,2), J. Lamprich (1,2), Y. Nojiri (1,2), A. Marx (1), F. Deppe (1,2,3), and R. Gross (1,2,3)
1. Walther-Meißner-Institut, Bayerische Akademie der Wissenschaften, 85748 Garching, German

2. Physik-Department, TU München, 85748 Garching, Germany
3. Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, 80799 Munich, Germany

Quantum mechanics offers interesting opportunities for illumination applications with accuracies beyond the classically obtainable limits. An especially interesting approach is based on using entangled microwave signals for radar applications [Las Heras et al., Sci. Rep., 7, 9333 (2017)]. We present a frequency-degenerate scheme for quantum illumination with superconducting microwave circuits. Our method is based on continuous variables that provide a convenient platform for quantum communication. In particular, we present a novel frequency-degenerate Josephson mixer, which is an important component in the quantum radar protocol. Our experimental results show very good agreement with the theoretical model.

All-Optical Single-Shot Readout of NV Centers in Diamond via Cryogenic Spin-to-Charge Conversion

Authors: Dominik M. Irber (1), Francesco Poggiali (1), Fei Kong (2), Michael Kieschnick (3), Tobias Lühmann (3), Damian Kwiatkowski (4), Jan Meijer (3), Jiangfeng Du (2), Fazhan Shi (2), Friedemann Reinhard (1).
(1) Walter Schottky Institut and Physik-Department, Technische Universität München.
(2) Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui, 230026, PR China.
(3) Felix-Bloch-Institut für Festkörperphysik Abteilung Angewandte Quantensysteme, Linnéstraße 5, 04103 Leipzig, Germany.
(4) Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, Poland.

Nitrogen-Vacancy (NV) centers in diamond are a front-runner candidate for quantum sensing and quantum computing. Many of the most disruptive applications, such as single-molecule magnetic resonance imaging, will require a speedup of measurements by two to three orders of magnitude over the state of the art. This is achievable by improved techniques for spin readout [1,2,3], which however require efficient collection optics, such as immersion optics or all-diamond micro-optics. For some of the most relevant applications, such as shallow implanted NV centers (less than 100nm close to the diamond surface) in a cryogenic environment, these tools are unavailable.
Here, we present an all-optical readout scheme for the NV spin-state that achieves single-shot fidelity even if photon collection is poor (delivering less than 103clicks/second). Our technique combines resonant excitation at low temperature with spin-to-charge conversion. Low temperature reduces the optical transition linewidth to a level where resonant laser excitation can selectively address the spin-sublevels [3,4]. In combination with a second laser pulse, a spin-to-charge conversion [5,6] protocol can be implemented, where the NV center is spin-selectively excited and converted to different charge-states. These are more stable than the initial spin-state and can be read-out with single-shot fidelity even using poor collection optics. The technique is applicable to shallow NV centers, and is more robust than competing approaches.
Our method accelerates measurements by three orders of magnitude over standard fluorescence readout, which will push the detection range for single electron spins to several tens of nanometers. It could therefore become a pivotal ingredient for nanoscale electron spin resonance experiments, as well as for two-qubit gates in scalable NV-based quantum registers.

[1] L. Robledo et al., Nature 477, 574 (2011)
[2] P. Neumann et al., Science 329, 542 (2010)
[3] D.A. Hopper et al., Micromachines 9, 437 (2018)
[4] A. Batalov et al., Physical Review Letters 102, 195506 (2009)
[5] M.W. Doherty et al., New Journal of Physics 13, 025019 (2011)
[6] B.J. Shields et al., Physical Review Letters 114, 136402 (2015)
[7] X.-D. Chen et al., Physical Review A 7, 014008 (2017)

Cavity-Enhanced Microscope for Cold Atoms

Authors: N. Sauerwein (EPFL), T. Cantat-Moltrecht and Jean-Philippe Brantut

We are setting a new experiment aimed at implementing a cavity-enhanced microscope for cold atoms. The core of our experiment is a high-finesse cavity combined with a high-numerical-aperture lens (0.37) in a single optical system. The cavity will be used to dispersively measure the presence of atoms in the cavity mode. A second short-wavelength laser addressing a transition between excited states is focused tightly into a set of lithium atoms, leading to a local enhancement of the coupling to the cavity and allowing for a non-destructive density measurement with a sub-micron spatial resolution [1]. With the control over the coupling, we are able to modify the cavity-mediated interactions both temporally and spatially.
Currently, we have finished the assembly of the combined cavity-microscope platform and laser system. The poster summarizes the concept, the design, and the current status of the setup and outlines possible future experiments.

[1] Yang, D., et al., PRL 120, 133601, (2018)

Dispersive read-out of room temperature spin qubits

Authors: Martin Schalk (1), Jasper Ebel (1), Timo Joas (1), Andreas Angerer (2), Johannes Majer (2) and Friedemann Reinhard (1)
1. Walter Schottky Institut and Physik-Department, Technische Universität München, Garching, Germany
2. Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien, Vienna, Austria

[1] J. Ebel et al., arXiv:2003.07562
[2] E. Eisenach et al., arXiv:2003.01104

On-chip quantum optics

Authors: Timo Sommer (1,2), Sebastian Müller (1), Giulio Terrasanta (3), Peter Wegmann (4), David Hoch (1,2,5), Menno Poot (1,2,5)
1. Department of Physics, Technical University Munich, Garching, Germany;
2. Munich Center for Quantum Science and Technology (MCQST), Munich, Germany;
3. Physics Section, Swiss Federal Institute of Technology in Lausanne (EPFL), Switzerland;
4. Department of Informatics, Technical University Munich, Garching, Germany;
5. Institute for Advanced Study, Technical University Munich, Garching, Germany"

With the growing interest in quantum optics and quantum computing, optical experiments have become increasingly complex in recent years. Currently, many of these experiments are done with bulk optics, but there is a limit of complexity and size of the experiments that soon will be hit. Integrated photonic circuits manufactured with modern semiconductor fabrication methods can overcome this limit and makes alignment of the individual components obsolete. Scaling thus becomes a matter of nanofabrication. This makes integrated photonic circuits a promising platform for quantum science. However, integration of the essential components for quantum optics experiments has been an ongoing quest. Here we present our approach, including integration of a single-photon sources, quantum circuitry, and single-photon detection into a single photonic chip. Single-photon sources can be realized with the nonlinearity of aluminum nitride, the circuity is implemented with waveguides made from silicon nitride, and single-photon detectors will be based on superconducting niobium-titanium nitride nanowires. Many interesting quantum optics challenges can be addressed with source, circuit, and detection working together.

Many body quantum phases of excitons and trions in an atomically thin semiconductor

Authors: A.V. Stier (1), J. Klein (1), A. Hötger (1), M. Florian (2), A. Steinhoff (2), A. Delhomme (3), T. Taniguchi (4), K. Watanabe (4), F. Jahnke (2), A.W. Holleitner (1), M. Potemski (3), C. Faugeras (3), and J.J. Finley (1)
1. Walter Schottky Institut, Technische Universität München, Garching, Germany
2. Institut für Theoretische Physik, Universität Bremen, Bremen, Germany
3. Universite Grenoble Alpes, INSA Toulouse, EMFL, CNRS, Grenoble, France
4. National Institute for Materials Science, Tsukuba, Japan"

A bosonic exciton surrounded by a Fermi gas represents a model system for the study of many-body correlated phases of interacting Bose-Fermi mixtures1–3. The strong Coulomb interactions in atomically thin quantum materials are ideal candidates for experimental studies since the carrier density can be electrically controlled and the robust symmetry and optical selection rules allow for distinct differentiation between spin, valley and charge degrees of freedom4. Here, we explore the phase space of neutral excitons and positively and negatively charged trions in the archetypal monolayer semiconductor MoS2 via density dependent, high field magneto-optical measurements.
For the neutral exciton, we observe unexpected nonlinear valley Zeeman shifts as we immerse it with electrons of distinct spin and valley Fermi textures. Our results show that the exciton only behaves as a true boson at charge neutrality and that contrary to conventional wisdom the total angular momentum J is the good quantum number for the magnetic energy. We explain the nonlinear valley Zeeman effects to arise from dipolar spin interactions with the spin-polarized Fermi sea.
For increasing electron density, we find quantum oscillations in the intravalley trion photoluminescence along with a non-uniform, but always linear, valley Zeeman shift in positive and negative magnetic fields. We explain this behavior from the different Landau level occupation in positive and negative magnetic fields and determine the effective electron and hole masses from this unequal valley Zeeman shift.
Further increasing the electron density, we observe an optical feature that is red shifted from the intravalley trion, which is currently discussed in the literature as a Mahan-type exciton. We find that this feature initially behaves as the intravalley trion interacting with the Fermi sea; specifically, the valley susceptibility mimics that of the total spin polarization of the sample. At the density where all spins and valleys are available for the particle, we find an abrupt deviation from this behavior towards a highly correlated manybody state, whose valley susceptibility is that of the neutral exciton bathed in a spin polarized Fermi sea. Our experiments unequivocally demonstrate that the exciton in monolayer semiconductors is a single particle boson only in the case close to charge neutrality and is otherwise an interacting many-body state with a spin-valley flavor that is defined by the Landau level quantized spin and valley texture.

1. Bloch, I., Dalibard, J. & Zwerger, W. Many-body physics with ultracold gases. Rev. Mod. Phys. 80, 885–964 (2008).
2. Greiner, M., Mandel, O., Esslinger, T., Hänsch, T. W. & Bloch, I. Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms. Nature 415, 39–44 (2002).
3. Fey, C., Schmelcher, P., Imamoglu, A. & Schmidt, R. Theory of exciton-electron scattering in atomically thin semiconductors. Phys. Rev. B 101, 195417 (2020).
4. Tang, Y. et al. Simulation of Hubbard model physics in WSe2/WS2 moiré superlattices. Nature 579, 353–358 (2020).

Superconducting Single-Photon Detectors made of NbTiN on SiO2 substrates

Authors: R. Flaschmann (1), S. Strohauer (1), L. Zugliani (1), N. Ploch (1), C. Schmid (1), M. Althammer (2), R. Gross (2), J.J. Finley (1), K. Müller (1)
1. Walter Schottky Institute
2. Walther Meißner Institute

In recent years, Superconducting Nanowire Single-Photon Detectors (SNSPDs) have raised tremendous attention as a possible key technology for optical quantum processing and faint light detection. With their ability to detect single photons with high efficiency and high count rates while providing low dark count rates and low timing jitter, further investigation of these detectors is of high interest [1,2,3]. Here, we present our latest results on detector fabrication at the Walter Schottky Institute, including thin film fabrication and characterization for NbTiN SNSPDs, a self-alignment method for fiber-to-detector coupling [4], and simulation results of SNSPDs equipped with a metal mirror and a SiO2 cavity to enhance the detection efficiency.

[1] C. M. Natarajan et al., Superconductor Science and Technology, 25(6), 063001 (2012).
[2] S. Ferrari et al., Nanophotonics, 7(11), 1725–1758 (2018).
[3] F. Marsili et al., Nature Photonics, 7(3), 210–214 (2013).
[4] A. Miller et al., Optics Express, 19(10), 9102 (2011).

Electrical Readout of NV Centers in Diamond

Authors: Lina Maria Todenhagen and Martin S. Brandt
Walter Schottky Institut, Technische Universität München

Quantum sensing with color centers in diamond is traditionally done purely optically. Recently, it was shown that the readout of NV centers can also be achieved electrically, enabling a significant miniaturisation of NV-based magnetometers.
In this contribution, we present a systematic study into the dependence of the EDMR contrast on the wavelength of the excitation used and compare it to the corresponding dependence of ODMR.

Full-field Terahertz imaging at kilohertz frame rates using atomic vapour

Authors: Lucy Downes, Andrew MacKellar, Shuying Chen, Nourah AlMuhawish, Matthew Jamieson, Charles Adams & Kevin Weatherill (Durham University)

There is much interest in using terahertz imaging technology across a wide range of applications including medical diagnostics, security scanning and production-line quality control. However, for many applications, THz imaging has not yet achieved the required speed and sensitivity for real-time analysis and therefore, it remains a long-standing goal to achieve true real-time and full-field terahertz imaging.
Here, we present a new method for terahertz imaging which uses room-temperature atomic vapor as an efficient terahertz-to-optical convertor, thereby allowing imaging to be collected using standard optical cameras. We demonstrate 2D imaging with near diffraction-limited resolution for a 1 cm^2 sensor and frame rates of several kilohertz. The system demonstrates a linear sensitivity scaling and we measure a minimum detectable intensity of ~0.1 mW m^-2 for 1 s integration. We expect that with some minor modifications to the setup, frame rates exceeding 50 kHz should be possible.

Towards Quantum Simulation of Light-Matter Interfaces with Strontium Atoms in Optical Lattices

Authors: Annie Jihyun Park, Jan Trautmann, Neven Santic, Andre Heinz, Valentin Klüsener, Eva Casotti, Florian Wallner, Immanuel Bloch, Sebastian Blatt
Max Planck Institute of Quantum Optics

In the last two decades, quantum simulators based on ultracold atoms in optical lattices have successfully emulated strongly correlated condensed matter systems. With the recent development of quantum gas microscopes, these quantum simulators can now control such systems with single-site resolution. Within the same time period, atomic clocks have also started to take advantage of optical lattices by trapping alkaline-earth-metal atoms such as Sr, and interrogating them with precision and accuracy at the 10^{-18} level. Here, we report on progress towards a new quantum simulator that combines quantum gas microscopy with optical lattice clock technology. We have developed in-vacuum buildup cavities with large mode volumes that will be used to overcome the limits to system sizes in quantum gas microscopes. We characterize the two-dimensional lattice created by the two orthogonal cavity modes of the in-vacuum buildup cavity by loading ultracold strontium atoms. By using such optical lattices that are state-dependent for the clock states, we aim to emulate strongly-coupled light-matter-interfaces in parameter regimes that are unattainable in real photonic systems.

Nanocavities for deterministically placed He-ion induced quantum emitters in atomically thin semiconductors

Authors: A. Lyamkina(1), C. Qian(1), L. Kühner(2), F. Sigger(1), M.M. Petrić(3), A. Nolinder(1), M. Kaniber(1), K. Müller(3), A.W. Holleitner(1), S.A. Maier(2) and J.J. Finley(1)
(1) Walter Schottky Institut and Fakultät für Physik, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany
(2) Lehrstuhl für Hybride Nanosysteme, Nanoinstitut München, Fakultät für Physik, LMU Königinstrasse 10, 80539 München, Germany
(3) Walter Schottky Institut and Fakultät für Electrotechnik und Informationstechnik, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany

Precise placement of discrete, spectrally homogeneous quantum emitters into solid-state nanophotonic architectures is required for cavity-QED experiments that go beyond the Jaynes-Cummings limit [1], exploring collective phenomena such as Dicke super- and sub-radiance [2] and integrated quantum photonic technologies [3]. Such emitters can be site-selectively generated in 2D-materials via local He-ion irradiation that produces luminescent defects with sub 10nm lateral precision [4, 5]. In this contribution, we consider prominent candidates for solid-state photonic architectures that are compatible with two-dimensional semiconductors. Examples include metallic bowtie antennas with extremely strong field enhancements and highly localized dipolar resonances, dielectric GaP dimer antennas and photonic crystal point defect nanocavities. Each of these systems offers a distinctly different Q/√(V_mode ) and, thereby, access to different regimes of light matter coupling. For example, the metallic bowtie antennas provide Q~4 due to strong Ohmic losses but have exceptionally low mode volumes V_mode~2×"10-6" µm^3 (Q/√(V_mode )=2.8×〖10〗^3 µm^(-3/2)). Dielectric GaP dimer antennas have significantly lower Ohmic losses and, thereby, Q~10, but have weaker mode confinement V_mode~3×"10-4" µm^3 (Q/√(V_mode )∼ 580 µm^(-3/2)). Finally, Si3N4 photonic crystal nanobeam cavities have experientially demonstrated Q>25000 but comparatively large mode volumes of V_mode~0.043 µm^3 (Q/√(V_mode )=1.2×〖10〗^5 µm^(-3/2). We demonstrate our current progress of nanofabrication for all three types of nanocavities and describe potential directions for our quantum optical experiments with discrete quantum emitters in 2D-semiconductors generated site-selectively using He-ions.

[1] S. Welte et al, Phys. Rev. X 8, 011018 (2018)
[2] S. Mignuzzi et al., Nano Lett. 19, 1613 (2019)
[3] P. Lodahl et al., Rev. Mod. Phys. 87, 347 (2015)
[4] J. Klein et al., Nature Comm. 10, 2755 (2019)
[5] J. Klein et al., arXiv:1901.01042 (2020)

We gratefully acknowledge the Alexander v. Humboldt foundation, the DFG via MCQST and e-Conversion and the EU via S2QUIP for financial support.

### F: Quantum Matter Posters

Overview of posters covering Quantum Matter projects.
You can find them in the room with the same name.

Synthesis of Atomically Thin Semiconductors

Authors: Ismail Bilgin (1), Zhijie Li (1), Jonathan Förste (1), Victor Funk (1), Michael Förg, Kenji Watanabe (2),Takashi Taniguchi (2) and A. Högele (1)
1. Fakultät für Physik, Munich Quantum Center, and CeNS, Ludwig-Maximilians-Universität München, Germany
2. National Institute for Materials Science, Tsukuba, Ibaraki,Japan

Two-dimensional transition metal dichalcogenide (TMD) heterostructures have gained a tremendous attention due to their interlayer exciton features such as long lifetime, dipole transition, Moiré physics, valleytronics etc. Fabrication of such materials has been limited to stacked exfoliated monolayer samples, and direct growth of various TMD heterostructures is required for diverse quantum optoelectronic applications. Here, we present high quality, large area monolayer TMD samples and heterobilayer samples that grow with perfect rotational alignment using a vapor phase chalcogenization(VPC) method. The encapsulation of VPC grown monolayer samples in h-BN exhibits narrow photoluminescence (PL) linewidth around 3 meV. We also explore interlayer excitons (IXs) on MoSe2-WSe2 heterobilayer samples with 0 and 60 degree aligned.

Confined phases of spinless fermions coupled to Z_2 gauge theory

Authors: Umberto Borla (Department of Physics, Technical University of Munich, 85748 Garching, Germany)
Ruben Verresen (Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA)
Fabian Grusdt (Ludwig-Maximilians-Universität Munich, Theresienstr. 37, Munich D-80333, Germany)
Sergej Moroz (Department of Physics, Technical University of Munich, 85748 Garching, Germany)

We investigate a quantum many-body lattice system of one-dimensional spinless fermions interacting with a dynamical Z2 gauge field. The gauge field mediates long-range attraction between fermions resulting in their confinement into bosonic dimers. At strong coupling we develop an exactly solvable effective theory of such dimers with emergent constraints. Even at generic coupling and fermion density, the model can be rewritten as a local spin chain. Using the Density Matrix Renormalization Group the system is shown to form a Luttinger liquid, indicating the emergence of fractionalized excitations despite the confinement of lattice fermions. In a finite chain we observe the doubling of the period of Friedel oscillations which paves the way towards experimental detection of confinement in this system. We discuss the possibility of a Mott phase at the commensurate filling 2/3.

Crossover from few- to many-body physics: Efimov physics and the Bose polaron

Authors: Arthur Christianen, Ignacio Cirac, Richard Schmidt
Max Planck Institute of Quantum Optics

We study the problem of an impurity immersed in a Bose-Einstein condensate (BEC). Using a Gaussian state variational Ansatz we describe the system in the frame of the quantum impurity. This Ansatz includes coherent deformations of the BEC and two-body correlations between the bosons. In this way our model correctly incorporates three-body physics and allows us to study the crossover from the few-body physics, featuring Efimov states, to the many-body physics, where Bose polaron formation sets in. We find that the Bose polaron breaks down at finite negative scattering lengths, due to the formation of a many-body bound state. The nature of this bound state and its link to Efimov physics are currently under investigation.

Unexpected experimental observations in the temperature dependence of subgap superconducting states

Authors: Juan Carlos Estrada Saldaña (University of Copenhagen), Alexandros Vekris, Victoria Sosnovtseva, Thomas Kanne, Peter Krogstrup, Kasper Grove-Rasmussen, Jesper Nygård

Yu-Shiba-Rusinov (YSR) excitations, called Andreev bound states in some of the early literature, arise when quantum magnetic impurities are coupled to superconductors. They are at the basis of recent demonstrations of topological superconductivity (Nadj-Perge et al., Science 346, 2014). Their dependence on gate voltage and external magnetic field has been well documented in initial transport experiments (Lee et al., Nat. Nanotechnol., 9, 2014). However, little is known about the details of their temperature dependence.
We show that this dependence is profoundly different from expectations by current models. We demonstrate this in superconductor-quantum dot-metal devices defined on semiconductor nanowires with epitaxial superconducting leads (Estrada Saldaña et al., arXiv, 2002.12882, 2020, Communications Physics, accepted).

Fermi Surface Reconstruction in Heavy Fermion Systems - Cellular Dynamical Mean-Field perspective

Authors: Andreas Gleis, LMU Munich; Seung-Sup B. Lee, LMU Munich; Andreas Weichselbaum, Brookhaven National Laboratory and LMU Munich; Gabriel Kotliar, Brookhaven National Laboratory and Rutgers University; Jan von Delft, LMU Munich

A cellular dynamical mean-field theory plus numerical renormalization group study of heavy fermion quantum criticality in the three dimensional periodic Anderson model is presented. We find a continuous Kondo breakdown quantum critical point, characterized by a vanishing Fermi liquid scale and a reconstruction of quasiparticles and the Fermi surface. We present a phase diagram and the evolution of the spectral function and the Fermi surface across the quantum critical point. To show the significance of our calculations, we compare our findings to experimental data.

In-plane anisotropy of the photon-helicity induced linear Hall effect in few-layer WTe2

Authors: Jonas Kiemle (1), Paul Seifert (1), Florian Sigger (1,29, Kenji Watanabe (3), Takashi Taniguchi (3), Christoph Kastl (1,4), Ursula Wurstbauer (1,2,5) and Alexander Holleitner (1,2)
1. Walter Schottky Institut, Technical University of Munich, Am Coulombwall 4, 85748 Garching, Germany
2. Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 Munich, Germany
3. Advanced Materials Laboratory, Tsukuba, Ibaraki 305-0044, Japan
4. Molecular Foundry, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California, USA
5. Institute of Physics, WWU Muenster, Wilhelm-Klemm-Str. 10, Muenster, Germany

Using Hall photovoltage measurements, we demonstrate that a linear transverse Hall voltage can be induced in few-layer WTe2 under circularly polarized light illumination [1,2]. By applying a bias voltage along different crystal axes, we find that the photon-helicity induced Hall effect coincides with a particular crystal axis. Our results are consistent with the underlying Berry curvature exhibiting a dipolar distribution due to the breaking of crystal inversion symmetry. Using time resolved optoelectronic autocorrelation spectroscopy [3], we find that the decay time of the detected Hall voltage exceeds the electron-phonon scattering time by orders of magnitude but is consistent with the comparatively long spin lifetime of carriers in the momentum-indirect electron and hole pockets in WTe2. Our observation suggests that a helicity induced non-equilibrium spin density on the Fermi surface after the initial charge carrier relaxation gives rise to a linear Hall effect [1,4].
We acknowledge financial support by the DFG via the German Excellence Strategy ‘Munich Center for Quantum Science and Technology’ (MCQST) and project HO3324/12.

[1] P. Seifert et al., Phys. Rev. B 99, 161403(R) (2019).
[2] P. Seifert et al., Nat. Comm. 9, 331 (2018).
[3] P. Seifert et al., Nano Lett. B 99, 17, 2, 973-979 (2017).
[4] P. Seifert et al., Phys. Rev. Lett. 122, 146804 (2019).

Microwave Spectroscopy Reveals the Quantum Geometric Tensor of Topological Josephson Matter

Authors: R. L. Klees [1], G. Rastelli [1,2], J. C. Cuevas [3], and W. Belzig [1]
[1] Fachbereich Physik, Universität Konstanz, D-78457 Konstanz, Germany
[2] Zukunftskolleg, Universität Konstanz, D-78457 Konstanz, Germany
in

"Quantization effects due to topological invariants such as Chern numbers have become very relevant in many systems, yet, key quantities as the quantum geometric tensor (QGT) [1] providing local information about quantum states remain experimentally difficult to access. Recently, it has been shown that multiterminal Josephson junctions (MJJs) constitute an ideal platform to synthesize topological systems in a controlled manner and that the time-averaged transconductance between two terminals is quantized in terms of the Chern number [2-5]. Although first experiments towards ballistic MJJs have been performed [6,7], a direct way to measure the local QGT of Andreev states has not been proposed yet. We address this theoretically by studing properties of Andreev states in topological Josephson matter and demonstrate that the QGT of Andreev states can be extracted by synthetically polarized microwaves [8]. The oscillator strength of the absorption rates provides direct evidence of topological quantum properties of the Andreev states.

[1] M. Kolodrubetz, D. Sels, P. Mehta, A. Polkovnikov, Phys. Rep. 697, 1–87 (2017).
[2] R.-P. Riwar, M. Houzet, J. S. Meyer, and Y. V. Nazarov, Nat. Comm. 7, 11167 (2016).
[3] J. S. Meyer and M. Houzet, Phys. Rev. Lett. 119, 136807 (2017).
[4] H.-Y. Xie, M. G. Vavilov, and A. Levchenko, Phys. Rev. B 96, 161406 (2017).
[5] H.-Y. Xie, M. G. Vavilov, and A. Levchenko, Phys. Rev. B 97, 035443 (2018).
[6] A. W. Draelos, M.-T. Wei, A. Seredinski, H. Li, Y. Mehta, K. Watanabe, T. Taniguchi, I. V. Borzenets, F. Amet, and G. Finkelstein, Nano Lett. 19, 1039 (2019).
[7] N. Pankratova, H. Lee, R. Kuzmin, M. Vavilov, K. Wickramasinghe, W. Mayer, J. Yuan, J. Shabani, and V. E. Manucharyan, arXiv:1812.06017.
[8] R. L. Klees, G. Rastelli, J. C. Cuevas, and W. Belzig, Phys. Rev. Lett. 124, 197002 (2020).

Discrete Interactions between Interlayer Excitons Trapped at a MoSe2-WSe2 Heterointerface

Authors:Malte Kremser(1), Mauro Brotons-Gisbert(2), Johannes Knörzer(3), Janine Gückelhorn(1), Moritz Meyer(1), Matteo Barbone(4), Andreas V. Stier(1), Brian D. Gerardot(2), Kai Müller(4) and Jonathan J. Finley(1)
1. Walter Schottky Institute, Physics Department and MCQST, TU München, Am Coulombwall 4, 85748 Garching, Germany
2. Institute for Photonics and Quantum Sciences, SUPA, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom
3. Max-Planck-Institute for Quantum Optics and MCQST, Hans-Kopfermann-Str. 1, 85748 Garching, Germany
4. Walter Schottky Institut, Department of Electrical and Computer Engineering and MCQST, TU München, Am Coulombwall 4, 85748 Garching, Germany

Interlayer excitons (IXs) in hetero-bilayers of transition metal dichalcogenides have remarkable properties that open up tantalizing prospects of Bose–Einstein condensation, excitonic superfluidity at elevated temperatures and novel quantum phases, such as Wigner crystallization at low temperatures. Here, we trap a tunable number of IXs within a nanoscale confinement potential induced by placing a MoSe2-WSe2 hetero-bilayer onto an array of nanopillars. We control the mean occupation of the trap via the optical excitation level, identify different localized IX states via their sharp-line emission and characteristic power dependencies and directly measure the hierarchy of dipolar IX-IX interactions. Emission from the interlayer biexciton, triexciton, quadexciton and quintexciton is found to be blue-shifted from the single exciton by (8.4 ± 0.6) meV, (12.4 ± 0.4) meV, (15.5 ± 0.6) meV and (18.2 ± 0.8) meV, respectively. We reproduce this energy spectrum by modelling the repulsive dipole-dipole interactions of maximally spaced excitons in a harmonic trapping potential. Furthermore, the trapped biexciton splits into doublet, separated by (1.2 ± 0.5) meV. We calculate the exchange energy for the biexciton in a configuration-space approach and match the observed splitting for realistic material parameters and a confinement length of 3 nm. Our results demonstrate probing of dipolar interactions at the few-exciton limit, potentially opening a route to explore emergent quantum phases.

Synthetic spin-orbit coupling mediated by a bosonic environment

Authors: Mikhail Maslov, Mikhail Lemeshko, Enderalp Yakaboylu
Institute of Science and Technology Austria

We examine the behavior a mobile quantum impurity, possessing internal rotational degrees of freedom, confined to a planar ring in the presence of a many-particle bosonic bath. By considering the recently introduced rotating polaron problem, we define the Hamiltonian of the system and examine its energy spectrum. The regime of weak impurity-bath coupling is studied by means of a variational ansatz in the truncated Fock space. The corresponding spectrum indicates that there emerges a coupling between the internal and orbital angular momenta of the impurity as a consequence of the phonon exchange. We interpret such coupling as a phonon-mediated spin-orbit coupling (SOC) and quantify it by using a correlation function between the internal and the orbital angular momentum operators. The strong-coupling regime is investigated within the Pekar approach, and it is shown that the correlation function of the ground state shows a kink at a critical coupling, that is explained by a sharp transition from the noninteracting state to the states that exhibit strong interaction with the surroundings. Together, two interaction regimes reveal the connection between the exchange of bath excitations and the spin-orbit interaction between angular momenta of the impurity. The results might find applications in such fields as spintronics or topological insulators where spin-orbit coupling is of crucial importance.

Cat states in a driven superfluid: role of signal shape and switching protocol

Author: Jesús Mateos¹, Gregor Pieplow¹², Charles Creffield¹, Fernando Sols¹,
²Department of Physics, Humboldt-Universität zu Berlin, Newtonstr. 15, D-12489 Berlin, Germany

We investigate the behavior of a one-dimensional Bose-Hubbard model whose kinetic energy is made to oscillate with zero time-average. The effective dynamics is governed by an atypical many-body Hamiltonian where only even-order hopping processes are allowed. At a critical value of the driving, the system passes from a Mott insulator to a superfluid formed by a cat-like superposition of two quasi-condensates with opposite non-zero momenta. We analyze the robustness of this un-conventional ground state against variations of a number of system parameters. In particular we study the effect of the waveform and the switching protocol of the driving signal. Knowledge of the sensitivity of the system to these parameter variations allows us to gauge the robustness of the exotic physical behavior.

Approaching Fermi mixtures with complex Langevin: EOS and pairing behavior

Authors: Lukas Rammelmüller [LMU Munich, TU Darmstadt], Joaquin Drut [UNC Chapel Hill], Jens Braun [TU Darmstadt]

Theoretical treatment of imbalanced Fermi systems is challenging. Exact analytic methods, if available, are limited to 1D setups and thus numerical treatment is often the only viable option. Among the most successful methods for balanced Fermi gases, in particular for systems beyond the few-body regime, are Quantum Monte Carlo (QMC) approaches. For imbalanced Fermi systems, however, these approaches suffer from an exponential scaling with system size: the infamous sign-problem. A way to circumvent this issue is provided by the complex Langevin method which we explore in the context of ultracold Fermi gases. More specifically, we employ the method to 1D imbalanced fermions in the ground state as well as the 3D unitary Fermi gas at finite temperature and finite spin asymmetry.

Hidden Symmetries of Exotic Magnets

Authors: Nihal Rao, Ke Liu, Lode Pollet
LMU Munich

We unravel hidden symmetry points and order parameters in the HK model of Kitaev Materials using unsupervised and interpretable machine learning via TK-SVM.

Hybridized indirect excitons in MoS2/WS2 heterobilayers

Authors: Florian Sigger (WSI - TU Munich)

Ensembles of indirect or interlayer excitons (IXs) are intriguing systems to explore classical and quantum phases of interacting bosonic ensembles. IXs feature enlarged lifetimes due to the reduced overlap of the electron-hole wave functions [1,2]. A field effect structure with few layer hexagonal boron nitrite (hBN) as insulator and few-layer graphene as gate-electrodes facilitates an electric field control of the IXs in a MoS2/WS2 heterobilayer [2]. A multiplet structure in the IX emission band can be observed even at room temperature. Stark shift measurements reveal the presence of a finite out-of plane dipole of the IXs. Due to a different strength of the dipole and a distinct temperature dependence, we identify the IXs to stem from optical interband transitions with electrons and holes in different valleys of the heterostructures [2]. We observe a field dependent level anti-crossing for the energetically lowest emission line, forming hybridized indirect excitons at low temperatures [2,3]. We discuss this behavior in terms of a finite coupling of the electronic states of the two TMDC monolayers. Our results demonstrate the design of novel nano-quantum materials prepared from artificial van der Waals solids with the possibility to in-situ control their physical properties via external stimuli such as electric fields.
The work is supported by the Deutsche Forschungsgemeinschaft (DFG) via excellence clusters NIM and eConversion as well as DFG projects WU 637/4-1 and HO3324/9-1.

[1] Bastian Miller, Alexander Steinhoff, Borja Pano, Frank Jahnke, Alexander Holleitner, Ursula Wurstbauer, Nano Lett., 17(9), 5229-5237 (2017).
[2] Jonas Kiemle, Florian Sigger, Michael Lorke, Bastian Miller, Kenji Watanabe, Takashi Taniguchi, Alexander Holleitner, Ursula Wurstbauer, Phys. Rev. B 101, 121404(R) (2020).
[3] Shiyuan Gao, Li Yang, Catalin D. Spataru, Nano Lett., 17(12), 7809-7813, (2017).

Condensation signatures of photogenerated interlayer excitons in a van der Waals heterostack

Authors: L. Sigl(1), F. Sigger(1), F. Kronowetter(1), J. Kiemle(1), J. Klein(1), K. Watanabe(2), T. Taniguchi(2), J. J. Finley(1,4), U. Wurstbauer(1,3), A.W. Holleitner(1,4)
1) Walter Schottky Institute and Physics Department, TU Munich, Am Coulombwall 4a, 85748 Garching, Germany.
2) National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan.
3) Institute of Physics, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str.10, 48149 Münster, Germany.
4) Munich Center for Quantum Science and Technology (MCQST), Schellingstr. 4, 80799 München, Germany.

Excitons are bound electron-hole pairs in semiconductors. At low temperature and high exciton density, the occupation numbers of the low-energy exciton states are predicted to increase leading eventually to quantum degeneracy and Bose-Einstein condensation. Atomistic van der Waals double layers with very strong exciton binding energies are ideal systems for the study of high-temperature exciton condensation, since the interlayer excitons therein exhibit long lifetimes up to several tens of nanoseconds. Here we present strong signatures for exciton condensation at elevated temperatures in a MoSe2-WSe2 heterostructure. We observe several criticalities in photogenerated exciton ensembles with respect to photoluminescence intensity, lifetime, linewidth, and temporal coherence on temperature and exciton density, accompanied by a relative occupation of the underlying exciton state of 100%. The phenomena exist above 10 kelvin consistent with the predicted critical condensation temperature. Our study provides a first phase-diagram of photogenerated many-body interlayer exciton states that paves ground for excitonic based integrated quantum optical circuits.

High-finesse tunable cavity in a closed-cycle cryogenic system

Authors: Samarth Vadia (1,2,4), Johannes Scherzer (1), Claudio Dal Savio (2), David Hunger (3), Khaled Karrai (2), and Alexander Högele (1,4)
(1) Fakultät für Physik, Munich Quantum Center, and CeNS, Ludwig-Maximilians-Universität München, Germany
(2) attocube Systems AG, Haar bei München, Germany
(3) Karlsruher Institut für Technologie, Karlsruhe, Germany
(4) Munich Center for Quantum Science and Technology (MCQST), München, Germany

The light-matter interactions in a solid-state based quantum emitter combined with an optical resonator is a vital tool for the realization of quantum technologies, as well as fundamental studies in the field of cavity quantum electrodynamics. Exciton-polaritons have been observed in two-dimensional monolayer transition metal dichalcogenides (TMDs) in the strong-coupling regime with a variety of resonators, including Fabry-Pérot cavities and plasmonic nanostructures. One promising experimental platform is based on tunable Fabry-Pérot cavity. The capability of spectral and spatial tuning enables control of the cavity resonance, and therefore the effective photon confinement, and interaction with individual quantum emitters. In the poster, we will demonstrate advances towards a high-finesse tunable open cavity, operated at low temperature in a closed-cycle cryostat. We will discuss various aspects of the setup, particularly mechanical stability, and show measurements characterizing its performance. Furthermore, we will present recent experimental progress investigating light-matter coupling between two-dimensional semiconductor monolayer and tunable open cavity.

### G: Explorative Directions Posters

Overview of posters covering innovative Explorative projects.
You can find them in the room with the same name.

Rayleigh edge waves in 2d chiral crystals

Authors: Claudio Benzoni
Physik-Department, Technische Universität München, 85748 Garching, Germany
Munich Center for Quantum Science and Technology (MCQST), 80799 München, Germany

We investigate, within the framework of linear elasticity theory, edge Rayleigh waves of a two-dimensional elastic solid which breaks time-reversal and parity symmetries due to the Coriolis--Lorentz force. We find that the direction of propagation of the Rayleigh modes is determined not only by the sign of the magnetic field but also by the Poisson ratio of the elastic system. We discover three qualitatively different regions distinguished by the chirality of the low-frequency edge waves and study their universal properties. To illustrate the Rayleigh edge-waves in real time, we have carried out finite-difference simulations of the model.

Confinement and lack of thermalization after quenches in the bosonic Schwinger model

Authors: Titas Chanda (Jagiellonian University), Jakub Zakrzewski, Maciej Lewenstein, Luca Tagliacozzo

We excite the vacuum of a relativistic theory of bosons coupled to a U(1) gauge field in 1+1 dimensions (bosonic Schwinger model) out of equilibrium by creating a spatially separated particle-antiparticle pair connected by a string of electric field. During the evolution, we observe a strong confinement of the bosons witnessed by the bending of their light cone, reminiscent of what has been observed for the Ising model [Nat. Phys. 13, 246 (2017)]. As a consequence, for the time scales we are able to simulate, the system evades thermalization and generates exotic asymptotic states. These states are made of two disjoint regions, an external deconfined region that seems to thermalize, and an inner core that reveals an area-law saturation of the entanglement entropy.

Long-range coherence and multistability of lossy qubits

Authors: Shovan Dutta and Nigel Cooper
TCM Group, Cavendish Laboratory, University of Cambridge

Environmental coupling typically drives a quantum system to a unique steady state with very little coherence, which is a major obstacle for quantum control and information processing. Here we identify a simple experimental setting of a locally pumped and lossy array of two-level quantum systems that can stabilize states with strong long-range coherence. Indeed, by analytic construction, we show there is an extensive set of steady-state density operators, from minimally to maximally entangled, despite this being an interacting open many-body problem. Such nonequilibrium states arise from a hidden symmetry that stabilizes Bell pairs over arbitrarily long distances, leading to controllable long-range entanglement. We show how to selectively prepare and observe the steady states in existing setups. Our conclusions apply to a broad range of two-level systems.

Gate-tunable single-photon emitters site-selectively generated in monolayer MoS2

Authors: Alexander Hötger, Julian Klein, Lukas Sigl, S. Gyger, Takashi Taniguchi, Kenji Watanabe, V. Zwiller, K.D. Jöns, Ursula Wurstbauer, Jonathan Finley, Alexander Holleitner

Controlling single-photon emission on a few nanometers plays an important role for the scalability of future quantum photonic circuits. Moreover, it is highly relevant to facilitate a gate-switchable emission for quantum information schemes. By irradiating MoS2 with helium ions, we generate single-photon sources with sharp emission lines at ~1.75 eV with a lateral position accuracy of only a few nanometers. The narrow linewidth of the reported emission peaks points toward single spatially trapped excitons at defect sites in MoS2. In second-order correlation measurements the nature of single-photon emission at several of these defects is unambiguously proofed. We demonstrate that a voltage between a graphite gate and the locally bombarded MoS2, encapsulated in hexagonal boron nitride, results in a controlled switching of the defect emission. The emission is stable for negative gate voltages (intrinsic regime), whereas for positive gate voltages (n-doped regime), the emission can be completely quenched. We show that switchable emitters can be precisely positioned in lateral matrices and arbitrary patterns. This controllability of light emission in spatial and temporal means paves the way for new integrated quantum photonic technologies.

Reduced Density Matrix Functional Theory for Dilute Bose Gases

Authors: Julia Liebert, Christian Schilling
Arnold Sommerfeld Centre for Theoretical Physics, LMU Munich
Munich Centre for Quantum Science and Technology (MCQST)

Based on a generalization of Hohenberg-Kohn's theorem, we propose a ground state theory for bosonic quantum systems. Since it involves the one-particle reduced density matrix as a variable but still recovers quantum correlations in an exact way it is particularly well suited for the accurate description of Bose-Einstein condensates. We derive the universal functional for dilute, homogenous Bose gases in three dimensions based on a particle-number conserving Bogoliubov theory. Remarkably, its gradient is found to diverge in the regime of complete condensation, providing a comprehensive explanation for the absence of complete condensation in nature.

Towards High-Precision Spectroscopy of the 1S-2S Transition in He+

Authors: Fabian Schmid (1), Akira Ozawa (1), Johannes Weitenberg (1), Theodor W. Hänsch (1, 2), and Thomas Udem (1, 2)
(1) Max Planck Institute of Quantum Optics, Garching, Germany
(2) Ludwig Maximilian University, Munich, Germany

Precise tests of a physical theory require a system whose properties can be both measured and calculated with very high precision. One famous example is the hydrogen atom which can be precisely described by bound-state quantum electrodynamics (QED) and whose transition frequencies can be accurately measured by laser spectroscopy. By comparing the experimental data to theory, fundamental constants, in particular the Rydberg constant and the nuclear charge radius, can be determined and the consistency of QED itself can be tested.
We are currently setting up an experiment to perform spectroscopy on the narrow 1S-2S two-photon transition in the simplest hydrogen-like ion, He+. Due to their charge, He+ ions can be held near-motionless in the field-free environment of a Paul trap, providing ideal conditions for a high precision measurement. By combining the 1S-2S transition frequency with an accurate value of the helium nuclear charge radius measured by muonic helium spectroscopy, we will be able to make an independent determination of the Rydberg constant that can be compared with the value obtained from hydrogen spectroscopy. Furthermore, interesting higher-order QED corrections scale with large exponents of the nuclear charge, making this measurement much more sensitive to these corrections compared to the hydrogen case.
The main challenge of the experiment is that driving the 1S-2S transition in He+ requires narrow-band radiation at 61 nm. This lies in the extreme ultraviolet (XUV) spectral range where no transparent solids and no cw laser sources exist. Our approach is to use two-photon direct frequency comb spectroscopy with an XUV frequency comb which is generated from an infrared high power frequency comb using intracavity high harmonic generation. The spectroscopy target will be a small number of He+ ions which are trapped in a linear Paul trap and sympathetically cooled by co-trapped Be+ ions.

A random unitary circuit model for black hole evaporation

Authors: Lorenzo Piroli*, Christoph Sünderhauf*, Xiao-Liang Qi (* contributed equally)
Max Planck Institute ofQuantum Optics

Inspired by the Hayden-Preskill protocol for black hole evaporation, we consider the dynamics of a quantum many-body qudit system coupled to an external environment, where the time evolution is driven by the continuous limit of certain 2-local random unitary circuits. We study both cases where the unitaries are chosen with and without a conserved U(1) charge and focus on two aspects of the dynamics. First, we study analytically and numerically the growth of the entanglement entropy of the system, showing that two different time scales appear: one is intrinsic to the internal dynamics (the scrambling time), while the other depends on the system-environment coupling. In the presence of a U(1) conserved charge, we show that the entanglement follows a Page-like behavior in time: it begins to decrease in the middle stage of the “evaporation”, and decreases monotonically afterwards. Second, we study the time needed to retrieve information initially injected in the system from measurements on the environment qudits. Based on explicit numerical computations, we characterize such time both when the retriever has control over the initial configuration or not, showing that different scales appear in the two cases.

The poster is based on JHEP 2020, 63 (2020), https://doi.org/10.1007/JHEP04(2020)063

Multiloop Functional Renormalization Group for Frustrated Quantum Magnets

Authors: Julian Thönniß, Fabian B. Kugler, Jan von Delft, Matthias Punk
LMU Munich

We present a multiloop pseudofermion functional renormalization group (mfRG) approach to study models of quantum magnets in arbitrary dimensions.

Our multiloop approach promises various advantages over previous pseudofermion fRG studies, most notably restoring independence from the choice of infrared regulator.

As a benchmark case, we present results for the spin-$\tfrac{1}{2}$ Heisenberg model on the kagome lattice, a prime example of a geometrically frustrated magnet believed to host a quantum spin-liquid phase.

We expect mfRG to be particularly useful for the study of frustrated quantum magnets in three dimensions, where hardly any numerically reliable methods for studying large system sizes are available.

Pulsed Auger Electron Detected Magnetic Resonance in Phosphorus-Doped 28Silicon (AEDMR)

Authors: David Vogl, Paul Steinacker, Martin S. Brandt
Walter Schottky Institut and Physik Department, Technische Universität München

Chemical purification and isotope engineering allow to obtain Silicon crystals effectively containing no background spins. This makes 28Si or 30Si an ideal material for spin-based quantum storage and coherence times of tens of minutes even at room temperature have already been demonstrated for ionized donors in 28Si. We set up and investigate a technique for the initialization, manipulation, and readout of electron and nuclear spins of shallow Phosphorus donors in 28Si utilizing the spin-dependent and resonant excitation of donor-bound excitons and their successive Auger decay and detecting the Auger electrons capacitively. This method was first implemented by Thewalt and coworkers. Here, we explore this spin system in greater detail and, e.g., observe a driven decay in the electron spin Rabi oscillation and are able to selectively generate spin hyperpolarization of the 31P nuclei via cross relaxation. This technique is also applicable to donors with nuclear spin higher than 1/2, which opens up the possibility of examining the quadrupolar interaction in more detail.

Detecting topology through dynamical experiments

Authors: Andreas Haller (Uni Mainz), Pietro Massignan, Matteo Rizzi

In our recent work https://arxiv.org/abs/2001.09074 (accepted in PR Research), we consider a particular protocol to read-out the winding-number of arbitrary 1D chiral models based on tracking the time-evolved density of an excitation. We show that a readily observable quantity named mean chiral displacement (MCD) gives direct access to the winding number of a non-interacting Fermi sea, and study the behavior of the MCD in interacting fermionic Su-Schrieffer-Heeger (SSH) wires by means of Matrix Product States simulations: If the wires display short-range correlations only, the MCD is shown to provide a faithful readout of the corresponding topological phase diagram. When longer-range correlations appear, the corresponding phase diagram contains trivial insulator, topological insulator, and a symmetry-breaking phase and the time-traces of the MCD are considerably different in each of the three phases thus providing a reliable tool to easily read-out the underlying phases of matter.

Many-body localization with synthetic gauge fields in disordered Hubbard chains

Author: Kuldeep Suthar(1), Piotr Sierant (1,2), and Jakub Zakrzewski (1,3)
1. Institute of Theoretical Physics, Jagiellonian University in Krakow, Łojasiewicza 11, 30-348 Kraków, Poland
2. ICFO- Institut de Sciences Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
3. Mark Kac Complex Systems Research Center, Jagiellonian University in Krakow, Łojasiewicza 11, 30-348 Kraków, Poland

The phenomenon of many-body localization (MBL) is attracting significant theoretical and experimental interest over the past few years. The signatures of MBL have been observed in recent cold-atom experiments in optical lattices. The recent experimental advances of synthetic gauge field allow to explore the MBL with magnetic flux. We discuss the role of synthetic magnetic field on the localization properties of disorderd fermions. The spectral statistics exhibits a transition from ergodic to MBL phase, and the transition shifts to larger disorder strengths with increasing magnetic flux. The dynamical properties indicate the charge excitation remains localized whereas spin degree of freedom delocalizes in the presence of synthetic flux. The full localization of spin excitation can be recovered when spin-dependent disorder potential is realized. Furthermore, we show the effect of quantum statistics on the local correlations and show that the long-time spin oscillations of a hard-core boson system are destroyed as opposed to the fermionic case.

Spin shuttling in a silicon double quantum dot

Authors: Florian Ginzel (1), Adam R. Mills (2), Jason R. Petta (2) and Guido Burkard(1)
1 Department of Physics, University of Konstanz, D-78457 Konstanz, Germany
2 Department of Physics, Princeton University, Princeton, New Jersey 08544, USA

The transport of quantum information between different nodes of a quantum device is among the challenging functionalities of a quantum processor. In the context of spin qubits, this requirement can be met by coherent electron spin shuttling between semiconductor quantum dots. Here we theoretically study a minimal version of spin shuttling between two quantum dots. To this end, we analyze the dynamics of an electron during a detuning sweep in a silicon double quantum dot (DQD) occupied by one electron. Possibilities and limitations of spin transport are investigated. This research is motivated both by the demand for long and intermediate range interactions in quantum information devices and by recent experimental progress [1,2]. Spin-orbit interaction and the Zeeman effect in an inhomogeneous magnetic field play an important role for spin shuttling and are included in our model. Interactions that couple the position, spin and valley degrees of freedom open a number of avoided crossings in the spectrum allowing for diabatic transitions and interfering paths. The outcomes of single and repeated spin shuttling protocols are explored by means of numerical simulations and an approximate analytical model based on the solution to the Landau-Zener problem. We find that fast high-fidelity spin-shuttling is feasible for optimal choices of parameters or protected by constructive interference. Relying on destructive interference between different paths the DQD can also act as a spin or valley filter.

[1] T. Fujita et al., npj Quantum Information 3, 22 (2017)
[2] A. R. Mills et al., Nature Communs 10, 1063 (2019)

Diagrammatic Monte Carlo Methods for the Fermi Polaron

Authors: Stefan Wittlinger (LMU Munich, Germany) , Andrey Mishchenko (RIKEN, Japan), Lode Pollet (LMU Munich, Germany)

We present an improved Monte Carlo algorithm for calculations of the Fermi polaron, an impurity immersed into a Fermi bath. In general, Monte Carlo simulations of the Fermi polaron problem suffer from the fermionic sign problem. For our scheme, we find a wide parameter range with a tractable sign problem. The perturbative expansion of the interaction is sampled using the diagrammatic determinant quantum Monte Carlo algorithm. The sampling is done in imaginary time, which might require analytical continuation. The scheme works at finite and zero temperature. For finite temperature, we present results for the mobility. For zero temperature, we present results for the ground state energies.

A passive, heralded quantum memory with single atoms in crossed fiber cavities

Authors: Pau Farrera, Dominik Niemietz, Manuel Brekenfeld, Gianvito Chiarella, Joseph Dale Christesen, and Gerhard Rempe
Max Planck Institute of Quantum Optics

Recent experimental advancement in the field of optical cavity QED comprises two directions of development. The first one consists on a further reduction of the mode volume of the resonators, as it is possible with the introduction of fiber based Fabry-Perot cavities [1]. The second one consists on an increase in the number of well-controlled modes the photon emitters can couple to [2,3].
We have set up a new experiment that combines these two experimental advancements in a single platform with single neutral atoms trapped at the center of two crossed fiber cavities. This novel setup provides new challenges and capabilities, such as the fabrication and assembling of high-finesse fiber cavities, the strong coupling of single atoms to both cavity modes for long trapping times, the atom imaging system, or the microwave manipulation of the atomic states.
Some of the mentioned capabilities were recently used to implement a passive, heralded quantum memory for photonic polarization qubits. The passive nature of the memory scheme requires neither amplitude- and phase-critical control fields [4] nor feedback loops that can be prone to errors [5]. The storage heralding capability is an important feature in the presence of photon loss, and improves the fidelity of the qubit storage.
In the future this novel platform will enable the development of other new quantum information processing schemes based on two-mode cavity QED.

[1] Hunger et al., New J. Phys. 12, 065038 (2010)
[2] Leonard et al., Nature 543, 87 (2017)
[3] Hamsen et al., Nat. Phys. 14, 885 (2018)
[4] Specht et al., Nature 473, 190 (2011)
[5] Kalb et al., Phys. Rev. Lett. 114, 220501 (2015)

Spin-Resolved Fermi Gas Microscopy

Authors: Sarah Hirthe (MPQ), S. Hirthe, D. Bourgund, P. Sompet, J. Koepsell1, T. Chalopin, J. Vijayan, P. Bojovic, G. Salomon, C. Gross, I. Bloch

The Fermi-Hubbard model is believed to capture the essential ingredients of many phenomena in high-Tc superconducting materials such as the cuprates, and yet its phases emerging upon doping are notoriously hard to compute numerically. In our setup we use ultracold fermionic lithium in an optical lattice to realize highly controllable Fermi-Hubbard systems. The simultaneous detection of spin and charge in our single-site resolved quantum gas microscope allows us to access almost arbitrary spin- and density-correlations in these systems.
We observe antiferromagnetic spin order in the Mott-insulating regime both in one- and two-dimensional systems. In the one-dimensional system a dopant leads to the formation of a domain wall of the antiferromagnetic order. Upon doping of the system we find an incommensurate spin-density wave with a wave vector that does not match the reciprocal lattice. In two-dimensional systems single dopants do not lead to domain walls. Instead a competition between magnetic and kinetic energy arises, leading to the formation of a magnetic polaron. We have identified such polarons by detecting reversed magnetic correlations locally around doublons. When varying the doping even further, we see how the polaron picture starts breaking down at around 20% doping. At the same time a strong singlet character emerges in the spin-environment of two close-distance hole-pairs.

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