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.

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.

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.

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."

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.

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.

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.

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

The accurate characterization of nonequilibrium strongly-correlated quantum systems has been a longstanding challenge in many-body physics. Notable among them are quantum impurity models, which appear in various nanoelectronic and quantum computing applications. Despite their seeming simplicity, they feature correlated phenomena, including emergent energy scales and non-Fermi-liquid physics, requiring renormalization group treatment. This has typically been at odds with the description of their nonequilibrium steady-state under finite bias, which exposes their nature as open quantum systems. We present a novel numerically-exact method for obtaining the nonequilibrium state of a general quantum impurity coupled to metallic leads at arbitrary voltage or temperature bias, which we call "RL-NESS" (Renormalized Lindblad-driven NonEquilibrium Steady-State). It is based on coherently coupling the impurity to discretized leads which are treated exactly. These leads are furthermore weakly coupled to reservoirs described by Lindblad dynamics which impose voltage or temperature bias. Going beyond previous attempts, we exploit a hybrid discretization scheme for the leads together with Wilson's numerical renormalization group, in order to probe exponentially small energy scales. The steady-state is then found by evolving a matrix-product density operator via real-time Lindblad dynamics, employing a dissipative generalization of the time-dependent density matrix renormalization group. In the long-time limit, this procedure converges to the steady-state at finite bond dimension due to the introduced dissipation, which bounds the growth of entanglement. We test the method against the exact solution of the noninteracting resonant level model. We demonstrate its power using an interacting two-level model, for which it correctly reproduces the known limits, and gives the full I-V curve between them.

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.

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.

Authors: Janis Nötzel, Stephen DiAdamo
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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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%.

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)

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.

Authors: Pablo Antonio Moreno Casares & Miguel Ángel Martín-Delgado
Universidad Complutense de Madrid

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.

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.

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.

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

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.

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.

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"

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.

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)"

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.

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.

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)

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)

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

Atom-like emitters coupled to a cavity are renowned for enabling experimental access to study and manipulate quantum systems. Here, we explore the extension of cavity quantum electrodynamics to room temperature solid state devices. A promising new approach to get improved access to room temperature quantum dynamics of Nitrogen vacancy (NV) centres is coupling them to a cavity [1,2]. In our experiment, we couple an ensemble of NV centres to a high-quality dielectric resonator. NV centres in diamond belong to the best studied room temperature solid state quantum systems since they provide long spin coherence times and offer access to both optical and microwave transitions. However, the standard optically detected magnetic resonance (ODMR) technique for NV spin state read-out suffers from a low collection efficiency and is destructive. Thus, we try to extend the ODMR detection using a high-quality dielectric resonator as a cavity. The cavity is then read out in a dispersive way using the microwave photons coupled to the collective NV spin motion. First, we characterize and optimize different resonator combinations, where we can measure resonator quality factors Q up to 5000 with a centre frequency at around 3 GHz. Then we investigate dispersive phase shifts of the microwave signals reflected from the NV-loaded cavity. Finally, we propose a pulsed microwave manipulation scheme to read-out and manipulate the NV spins. A dispersive access to room temperature spin qubits could improve many quantum devices such as sensors or memory registers for communication. We could imagine that our methods can soon be applied directly to develop and built parts of a room temperature quantum signal analyser driven by dynamical decoupling pulses on the NV cavity states.

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

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.

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).

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.

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)

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.

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)

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.