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Fri ** May 20th****, 2022 - 11.00 am**

ExQM Seminar via Zoom by Max Bramberger on `Cellular Dynamic Mean Field Theory Study of Tetragonal CuO´

ExQM Seminar via Zoom by Max Bramberger on `Cellular Dynamic Mean Field Theory Study of Tetragonal CuO´

On** Fri May 20th, 2022 – 11.00 am **via zoom

**Max Bramberger** will give an ExQM talk on

*`Cellular Dynamic Mean Field Theory Study of Retragonal CuO**´*

*The treatment of macroscopic systems of strongly correlated electrons*

is a very challenging problem as the non-local kinetic part of the

Hamiltonian competes with local interaction effects.

Dynmical Mean Field Theory (DMFT) has over the last decades become the

state of the art method to treat these kind of problems. However as

DMFT maps the problem onto a single impurity non-local correlation

effects are ignored.

The first part of this talk aims to give an intuitive introduction to

the concept of Cellular Dynamical Mean Field Theory (CDMFT), an

extension of DMFT that allows for the treatment of short ranged

correlations and thereby systematically improves upon DMFT.

In the second part we want to present results obtained in the context

of our recent application of CDMFT to the tetragonal phase of CuO, an

exotic 2D material, that was experimentally found to consist of weakly

coupled CuO2 sublattices. We make explicit use of the CDMFT formalism

and investigate the effect short ranged correlations have on the

decoupling of those sublattices.

Fri ** May 6th****, 2022 - 11.00 am**

ExQM Seminar via Zoom by Frederik Bopp on `Electric Field Tunable Transition Dipole Moments and Selective Optical Charging of a Single QD Molecule´

ExQM Seminar via Zoom by Frederik Bopp on `Electric Field Tunable Transition Dipole Moments and Selective Optical Charging of a Single QD Molecule´

On** Fri May 6th, 2022 – 11.00 am **via zoom

**Frederik Bopp** will give an ExQM talk on

*`**Electric Field Tunable Transition Dipole Moments and Selective Optical **Charging of a Single QD Molecule**´*

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.

For pairs of vertically stacked QDs, forming a QD-molecule (QDM), these single dot properties are retained. Additionally, interdot charge tunneling opens the way to enhanced spin coherence times (T_{2}^{*}) compared so single dots [1] and to use single QDMs as an interface for the deterministic generation of 2D photonic cluster states [2]. To use single QDMs as flexible spin-photon interfaces, the ability to prepare one and two spin states and electrically gate the spin-spin coupling is desirable.

Here, we realize devices consisting of a single QDM embedded within the intrinsic region of a ~10 x 25 µm micro-photodiode The efficiency of the spin-photon coupling is enhanced using an annular Bragg grating placed around the QDM. Such approaches enhance the photon extraction efficiency to >20% over a >7 nm bandwidth [3], whilst having capacitance that is small enough (<200 fF) to facilitate ultrafast (>500 MHz) electro-optical switching of the orbital tunnel couplings. Our devices demonstrate optical control of the QDM charge status and electronic control of tunnel coupling. Figure 1 shows typical field dependent Rabi oscillations recorded using resonance fluorescence from direct and indirect exciton branches. The exciton optical dipole moment can be electrically tuned from 4.8×10^{-27} – 1.4×10^{-26} Cm, as the orbital state admixtures in the two dots are tuned. Comparison with the detuning dependent exciton decay lifetimes, elucidates the interplay between radiative recombination and phonon induced relaxation between direct and indirect exciton branches as interdot tunnel coupling is tuned. Finally, we demonstrate selective, all optical electron spin preparation in the QDM by resonantly exciting an e-h pair and allowing the hole to tunnel escape, leaving the electron trapped.

Optical electron spin preparation and the ability to control interdot orbital couplings provides a perspective to use our devices for the deterministic generation of 2D cluster states.

**References**

[1] K. M. Weiss et al., Phys. Rev. Lett. 109, 107401 (2012)

[2] S. E. Economou et al., Phys. Rev. Lett. 105, 093601 (2010)

[3] J. Schall et al. Adv. Quant. Tech., 4, 6, (2021)

German Physics Society Spring Meeting in Heidelberg (MP)

Online Conference hosted by DPG with contributions by ExQM in Mathematical Physics (MP) Section.

German Physics Society Spring Meeting in Erlangen (SAMOP)

Online Conference hosted by DPG with contributions by ExQM in Atoms, Molecules, Quantum Optics and Photonics (SAMOP) Section .

Fri ** Feb 11th****, 2022 - 11.00 am**

ExQM Seminar via Zoom by Eugenio Pozzoli (INRIA, Paris & U. Bourgogne) on `Controlling Closed Quantum Systems: Multipolarization, Perturbation, and Application to Molecular Rotation´

ExQM Seminar via Zoom by Eugenio Pozzoli (INRIA, Paris & U. Bourgogne) on `Controlling Closed Quantum Systems: Multipolarization, Perturbation, and Application to Molecular Rotation´

On** Fri Feb 11th, 2022 – 11.00 am **via zoom

**Eugenio Pozzoli** (INRIA, Paris & U. Bourgogne) will give an ExQM talk on

*` Controlling Closed Quantum Systems: Multipolarization, Perturbation, and Application to Molecular Rotation´*

The control of quantum systems is a fundamental problem in physics, chemistry and quantum information. In an ideal setting, we model the evolution of the system via its wave function that follows a bilinear Schroedinger equation in some infinite-dimensional Hilbert space. The control is represented by an external field interacting with the dipole of the system.

This description includes systems such as qubits, harmonic oscillators, rotovibrating molecules, and many others. The controllability problem is the capability of driving an initial wave function to a target one, by suitably choosing the external field. Several questions may be addressed, for example: (i) can the system be steered everywhere in the state space? (ii) What is the shape of a control needed to achieve a transfer of population? (iii) What is the minimal time required for the transfer? In this talk we first present two abstract algebraic and perturbative criteria to answer (i) and (ii).

The techniques are based on tools of dynamical systems on Lie groups combined with analysis of Hermitian operators. We then apply the results to rotating molecules as prototypes of degenerate infinite-dimensional quantum systems. We also present an application to the problem of enantiomer-selection in chiral molecules.

This is a series of collaborations with Ugo Boscain, Mario Sigalotti, Monika Leibscher, Christiane Koch, and Thomas Chambrion.

Fri ** Jan 28th****, 2022 - 12.30 am**

ExQM Seminar via Zoom by Emanuel Malvetti on `The Structure of the GKS-Lindblad Equation´

ExQM Seminar via Zoom by Emanuel Malvetti on `The Structure of the GKS-Lindblad Equation´

On** Fri Jan 28th, 2022 – 12.30 am **via zoom

**Emanuel Malvetti** will give an ExQM talk on

*`The Structure of the GKS-Lindblad Equation‘*

The Hamiltonian time evolution of an isolated quantum system has a simple structure, given by the eigendecomposition of the Hamiltonian. We will recall this in some detail before presenting a generalization to open quantum systems. Some results hold for arbitrary dynamical semigroups of completely positive maps, but the special case of the GKS-Lindblad equation has a much richer structure. Finally we illustrate how this information can be applied to control problems with a fixed GKS-Lindblad generator and an arbitrary control Hamiltonian.

Fri ** Jan 21th****, 2022 - 11.00 am**

ExQM Seminar via Zoom by Qiming Chen on `Quantum Behaviours of Duffing Oscillators at Dissipative Phase Transitions´

ExQM Seminar via Zoom by Qiming Chen on `Quantum Behaviours of Duffing Oscillators at Dissipative Phase Transitions´

On** Fri Jan 21th, 2022 – 11.00 am **via zoom

**Qiming Chen** will give an ExQM talk on

*`Quantum Behaviours of Duffing Oscillators at Dissipative Phase Transitions‘*

Understanding the indeterministic phenomena in a deterministic nonlinear system has been an implicit dream since Lorenz named it the „butterfly effect“. A prominent example is the hysteresis and bistability of a Duffing oscillator, which is traditionally attributed to the competition between two coexisting steady states localized in a double-well potential. However, this interpretation fails in a quantum-mechanical perspective where only a single unique steady state is allowed in the whole parameter space. Here, we measure the non-equilibrium dynamics of a superconducting Duffing oscillator and reconcile the classical and quantum theories in a unified picture. We demonstrate that the classically regarded steady states are metastable, which have a remarkably long lifetime but should eventually relax to the unique steady state allowed by quantum mechanics. By engineering the metastability lifetime increasingly large, we observe a first-order dissipative phase transition which mimics a sudden change of the photon density in a 11-site Bose-Hubbard lattice. We also reveal the two phases of the transition, i.e., a coherent phase and a squeezed phase, by quantum state tomography. Our results provide a comprehensive understanding of the Duffing oscillator. They form an essential step towards understanding the critical and intriguing effects in non-equilibrium systems.

Fri ** Jan 14th****, 2022 - 11.00 am**

ExQM Seminar via Zoom by Frederik vom Ende on `The Geometry of Entanglement´

ExQM Seminar via Zoom by Frederik vom Ende on `The Geometry of Entanglement´

On** Fri Jan 14th, 2022 – 11.00 am **via zoom

**Frederik vom Ende** will give an ExQM talk on

*`The Geometry of Entanglement‘*

In an attempt to understand entanglement in a more geometric way, we collect topological facts about separable and entangled states and try to combine them into a single image. More precisely, we first investigate path-connectedness, denseness, and relative dimensons for the pure-state case, before doing the same — and more, such as interior points and semigroup properties — for general mixed states.

While we will show that no three-dimensional picture can illustrate all the geometric and topological facts we collect, we still manage to include as many as (mathematically) possible in a visual representation of how entanglement „looks like“ as a subset of all density matrices.

ENB Graduation Virtual Ceremony

**Maximilian Buser **receives his honours document.

2nd European Quantum Technologies Virtual Conference EQCT

Online Conference hosted by the European Quantum Flagship via University of Galway, Ireland with contributions by ExQM.

ExQM Seminar via Zoom by Christian Arenz on `Progress Toward Favorable Landscapes in Quantum Combinatorial Optimization´

On **Tue Oct 5th, 2021 – 11.00 am **via zoom

**Christian Arenz **(Princeton University) will give an ExQM talk on

*`Progress Toward Favorable Landscapes in Quantum Combinatorial Optimization‘*

The performance of variational quantum algorithms relies on the success of using quantum and classical computing resources in tandem. In this talk, I explore how these quantum and classical components interrelate. In particular, I focus on algorithms for solving the combinatorial optimization problem MaxCut, and study how the structure of the classical optimization landscape relates to the quantum circuit used to evaluate the MaxCut objective function.

In order to analytically characterize the impact of quantum features on the landscape critical point structure, I consider a family of quantum circuit ansätze composed of mutually commuting elements. I identify multiqubit operations as a key resource, and show that overparameterization allows for obtaining favorable landscapes. Namely, I prove that an ansatz from this family containing exponentially many variational parameters yields a landscape free of local optima for generic graphs.

However, I further show that these ansätze do not offer superpolynomial advantages over purely classical MaxCut algorithms. I then present a series of numerical experiments illustrating that non-commutativity and entanglement are important features for improving algorithm performance.

Based on *Phys. Rev. A* **104** (2021), 032401.

DPG Physics Autumn Meeting

Online Conference hosted by German Physics Society and University of Jena.

Munich (Virtual) Conference on Quantum Science and Technology (QST)

3D Online Conference hosted by the *Munich Centre of Quantum Science and Technology* (MCQST) and taking place in a virtual model of „Deutsches Museum“.

ExQM Seminar via Zoom by Emanuel Malvetti on `Markovian Quantum Systems with Fast and Full Hamiltonian Control´

On **Fri Jul 9th, 2021 – 11.00 am **via zoom

**Emanuel Malvetti **will give an ExQM talk on

*` Markovian Quantum Systems with Fast and Full Hamiltonian Control‚*

Markovian quantum systems, described by a Lindblad equation, with fast and full Hamiltonian control can effectively be reduced to a control system on the eigenvalues of the density matrix of the state. We will illustrate this procedure explicitly for the qubit where optimal control problems can in some cases be solved explicitly. Then we state the main control theoretic result for arbitrary quantum systems and show some applications. Finally we sketch a generalization of this result to the abstract setting of symmetric Lie algebras.

ExQM Seminar via Zoom by Prof. Kavan Modi (Monash U., Melbourne) on `Markovianization: How Quickly Does Nature Forget?´

On **Fri Jun 25th, 2021 – 10.30 am **via zoom

**Prof. Kavan Modi **will give an ExQM talk on

*` Markovianization: How Quickly Does Nature Forget?‚*

Memoryless processes are ubiquitous in nature, in contrast with the mathematics of open systems theory, which states that non-Markovian processes should be the norm.

This discrepancy is usually addressed by subjectively making the environment forgetful. Here we prove that there are physical non-Markovian processes that with high probability look highly Markovian for all orders of correlations; we call this phenomenon Markovianization. Formally, we show that when a quantum process has dynamics given by an approximate unitary design, a large deviation bound on the size of non-Markovian memory is implied.

We exemplify our result employing an efficient construction of an approximate unitary circuit design using only two-qubit interactions, showing how seemingly simple systems can speedily become forgetful. Conversely, since the process is closed, it should be possible to detect the underlying non-Markovian effects. However, for these processes, observing non-Markovian signatures would require highly entangling resources and hence be a difficult task.

International (Zoom) Conference: 52th Symposion on Mathematical Physics (Virtually) in Torun

The 52th SMP focusses on „*Channels, Maps and All That*„. It specially commemorates the late **Andrzej Kossakowski.**

ExQM Seminar via Zoom by Markus Hasenöhrl on `On the Generators of Dynamical Semigroups of Superchannels and Semicausal Channels´

On **Fri Mar 5th, 2021 – 11.00 am **via zoom

**Markus Hasenöhrl** will give an ExQM talk on

*` On the Generators of Dynamical Semigroups of Superchannels and Semicausal Channels‚*

A superchannel is a linear map that maps quantum channels to quantum channels, while satisfying suitable consistency relations. If the input and output quantum channels act on the same space, then the composition of two superchannels is again a superchannel. Hence, superchannels form a semigroup and one can look at continuous one-parameter semigroups of superchannels and their generators.

In our work, we characterize these generators in two ways: firstly we find a computationally efficiently checkable criterion, for when a given map generates a dynamical semigroup of superchannels; and secondly we find a normal form (like the Lindblad form) for these generators, which allows us to enumerate all generators and also to study analytic properties ofthe corresponding evolution equation.

ExQM Seminar via Zoom by Maximilian Buser on `Probing Quantum Phases and the Hall Response in Bosonic Flux Ladders´

On **Fri Feb 12th, 2021 – 11.00 am **via zoom

**Maximilian Buser** will give an ExQM talk on

*`Probing Quantum Phases and the Hall Response in Bosonic Flux Ladders‘*

The focus of this talk is on bosonic flux ladders. First, we touch on a model which is envisioned to be realized in a future quantum gas experiment exploiting the internal states of potassium atoms as a synthetic dimension. Considering specifics of the future experiment, we map out the ground-state phase diagram and report on Meissner and biased-ladder phases. We show that quantum quenches of suitably chosen initial states can be used to probe the equilibrium properties of the dominant ground-state phases.

Second, we concentrate on the Hall response. While flux ladders are the most simple lattice models giving rise to the Hall effect, the theoretical description of the ground-state Hall response in these systems remains a tricky problem and an active line of research. We discuss feasible schemes to extend measurements of the Hall polarization to a study of the Hall voltage, allowing for direct comparison with solid state systems. Most importantly, we report on characteristic zero crossings and a remarkable robustness of the Hall voltage with respect to interaction strengths, particle fillings, and ladder geometries, which is unobservable in the Hall polarization.

International Conference: Quantum Information Processing (QIP)

The 24th QIP is hosted and chaired by **Profs. Michael Wolf **and** Robert König.**

ExQM Seminar via Zoom by Till Klostermann on `Fast, Long Distance Optical Transport of Cs´

On **Fri Jan 15th, 2021 – 11.00 am **via zoom

**Till Klostermann** will give an ExQM talk on

*`Fast, Long Distance Optical Transport of Cs‘*

As experimental setups for quantum gas experiments become more complicated, access around the experimental chamber for lasers, microscopes or magnetic field coils becomes a limiting factor. One way to address this issue is optical transport, to spacially separate the pre-cooling stages from the place of the final experiment.

I present our implementation of such an optical transport scheme, using a running wave optical lattice. Using a Bessel beam and a double pass AOM setup to control the laser frequencies, we manage to transport our atoms over a distance of 43 cm in less than 26 ms. We measured the dependence of the transport efficiency on laser power and acceleration and find that it is predominantly limited by the trap depth. After transport we produce a pure Bose-Einstein condensate of 20.000 atoms.

ExQM Seminar via Zoom by Max Bramberger on `Dynamical Mean Field Theory with Matrix Product States´

On **Fri Dec 11th, 2020 – 10.30 am **via zoom

**Max Bramberger** (group Prof. Schollwöck) will give his inaugural ExQM talk on

*`Dynamical Mean Field Theory with Matrix Product States‘*

When electrons become strongly correlated there is no straightforward way of treating macroscopic systems, as the local interaction between electrons competes with the non-local band structure effects.

In the last decades Dynamical Mean Field Theory (DMFT) has proven to be an appropriate method to treat both these effects. It approximates the problem by embedding an interacting impurity in a bath of non-interacting fermions.

The first part of this talk aims at explaining DMFT from a theoretical

point of view, as well as at introducing the algorithmic implementation using Matrix Product States (MPS) as an impurity solver. The second part gives an introduction to a recent application of the method to BaOsO3, a transition metal oxide in which Hund’s coupling and spin-orbit coupling as well as the van Hove singularity play a fundamental role.

ExQM Seminar via Zoom by Adomas Baliuka on `Post-Processing for Discrete Variable Quantum Key Distribution´

On **Fri Dec 4th, 2020 – 10.30 am **via zoom

**Adomas Baliuka** (group Prof. Weinfurter) will give his inaugural ExQM talk on

*`Post-Processing for Discrete Variable Quantum Key Distribution‘*

Two communicating parties can use quantum key distribution (QKD) to establish a shared secret key: First, they exchange quantum signals and measure them. Second, they perform post-processing, during which the secret key is extracted from the measurement results by making use of classical communication.

Key tasks of post-processing are authentication of the communication, error reconciliation and privacy amplification.

One solution to the error reconciliation problem is distributed source coding using low-density parity-check codes. Their decoding via iterative message passing algorithms is an example of inference on probabilistic graphical models. In our work, we further develop such code constructions and decoding algorithms suited for application-oriented QKD systems.

ExQM Seminar via Zoom by

On **Fri Nov 27th, 2020 – 10.30 am **via zoom

**Dr Frederik vom Ende** will give an extended version of his PhD-defense talk on

*`**Reachability in Controlled Markovian Quantum Systems: **An Operator-Theoretic Approach**‚*

In quantum systems theory one of the fundamental problems boils down to: Given an initial state, which final states can be reached by the dynamic system in question? While for closed systems this is a well-studied question with a number of structural results, as soon as the system interacts with its environment this problem becomes pretty difficult to analyze.

In this talk we will settle on the intermediate case of switchable Markovian coupling of the system to the environment (e.g., a thermal bath of arbitrary temperature), a scenario relevant to certain physical setups. Assuming full unitary control of the uncoupled system, we prove the following three results:

1. Local switchable coupling of an arbitrary system of qudits to a temperature-zero bath allows to (approximately) generate every quantum state from every initial state.

2. For non-zero temperatures we give a non-trivial upper bound as a consequence of new results on d-majorization.

3. For infinite-dimensional systems, if the switchable noise term is generated by a single compact normal operator—which is a special case of a unital system—this allows to approximately reach any target state majorized by the initial state, as up to now only has been known in finite-dimensional analogues.

ExQM Distinguished-Guest Lecture via Zoom by

On **Fri Nov 20th, 2020 – 10.30 am **via zoom

**Prof. Dariusz Chruściński** will give a distinguished-speaker ExQM lecture on

*`On the Universal Constraints for Relaxation Rates for Quantum Dynamical Semigroups‘*

A conjecture for the universal constraints for relaxation rates of a quantum dynamical semigroup is proposed. It is shown that it holds for several interesting classes of semigroups, e.g., unital semigroups and semigroups derived in the weak coupling limit from the proper microscopic model.

Moreover, the conjecture proposed is supported by numerical analysis. This conjecture has further interesting implications: it allows to provide universal constraints for spectra of quantum channels and it provides a necessary condition to decide whether a given channel is consistent with Markovian evolution.

ExQM Seminar via Zoom by

On **Fri Nov 13th, 2020 – 10.30 am **via zoom

**Emanuel Malvetti** will give his inaugural ExQM seminar on

*`Optimal Cooling of Markovian Quantum Systems with Unitary Control‘*

We consider quantum systems described by Lindblad dynamics with unitary control. First we derive a master equation describing the evolution of the spectrum of the state, and give some polyhedral bounds on the achievable derivatives, leading to speed limits for the eigenvalues of the state. We characterize the Lindblad operators of systems that can always be asymptotically cooled to a pure state using unitary control, and systems for which any state can be reached from a pure state.

Then we show that the set of achievable derivatives of the spectrum has a subset of optimal derivatives for cooling, and we explore cooling strategies for some selected systems.

In particular we give an optimal cooling scheme for a system consisting of two spins, one of which decays at a fixed rate, and we present a cooling scheme for truncated spin-boson systems of arbitrary size.

ExQM Seminar via Zoom by

On **Fri July 17th, 2020 at 10.30 am **via zoom

**Bo Wang** will give an ExQM seminar on

*`Continuous Quantum Light from a Dark Atom‘*

Single photons can be generated from a single atom strongly coupled to a optical cavity via a stimulated Raman adiabatic passage between two atomic ground states [1]. During the generation of the photon, the atom stays within the dark state of electromagnetically induced transparency (EIT) avoiding spontaneous decay from the excited state.

In contrast to this well-know scenario, here we present the result to generate quantum light continuously from an atom in the dark state. A coherent coupling is added between the atomic ground states to allow the coherent generation of multiple photons. This would usually result in the destruction of the dark state and the reappearance of spontaneous decay.

However, the dark states of the strongly coupled cavity EIT result from the interference between two atomic ground states entangled with different photonic states [2]. Such dark states are preserved from the local coupling that is applied only within the atomic Hilbert space. Additionally, the nonlinearity of the system allows us to control the quantum fluctuations of the generated light via a quantum Zeno effect.

[1] Kuhn, A. et al., *Phys. Rev. Lett.* **89**, 067901 (2002)

[2] Souza, J.A. et al., *Phys. Rev. Lett.* **111**, 113602 (2013).

ExQM Seminar via Zoom by

On **Fri July 10th, 2020 at 10.30 am **via zoom

**Dr Bálint Koczor** will give an ExQM seminar on

*`Measurement Cost of Metric-Aware Variational Quantum Algorithms‘*

Variational quantum algorithms are promising tools for near-term quantum computers as their shallow circuits are robust to experimental imperfections. Their practical applicability, however, strongly depends on how many times their circuits need to be executed for sufficiently reducing shot-noise.

In my talk I will introduce metric-aware quantum algorithms which are variational algorithms that use a quantum computer to efficiently estimate both a matrix and a vector object. I will discuss in detail the recently introduced quantum natural gradient approach which uses the quantum Fisher information matrix as a metric tensor to correct the gradient vector for the co-dependence of the circuit parameters.

I will finally present our rigorous characterisation of the number of measurements required to determine an iteration step to a fixed precision, and propose a general approach for optimally distributing samples between matrix and vector entries. In particular, we establish that the number of circuit repetitions needed for estimating the quantum Fisher information matrix is asymptotically negligible for an increasing number of iterations and qubits.

This talk is based on the recent preprint arXiv:2005.05172 which is joint work with Barnaby van Straaten.

July 6-8th, 2020

Virtual Conference of Munich Centre for Quantum Science and Technology (MCQST)

Virtual Conference of Munich Centre for Quantum Science and Technology (MCQST)

MCQST Virtual-Conference Programme

Held via *meetanyway* due to covid-19.

ExQM Seminar via Zoom by

On **Fri July 3rd, 2020 at 10.30 am **via zoom

**Lukas Knips** will give an ExQM seminar on

*` How Random Measurements Can Reveal Entanglement‚*

In my talk, I’d like to present a method to use measurements in

arbitrary – and possibly even unknown – directions for detecting

entanglement.

Usually, quantum entanglement is revealed via a well aligned, carefully chosen set of measurements. Yet, under a number of experimental conditions, for example in communication within multiparty quantum networks, noise along the channels or fluctuating orientations of reference frames may ruin the quality of the distributed states.

In this talk and the corresponding paper [1] it is shown that even for strong fluctuations one can still gain detailed information about the state and its entanglement using random measurements. Correlations between all or subsets of the measurement outcomes and especially their distributions provide information about the entanglement structure of a state. We analytically derive an entanglement criterion for two-qubit states and provide strong numerical evidence for witnessing genuine multipartite entanglement of three and four qubits. Our methods take the purity of the states into account and are based on only the second moments of measured correlations.

Extended features of this theory are demonstrated experimentally with four photonic qubits. As long as the rate of entanglement generation is sufficiently high compared to the speed of the fluctuations, this method overcomes any type and strength of localized unitary noise.

[1] Knips, L., Dziewior, J., Kłobus, W. et al. Multipartite

entanglement analysis from random correlations. npj Quantum Inf 6, 51 (2020).

ExQM Seminar via Zoom by

On **Fri June 26th, 2020 at 10.30 am **via zoom

**Zoltán Zimborás** will give an ExQM seminar on

*`Fermionic Superselection Rules and the Concept of Orbital Entanglement and Correlation in Quantum Chemistry‘*

A recent development in quantum chemistry has established the quantum mutual information between orbitals as a major descriptor of electronic structure. This has already facilitated remarkable improvements of numerical methods and may lead to a more comprehensive foundation for chemical bonding theory.

Building on this promising development, our work provides a refined discussion of quantum information theoretical concepts by introducing the physical correlation and its separation into classical and quantum parts as distinctive quantifiers of electronic structure. In particular, we succeed in quantifying the entanglement. Intriguingly, our results for different molecules reveal that the total correlation between orbitals is mainly classical, raising questions about the general significance of entanglement in chemical bonding.

Our work also shows that implementing the fundamental particle number superselection rule, so far not accounted for in quantum chemistry, removes a major part of correlation and entanglement previously seen. In that respect, realizing quantum information processing tasks with molecular systems might be more challenging than anticipated.

Based on joint work including the Schollwöck group, see arXiv:2006.00961 .

ExQM Seminar via Zoom by

On **Fri June 5th, 2020 at 10.30 am **via zoom

**Frederik Bopp** will give an ExQM seminar on

*`**Towards Singlet-Triplet Qubits in Quantum-Dot Molecules**‚*

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 (T_{2}^{*} ~ 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 T_{2}^{*} 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]. However, these studies have also shown it is very challenging to *simultaneously* maintain the electric field needed to reach the sweet spot condition whilst simultaneously operating in the required charge stability condition, where the QDM is populated by two spins (electron or hole), one in each of the dots forming the QDM.

We present a different 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.

ExQM Seminar via Zoom by

On **Fri May 29th, 2020 at 10.30 am **via zoom

**David Castells Graells** will give his inaugural ExQM seminar on

*`**Tunable Enhanced Atom-Light Interaction Using Atomic Subwavelength Arrays‘*

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

ExQM Seminar via Zoom by

On **Fri May 15th, 2020 at 10.30 am **via zoom

**Frederik vom Ende** **will give** an ExQM seminar on

*`**The Role of Strict Positivity in Quantum Dynamics‘*

Motivated by quantum thermodynamics we investigate the notion of strict positivity, that is, linear maps which map positive definite states to something positive definite again.

We show that strict positivity is decided by the action on any full-rank state, and that the image of not-strictly positive channels—up to something unitary—lives inside a lower-dimensional block. This implies that such channels have maximal distance from the identity channel.

We use this to conclude that Markovian dynamics are strictly positive and investigate connections between strict positivity and other notions of divisibility.

ExQM Seminar by

On **Fri May 8th, 2020 at 10.30 am **via zoom

**Julian Roos** **will give** an ExQM seminar on

*`**Markovian Regimes in Quantum Many-Body Systems‘*

Long time evolution of the full state of quantum many body systems is generally out of reach due to build-up of entanglement. However, the computation of local observables only requires knowledge of the state of (small) subsystems. Is it possible to obtain a description of the reduced dynamics similarly to what is done in the fields of Quantum Optics and Open Quantum Systems (OQS)?

We expect that such an endeavour is most promising in the simplest case, i.e. when the dynamics are Markovian (memoryless), and we thus study if such regimes do also exist in a many body setup. Here, the conditions that allow for the derivation of a Markovian master equation in the theory of OQS (Born-Markov) are not satisfied.

In this talk I thus adopt a Quantum Information Theory perspective to identify interesting Markovian regimes (of a spin coupled to a XY spin chain) and explain the underlying physics responsible for Markovian/non-Markovian dynamics.

**You may wish to install www.zoom.us for preparation. Stay healthy!**

Next ExQM Seminar by

On **Fri Mar. 20th, 2020 in the Mathematics Building, 3rd floor Seminar Room 03.10.011 (Wolf group) at 10.30 am **[note room shift due to MPQ Corona-virus policy!]

**Julian Roos** **would have given** an ExQM seminar on

*`**Markovian Regimes in Quantum Many-Body Systems‘*

Long time evolution of the full state of quantum many body systems is generally out of reach due to build-up of entanglement. However, the computation of local observables only requires knowledge of the state of (small) subsystems. Is it possible to obtain a description of the reduced dynamics similarly to what is done in the fields of Quantum Optics and Open Quantum Systems (OQS)?

We expect that such an endeavour is most promising in the simplest case, i.e. when the dynamics are Markovian (memoryless), and we thus study if such regimes do also exist in a many body setup. Here, the conditions that allow for the derivation of a Markovian master equation in the theory of OQS (Born-Markov) are not satisfied.

In this talk I thus adopt a Quantum Information Theory perspective to identify interesting Markovian regimes (of a spin coupled to a XY spin chain) and explain the underlying physics responsible for Markovian/non-Markovian dynamics.

ExQM Seminar by

On **Tue Mar. 10th, 2020 in Chemistry Dept. (6th level in yellow section) Lecture Room CH 63.214 at 1.15 pm **

**Emanuel Malvetti** (ETH Zurich, Renner group) will give an ExQM seminar on

*`**Quantum Circuits for Sparse Isometries‘*

We consider the task of breaking down a quantum computation given as an isometry into C-Nots and single-qubit gates, while keeping the number of C-Not gates small. Although several decompositions are known for general isometries, here we focus on a method based on Householder reflections that adapts well in the case of sparse isometries.

We show how to use this method to decompose an arbitrary isometry before illustrating that the method can lead to significant improvements in the case of sparse isometries. We also discuss the classical complexity of this method and illustrate its effectiveness in the case of sparse state preparation by applying it to randomly chosen sparse states.

ExQM Seminar by

On **Fri Mar. 6th, 2020 in the Mathematics Building, 3rd floor Seminar Room 03.10.011 (Wolf group) at 10.30 am **[note room shift due to MPQ Corona-virus policy!]

**Nicola Pancotti** will give an ExQM seminar on

*`Quantum East Model: Localization, Non-Thermal Eigenstates*

*and Slow Dynamics‘*

We study in detail the properties of the quantum East model, an interacting quantum spin chain inspired by simple kinetically constrained models of classical glasses.

Through a combination of analytics, exact diagonalization and tensor network methods we show the existence of a fast-to-slow transition throughout the spectrum that follows from a localization transition in the ground state.

On the slow side, we explicitly construct a large (exponential in size) number of non-thermal states which become exact finite-energy-density eigenstates in the large size limit, as expected for a true phase transition.

A „super-spin“ generalization allows us to find a further large class of area-law states proved to display very slow relaxation.

Under slow conditions, many eigenstates have a large overlap with product states and can be approximated well by matrix product states at arbitrary energy densities.

We discuss implications of our results for slow thermalization and non-ergodicity more generally for quantum East-type Hamiltonians and their extension in two or higher dimensions.

ExQM Seminar by

On **Wed Feb. 19th** in** Chemistry Dept. (6th level in yellow section) Lecture Room CH 63.214 **at 10.30 am

**Nicolas Augier (CNRS, Paris)** will give a special ExQM seminar on

*`Results for the Ensemble Controllability of Quantum Systems´*

The principal issue that will be developed in this talk is how to control a parameter-dependent family of quantum systems with a common control input, that is, the ensemble controllability problem. Thanks to the study one-parametric families of Hamiltonians and their generic singularities when the system is driven by two real inputs, we will give an explicit adiabatic control strategy for the ensemble controllability problem when geometric conditions on the spectrum of the Hamiltonian are satisfied, in particular, the existence of conical or semi-conical intersections of eigenvalues.

Then, in order to understand which controllability properties can be extended to the case where the system is driven by a single real input, we will study the compatibility of the adiabatic approximation with the rotating wave approximation.

ExQM Seminar by

On **Wed Feb. 5th** in** MPQ Lecture Hall **at 2.00 pm

**Prof. Ugo Boscain (CNRS, Paris)** will give a special ExQM seminar on

*`Ensemble Control of Spin Systems´*

In this talk I will discuss how to control an ensemble of spin systems (depending on one parameter) using two controls. The technique is based on the adiabatic approximation and the presence of conical eigenvalue intersections. I will then discuss the compatibility of the rotating wave approximation (RWA) and of the adiabatic theory to realize this technique with only one control.

Show-Case Lecture by Prof. Bloch at 'CAS' Centre for Advanced Studies of LMU Munich

On **Thu Jan. 16th **in** the LMU Centre for Advanced Studies (CAS), Seestr. 13, 80802 Munich at 6.30 pm **

**Prof. Immanuel Bloch** will be giving a show-case lecture on the state-of-the-art of quantum simulation in optical lattices. Registration is recommended under info@cas.lmu.de .

ExQM Seminar by

On **Fri Jan. 10th** in** MPQ Lecture Hall **at 10.30 am

**Qiming** **Chen** will give his inaugural ExQM seminar on

*`Quantum Fourier Transform in Oscillating Modes´*

Quantum Fourier transform (QFT) is a key ingredient for 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 that uses two ancilla resonators to form a large dimensional Hilbert space for the realization of QFT. By employing the perfect state-transfer method, we map an unknown multi-qubit state to one resonator, and generate the QFT state in the second oscillator through cross-Kerr interaction and projective measurement. Quantitative analyses show that our method enables relatively high-dimensional and fully-quantum QFT in the state-of-the-art superconducting quantum circuits, which paves the way for implementing various QFT related quantum algorithms in the near future.

ENB Graduation Ceremony at 'Alte Kongresshalle' in Munich

**Moritz August, **** Anna-Lena Hashagen, Bálint Koczor, Lukas Knips, David Leiner, Stephan Welte, Jakob Wierzbowski **receive their honours documents during the ceremony.

ExQM Seminar by

On **Mon Dec. 9th** in** MPQ Lecture Hall B0.32 **at 10.30 am

**Dr Bálint Koczor** (now Oxford University) will talk on

*`Variational-State Quantum Metrology´*

Quantum metrology aims to increase the precision of a measured quantity that is estimated in the presence of statistical errors using entangled quantum states.

We present a novel approach for finding (near) optimal states for metrology in the presence of noise, using variational techniques as a tool for efficiently searching the classically intractable high-dimensional space of quantum states. We comprehensively explore systems consisting of up to 9 qubits and find new highly entangled states that are surprisingly not symmetric under permutations and non-trivially outperform previously known states up to a constant factor 2. We consider a range of environmental noise models; while passive quantum states cannot achieve a fundamentally superior scaling (as established by prior asymptotic results) we do observe a significant absolute quantum advantage.

We finally outline a possible experimental setup for variational quantum metrology which can be implemented in near-term hardware.

This talk is based on a joint work (arXiv:1908.08904) with Suguru Endo, Tyson Jones, Yuichiro Matsuzaki and Simon C. Benjamin.

ExQM Seminar by

On **Fri Nov. 29th** in** MPQ Lecture Hall **at 10.30 am

**Maximilian Buser** will talk on

*`’Ground-State Phases and Quench Dynamics in Interacting Bosonic Flux-Ladders´*

Flux-ladders constitute the minimal setup enabling a systematic

understanding of the rich physics of interacting particles in strong

magnetic fields and in lattices. These systems were also realized in a

series of experiments and therefore, they have attracted great

interest. An experimental realization with synthetic dimensions and

tunable inter-particle interactions based on ultracold 39K and 41K

atoms is expected to be realized by the quantum gases group at ICFO.

understanding of the rich physics of interacting particles in strong

magnetic fields and in lattices. These systems were also realized in a

series of experiments and therefore, they have attracted great

interest. An experimental realization with synthetic dimensions and

tunable inter-particle interactions based on ultracold 39K and 41K

atoms is expected to be realized by the quantum gases group at ICFO.

In this work, the ground-state phase diagram of the synthetic

flux-ladder model is mapped out using extensive density-matrix

renormalization-group simulations and putting the emphasis on

parameters which can be realized in the future experiment. The focus

is on accessible observables such as the chiral current and the

leg-population imbalance; typical particle-current patterns and

momentum-distribution functions in various ground-state phases are

exemplified. For a particle filling of one boson per rung, the

Mott-insulating Meissner phase, as well as biased-ladder phases,

existing on top of superfluids and Mott insulators, are identified and

located. Considering a particle filling of one boson per two rungs, we

report on the appearance of a stable vortex-lattice phase.

flux-ladder model is mapped out using extensive density-matrix

renormalization-group simulations and putting the emphasis on

parameters which can be realized in the future experiment. The focus

is on accessible observables such as the chiral current and the

leg-population imbalance; typical particle-current patterns and

momentum-distribution functions in various ground-state phases are

exemplified. For a particle filling of one boson per rung, the

Mott-insulating Meissner phase, as well as biased-ladder phases,

existing on top of superfluids and Mott insulators, are identified and

located. Considering a particle filling of one boson per two rungs, we

report on the appearance of a stable vortex-lattice phase.

Furthermore, we demonstrate that quantum quenches from suitably chosen initial states can be used to probe the equilibrium properties in the transient dynamics. Concretely, we consider the instantaneous turning on of hopping matrix elements along the rungs or legs in the synthetic flux-ladder model, with different initial particle distributions.

Specifically, we show that clear signatures of the biased-ladder phase

can be observed in the transient dynamics. Moreover, the behavior of

the chiral current in the transient dynamics is discussed. The results

presented in this work might provide relevant guidelines for future

implementations of flux-ladders in experimental setups exploiting a

synthetic dimension.

can be observed in the transient dynamics. Moreover, the behavior of

the chiral current in the transient dynamics is discussed. The results

presented in this work might provide relevant guidelines for future

implementations of flux-ladders in experimental setups exploiting a

synthetic dimension.

ExQM Inaugural Seminar by

On **Fri Oct. 25th** in** MPQ Lecture Hall **at 10.30 am

**Markus Hasenöhrl **(Group Michael Wolf) will talk on

*`’Interaction-Free‘ Channel Discrimination´*

In this work, we reinterpret the famous Elitzur-Vaidman bomb-tester experiment as a quantum channel discrimination problem. To this end, we generalize the notion of ‚interaction-free‘ measurement to arbitrary quantum channels via a game theoretic model.

Our main result is a sufficient and necessary criterion for when it is possible or impossible to discriminate quantum channels in an ‚interaction-free‘ manner (i.e. such that the error probability and the ‚interaction‘ probability can be made arbitrarily small). For the case where our condition holds, we devise an explicit protocol with the property that both probabilities approach zero with an increasing number of channel uses. We also show that this protocol is optimal in a certain (asymptotic) sense. Furthermore, our protocol only needs one ancillary qubit and might thus be be implementable in near-term experiments. For the case where our condition does not hold, we prove an inequality that quantifies the trade-off between the error probability and the ‚interaction‘ probability.

MCQST Distinguised Guest Lecture by

On **Tue Oct. 8th** in** MPQ Lecture Hall **at 2.30 pm

**John Preskill **will talk on

*`Quantum Computing in the NISQ Era and beyond´*

Noisy Intermediate-Scale Quantum (NISQ) technology will be available in the near future. Quantum computers with 50-100 qubits may be able to perform tasks which surpass the capabilities of today’s classical digital computers, but noise in quantum gates will limit the size of quantum circuits that can be executed reliably.

NISQ devices will be useful tools for exploring many-body quantum physics, and may have other useful applications, but the 100-qubit quantum computer will not change the world right away – we should regard it as a significant step toward the more powerful quantum technologies of the future.

Quantum technologists should continue to strive for more accurate quantum gates and, eventually, fully fault-tolerant quantum computing.

PhD Defense Seminar by

On **Wed. 25th Sept.** at 10.00am in** MPQ Lecture Hall **

**Stephan Welte **will give his PhD defense talk on

*`Generation of Optical Cat States Entangled with an Atom´*

Schrödinger’s cat is a famous gedanken experiment on the existence of quantum mechanical superposition states of macroscopic objects [1]. Experimental implementations in quantum optics employ the superposition of two coherent states with an opposite phase, so-called cat states. These continuous-variable states can be tuned to vary the degree of macroscopicity and to study decoherence effects.

Our experiment implements a strong interaction of a coherent light pulse with a single trapped Rubidium atom, provided by an optical cavity [2]. We deterministically produce a hybrid entangled state between the atomic spin and the phase of the propagating light pulse. A projective measurement of the atomic spin projects the optical state and prepares it in an optical cat state. We study the non-classical properties of the produced states and demonstrate control over all relevant degrees of freedom, using coherent control of the atomic qubit. In the future, cat states may find applications in fiber-based optical quantum networks.

*Joint work with Bastian Hacker, Severin Daiss, Lin Li, Lukas Hartung, Emanuele Distante, and Gerhard Rempe.*

July 22-26th, 2019

IMPRS Summer School in Bad Aibling

IMPRS Summer School in Bad Aibling

NB: will be held in Bad Aibling, see Conference Schedule.

Fri Jul 19th, 2019 - 10:30 am

Planning Seminar for next ExQM Workshop etc.

Planning Seminar for next ExQM Workshop etc.

MPQ, Lecture Hall at 10.30 am.

We will plan** activities in 2019/20 **including

- ExQM Workshop
- guest programmes
- next seminars
- ideas of second ExQM generation

**Please all attend**.

ExQM Seminar by

In** MPQ Lecture Hall **at 10.30 am

**Margret Heinze **will talk on

*`Universal Uhrig Dynamical Decoupling for Bosonic Systems´*

We construct efficient deterministic dynamical decoupling schemes protecting continuous variable degrees of freedom from decoherence.Our schemes target decoherence induced by quadratic system-bath interactions with analytic time dependence. We show how to suppress such interactions to ?-th order using only ? pulses. Furthermore, we show to homogenize a 2m-mode bosonic system using only (? + 1)^(2?+1) pulses, yielding – up to ?-th order – an effective evolution described by non-interacting harmonic oscillators with identical frequencies.

The decoupled and homogenized system provides natural decoherence-free subspaces for encoding quantum information. Our schemes only require pulses which are tensor products of single-mode passive Gaussian unitaries and SWAP gates between pairs of modes.

PRL_123 (2019), 010501 (also https://arxiv.org/abs/1810.07117v2)

July 8-9th, 2019

Conference of Munich Centre for Quantum Science and Technology at Deutsches Museum, Centre for New Technologies (ZNT), Museumsinsel 1

Conference of Munich Centre for Quantum Science and Technology at Deutsches Museum, Centre for New Technologies (ZNT), Museumsinsel 1

Held at *Deutsches Museum*, Centre for New Technologies (ZNT), Museumsinsel 1, see map.

July 5th, 2019

Youngsters' Conference of Munich Centre for Quantum Science and Technology at MPQ

Youngsters' Conference of Munich Centre for Quantum Science and Technology at MPQ

ExQM Seminar by

In** MPQ Lecture Hall **at 10.30 am

**Frederik vom Ende **will talk on

*` Reachability in Infinite-Dimensional Unital Open Quantum Systems with Switchable GKS-Lindblad Generators´*

In quantum systems theory one of the fundamental problems boils down to: given an initial state, which final states can be reached by the dynamic system in question?

Here we consider *infinite-dimensional* open quantum dynamical systems following a unital Kossakowski-Lindblad master equation extended by controls. More precisely, their time evolution shall be governed by an inevitable (potentially unbounded) Hamiltonian drift term, finitely many bounded control Hamiltonians allowing for (at least) piecewise constant control amplitudes plus a bang-bang switchable noise term in GKS form (generated by some compact V).

Generalizing standard majorization results from finite to infinite dimensions, we show that such bilinear quantum control systems allow to approximately reach any target state majorized by the initial one — as up to now only has been known in finite-dimensional analogues.

ExQM Seminar by

In** MPQ Lecture Hall **(back again to left of entrance) at 10.30 am

**Stephan Welte **will talk on

*`Generation of Optical Cat States Entangled with an Atom´*

Schrödinger’s cat is a famous gedanken experiment on the existence of quantum mechanical superposition states of macroscopic objects [1]. Experimental implementations in quantum optics employ the superposition of two coherent states with an opposite phase, so-called cat states. These continuous-variable states can be tuned to vary the degree of macroscopicity and to study decoherence effects.

Our experiment implements a strong interaction of a coherent light pulse with a single trapped Rubidium atom, provided by an optical cavity [2]. We deterministically produce a hybrid entangled state between the atomic spin and the phase of the propagating light pulse. A projective measurement of the atomic spin projects the optical state and prepares it in an optical cat state. We study the non-classical properties of the produced states and demonstrate control over all relevant degrees of freedom, using coherent control of the atomic qubit. In the future, cat states may find applications in fiber-based optical quantum networks.

*Joint work with Bastian Hacker, Severin Daiss, Lin Li, Lukas Hartung, Emanuele Distante, and Gerhard Rempe.*

ExQM Seminar by

'Unitary Control of Quantum Systems in Finite and Infinite Dimensions'

In** MPQ Lecture Hall **(back again to left of entrance) at 10.30 am

**Prof. Michael Keyl **will talk on

*‚Unitary Control of Quantum Systems in Finite and Infinite Dimensions‘*

Quantum control theory studies the dynamics of quantum systems which can be manipulated, e.g., by external controls like electro-magnetic fields. One of the central topics in this framework is the decision problem of *controllability*: can we drive the system from any given initial (pure) state into any final (pure) state, or more generally, from any given initial state to any desired final state with the same spectrum of eigenvalues?

In finite dimensions this question can be completely answered in a Lie-algebraic framework. Infinite dimensions, on the other hand, trigger more challenging mathematics and require methods from operator analysis and (extensions of) infinite dimensional Lie theory.

This talk will provide an introduction into this topic, and an overview on some of its central questions and results. In finite dimensions we show in particular how a system Lie algebra can be associated to a quantum control system, which leads to an easy condition for deciding controllabiliy: the celebrated Lie algebra rank conditon.

The second part of the talk shows under which assumptions the finite dimensional results can be translated more or less directly to infinite dimensions — now using infinite dimensional algebras and groups, and the strong instead of the norm topology. Beyond these assumptions a number of interesting new open questions arise, which we briefly sketch in an outlook.

ExQM Seminar by

'Recent Developments in Tensor Networks'

In** MPQ Lecture Hall **(back again to left of entrance) at 10.30 am

**Dr Claudius Hubig **will talk on

*‚Recent Developments in Tensor Networks‘*

In the first half of the talk, I will report on recent progress made in the description of finite-dimensional quantum systems with non-local interactions using tensor network approaches. Such systems include molecular systems of interest in quantum chemistry as well as effective systems arising as the to-be-solved inner problems of the dynamical mean-field theory or the density matrix embedding theory. In all cases, using loop-free MPS or tree tensor networks, sufficient progress can be made over standard solvers using exact diagonalisation.

In the second half of the talk, we will summarise the relatively novel application of real-time evolution to infinite two-dimensional tensors networks to obtain time-dependent observables. The evolution is applied to the 2D S=1/2 Néel state on the square lattice in a disorder averaged Hamiltonian, where we find hints towards many-body localisation in the spin dynamics as the disorder strength is increased.

Refs.: arXiv:1811.00048, arXiv:1901.05824 and arXiv:1812.03801.

Guest Talk by

'On the Tension between Mathematics and Physics'

**Note Location: MPI for Astrophysics, MPA, Lecture Hall E.0.11 at 10.30 am (MPA is next door to MPQ, Karl-Schwarzschild-Str. 1, see room finder here.)**

**Prof. Rédei **will talk on

*‚On the Tension between Mathematics and Physics‘*

Because of the complex interdependence of physics and mathematics their relation is not free of tensions. The talk looks at how the tension has been perceived and articulated by some physicists, mathematicians and mathematical physicists.

Some sources of the tension are identified and it is claimed that the tension is both natural and fruitful for both physics and mathematics. An attempt is made to explain why mathematical precision is typically not welcome in physics.

Inaugural Talk by

'Building a New Caesium Quantum Gas Microscope'

**MPQ Lecture Hall B0.32 at 10.30 am **

**Till Klostermann **will talk on

*‚Building a New Caesium Quantum Gas Microscope‘*

I will present my PhD-thesis project, setting up a new experiment utilizing Caesium for investigating artificial gauge fields. The new experiment will use Raman assisted tunneling in a state-dependent lattice instead of shaking to engineer these gauge fields. A single-site resolution objective will give access to the individual particles position.

Due to Caesium’s large and accessible Feshbach resonance, it is a good candidate to investigate interactions in systems influenced by artifical gauge fields. I will also talk about the current status of the setup.

IAS-Talk by

'Quantum Advantage of Shallow Circuits'

**IAS, Lichtenbergstr. 2a, Wed Jan. 16th ** at 1.00 pm.

**Prof. Robert König **will talk on

*‚Quantum Advantage with Shallow Circuits‘*

Prof. Robert König (Theory of Complex Quantum Systems) will talk about the advantage of quantum computers as compared to conventional computers.

Quantum computers can perform operations on many values in one fell swoop whereas a single conventional computer typically must execute these operations sequentially. The promise of quantum computing lies in the ability to solve certain problems significantly faster (TUM press release).

**Relevant Publication:**

S. Bravyi, D. Gosset, R. König, “Quantum advantage with shallow circuits”, Science, 19. October 2018. DOI: 10.1126/science.aar3106

Talk by

'Symplectic Coarse-Grained Dynamics: Chalkboard Motion in Classical and Quantum Mechanics'

**MPQ Lecture Hall B0.32, Tue Jan. 15th ** at 10.30 am.

**Prof. Maurice de Gosson (Univ. Vienna) **will talk on

*‚Symplectic Coarse-Grained Dynamics: Chalkboard Motion in
Classical and Quantum Mechanics‘*

In the usual approaches to mechanics (classical or quantum) the primary object of interest is the Hamiltonian, from which one tries to deduce the solutions of the equations of motion (Hamilton or Schrödinger).

In the present talk, we reverse this paradigm and view the motions themselves as being the primary objects. This is made possible by studying arbitrary phase space motions, not of points, but of ellipsoids with the requirement that the symplectic capacity of these ellipsoids is preserved. This allows us to pilot and control these motions as we like. In the classical case these ellipsoids correspond to a symplectic coarse-graining of phase space, and in the quantum case they correspond to the „quantum blobs“ we have defined in previous work, and which can be viewed as minimum uncertainty phase-space cells which are in a one-to-one correspondence with Gaussian pure states.

PhD-Defense Talk by

'Symmetry Methods in Quantum Information Theory'

** Mathematics Building MI, Boltzmann Str. 3, Room 00.10.011 at 12.30 noon.**

**Anna-Lena Hashagen **will talk on

*‚Symmetry Methods in Quantum Information Theory‘*

ENB Graduation Ceremony at Conference Centre in Augsburg

** Conference Centre, Gögginger Str. 10, Augsburg**

**Michael Lohse **and **Christian Sames **(QCCC) receive their honours documents during the ceremony.

Talk by

'On Phase-Space Representations of Spin Systems and their Relations to Infinite-Dimensional Quantum States'

**MPQ Seminar Room Cirac Group in Third Floor at 10.30am**

**Bálint Koczor **will talk on

*‚On Phase-Space Representations of Spin Systems and their Relations to Infinite-Dimensional Quantum States‘*

Classical phase spaces have been widely applied in physics, engineering, economics or biology. I will give an overview of our recent works considering phase spaces of quantum systems, which have become a powerful tool for describing, analyzing, and tomographically reconstructing quantum states.

We provide a complete phase-space description of (coupled) spin systems including their time evolution, tomography, large-spin approximations and their infinite-dimensional limit, which recovers the well-known case of quantum optics. Finally, Born-Jordan distributions of infinite-dimensional quantum systems are discussed.

For more detail see also arXiv:1808.02697 and arXiv:1811.05872 .

PhD-Defense Talk by

'Tensor Networks and Machine Learning for Approximating and Optimizing Functions in Quantum Physics'

**Mathematics Building MI, Boltzmann Str. 3, Room 03.09.012 ****at 3.00 pm.**

**Moritz August **will talk on

*‚Tensor Networks and Machine Learning for Approximating and Optimizing Functions in Quantum Physics‘*

We explore the intersection of computer science and mathematics to address challenging problems in numerical quantum physics. We introduce, analyze and evaluate novel methods for the approximation of physical quantities of interest as well as the optimization of performance criteria in quantum control. These methods are based on techniques from the fields of tensor networks, numerical analysis and machine learning. Furthermore, we present work on the relation between machine learning and tensor network methods for the representation of quantum states.

We introduce a general algorithm which for the first time allows to approximate global functions Trf (A) of matrix product operators A which represent Hermitian matrices of very high dimensionality. Following this, we present an analytical analysis of the partial results computed by the procedure. This analysis leads us to the discovery of a more efficient variant of the algorithm and we subsequently show that it can be applied to a large class of spin Hamiltonians in quantum physics. We finally demonstrate how our method yields a novel strategy to approximate properties of thermal equilibrium states, some of which were so far inaccessible for numerical methods.

In the second part, we present a novel and broadly applicable method for solving quantum control scenarios. The method employs a particular class of recurrent neural networks, the long short-term memory network, to probabilistically model control sequences and optimize these models with tools from supervised and reinforcement learning. In a first version, we use an optimization procedure inspired by evolutionary algorithms to train the networks. We demonstrate in a quantum memory setting that the method can produce better results than certain analytical solutions. We then improve on these results by introducing a different optimization strategy based on insights from reinforcement learning known as policy gradient algorithms. The combination of long short-term memory networks and policy gradient optimization schemes allows us to tackle a wide variety of control problems, which we demonstrate numerically.

Finally, we show results on the relation between tensor networks and a particular class of machine learning models, the restricted Boltzmann machine. We find that restricted Boltzmann machines can be generalized in the tensor network framework and gain insight about their efficiency in representing states of many-body quantum systems.

Fri Nov. 9th, 2018 - 10.30 am

Harvard-Report Talk by**Nicola Pancotti** on

'Machine Learning and Tensor Networks for Quantum Many Body Physics '

Harvard-Report Talk by

'Machine Learning and Tensor Networks for Quantum Many Body Physics '

**MPQ Lecture Hall B0.32 ** at 10.30 am.

**Nicola Pancotti** (back from Harvard) will talk on

*‚Machine Learning and Tensor Networks for Quantum Many Body Physics‘*

In this talk, I will give a simple introduction to Machine Learning and Tensor Network techniques for the ground state search problem in Quantum Many Body physics. I will show how one can use Neural Network States as a powerful *ansatz* for the description of many body quantum spin systems and how to map a sub class of them to some well known Tensor Network families. I will show applications to classical pattern recognition and how to combined those families to existing Machine Learning techniques in order to improve their performances.

Finally I will discuss possible directions to extend these methods to fermionic systems and, in particular, to the framework of Gaussian states.

Fri Oct. 26th, 2018 - 10.30 am

Inaugural Talk by**Frederik Bopp** on

'Hybrid Photonic-Plasmonic Biosensing'

Inaugural Talk by

'Hybrid Photonic-Plasmonic Biosensing'

**MPQ Lecture Hall B0.32 ** at 10.30 am.

**Frederik Bopp** (who has just joined the Finley group as ExQM student) will talk on

*‚Hybrid Photonic-Plasmonic Biosensing‘*

Cavity-enhanced optical and plasmonic sensing are two commonly utilised techniques to analyse nanoparticle. Combining them into a hybrid system potentially allows to achieve high finesses and sub diffraction limited mode volumes simultaneously, leading to enhanced detection sensitivities and an increased range of detectable biomolecules. On the long term, these biosensors could form a new set of medical tools for the discovery, the study and the detection of biomarkers. My Master research aims at studying the coupling of an open microcavity to a gold plasmonic nanorod and to establish their potential for single molecule detection.

In my presentation I will provide a theoretical and experimental description of the coupling mechanism, linking these results to the potential sensitivity limit of this system.

Wed July 11th, 2018 - 2 pm

IMPRS Career Talk by the Secretary General of the Humboldt-Foundation**Dr Aufderheide** on

'Postdoc Opportunities in the Humboldt-Foundation`s Global Network'

IMPRS Career Talk by the Secretary General of the Humboldt-Foundation

'Postdoc Opportunities in the Humboldt-Foundation`s Global Network'

**MPQ Lecture Hall B0.32 ** at 2 pm.

**Dr. Enno Aufderheide** (Secretary General of Humboldt-Foundation) will talk on

*‚Postdoc Opportunities in the Humboldt-Foundation`s Global Network‘*

*Dr. Enno Aufderheide*

On 1 July 2010, Enno Aufderheide became the new Secretary General of the Humboldt Foundation. From 2006 to 2010, he was head of the Research Policy and External Relations Department at the Max Planck Society in Munich where he played a key role in the Society’s internationalisation strategy. From December 2008 onwards, he also took on responsibility for managing the Minerva Foundation for the promotion of German-Israeli academic cooperation.

June 14th through June 18th, 2018

14th Workshop on Numerical Ranges and Radii (WONRA) is co-hosted by ExQM featuring Man-Duen Choi, David Gross, Chi-Kwong Li, and Michael Wolf et al.

14th Workshop on Numerical Ranges and Radii (WONRA) is co-hosted by ExQM featuring Man-Duen Choi, David Gross, Chi-Kwong Li, and Michael Wolf et al.

The 14th International *Workshop on Numerical Ranges and Radii* (WONRA) is **co-hosed by ExQM** (and organised by Th. Schulte-Herbrüggen) under the motto

The **numerical range**, i.e. the set W(A):={<x|Ax> | <x|x> =1} plays a crucial role in spectral theory and, e.g., in the search of ground-state energies (Rayleigh quotient). In 1918/1919, by the celebrated **Toeplitz-Hausdorff Theorem**, it was shown to form a convex set. Clearly, the numerical range W(A) comprises the spectrum spec(A). In quantum-many-body systems, the important question arises, whether the spectrum of the underlying Hamiltonian is gapped — this decision problem was addressed in a seminal paper by Cubitt, Perez-Garcia, and Wolf, Nature **528**, 207 (2015). **Michael Wolf** will talk on *Undecidebility of the Spectral Gap* in a **special ExQM Lecture on Fri Jun 15th at 3pm in MPQ Lecture Hall B0.32 **as one highlight in this conference.

**
Schedule in Overview**,

**June 14 (Thursday), MPQ, Lecture Hall B0.32**

9:30 to 16:30 talks by **Choi**, Spitkovsky, Tam, Farenick, Nakazato, Chien, Osaka, Taheri

**June 15 (Friday) MPQ, Lecture Hall B0.32**

9:30 to 17:30 talks by Życzkowski, Schulte-Herbrüggen, **Gross**, Psarrakos, **Schuch**, **Wolf**, Weis, Huckle

**June 16 (Saturday)**

Social event and discussion

Afternoon: Visit in Munich downtown museums, e.g., Blue Rider in Lenbachhaus

18:30 optional dinner in a Munich beergarden downtown

**June 17 (Sunday), IAS, Lichtenbergstr. 2a, Auditorium on ground floor**

10:00 to 16:30 talks by Bebiano, Badea, vom Ende, Diogo, Crouzeix, Sze, Bračic

18:00 to 20:30 conference dinner at IAS faculty club

**June 18 (Monday), IAS, Lichtenbergstr. 2a, Auditorium on ground floor**

9:45 to 12:00 talks by **Kressner**, Lau, **Li**

12:00 to 14:00 Lunch on campus at IPP mensa

Fri May 18th, 2018 - 1:30 pm

Lecture by**Shai Machnes** on

'Control of Quantum Devices: Merging Pulse Calibration and System Characterization using Optimal Control'

Lecture by

'Control of Quantum Devices: Merging Pulse Calibration and System Characterization using Optimal Control'

TUM Campus, **Walther-Meissner Institute**, Walther-Meissner-Straße 8, 2nd floor, **Seminar Room 143** (or, if too noisy, 128) at 1.30 pm.

**Dr. Shai Machnes** (University of Saarbrücken) will talk on

*‚Control of Quantum Devices: Merging Pulse Calibration and System Characterization using Optimal Control‘*

The current methodology for designing control pulses for quantum devices circuits often results in a somewhat absurd situation: pulses are designed using simplified models, resulting in initially poor fidelities. The pulses are then calibrated in-situ, achieving high-fidelities, but without a corresponding model. We are therefore left with a model we know is inaccurate, working pulses for which we do not have a matching model, and a calibration process from which we learned nothing about the system.

Here, we propose a novel procedure to rectify the situation, by merging pulse design, calibration and system characterization: Calibration is recast as a closed-loop search for the best-fit model parameters, starting with a detailed, but only partially characterized model of the system. Fit is evaluated by fidelity of a complete set of gates, which are optimized to fit the current system characterization. The end result is a best-fit characterization of the system model, and a full set of high-fidelity gates for that model.

We believe the new approach will greatly improve both gate fidelities and our understanding of the systems they drive.

Fri May 18th, 2018 - 10:30 am

Inaugural Seminar by Bo Wang on

'Strong Coupling between Photons via a Four-Level N-type Atom'

Inaugural Seminar by Bo Wang on

'Strong Coupling between Photons via a Four-Level N-type Atom'

MPQ, Lecture Hall (**moved to B0.32**) at 10.30 am

**Bo Wang** (Rempe group) will talk on

*‚Strong Coupling between Photons via a Four-Level N-type Atom‘*

Four-level N-type atomic systems have been investigated for effects like the electromagnetically induced absorption (EIA) and cross-phase modulation (XPM) when interacting with classical light fields. Despite the giant non linearity, the interaction strengths are negligible at the level of individual quanta. However with the strong light matter coupling provided by cavity quantum electrodynamics, a significant interaction between single photons can be reached.

Here I will give a brief introduction on our experimental setup and the experiment where the photons of two light fields are strongly coupled via a single four-level N-type atom. The fields drive two modes of an optical cavity, which are strongly coupled to two separate transitions. A control laser drives one transition’s ground state to the other transition’s excited state, the inner transition of the N-type atom. It induces a tunable coupling between the modes and results in a doubly nonlinear energy-level structure of the photon-photon-atom system. The strong correlation between the light fields is observed via photon-photon blocking and photon-photon tunneling. With this system, nondestructive counting of photons and heralded *n*-photon sources might be within reach.

Wed May 16th, 2018 - 4:00 pm

Inaugural Lecture by Prof. Stefan Weltge (TUM Maths Dept) on

'A Barrier to P=NP Proofs'

Inaugural Lecture by Prof. Stefan Weltge (TUM Maths Dept) on

'A Barrier to P=NP Proofs'

TUM Mathematics Building, Boltzmannstr. 8, **Lecture Hall 3** at 4.00 pm.

**Prof. Stefan Weltge** will give his inaugural lecture on

*‚A Barrier to P=NP Proofs‘*

The P-vs-NP problem describes one of the most famous open questions in mathematics and theoretical computer science. The media are reporting regularly about proof attempts, all of them being later shown to contain flaws. Some of these approaches where based on small-size linear programs that were designed to solve problems such as the traveling salesman problem efficiently.

Fortunately, a few years ago, in a breakthrough result researchers were able to show that no such linear programs can exist and hence that all such attempts must fail, answering a 20-year old conjecture.

In this lecture, I would like to present a quite simple approach to obtain such a strong result. Besides an elementary proof, we will hear about (i) the review of all reviews, (ii) why having kids can boost your career, and (iii) a nice interplay of theoretical computer science, geometry, and combinatorics.

May 15th to June 20th, 2018

von-Neumann Lecture Series by Prof. Marius Junge (University of Illinois, USA) on Operator Algebraic Methods in Quantum Information Theory

von-Neumann Lecture Series by Prof. Marius Junge (University of Illinois, USA) on Operator Algebraic Methods in Quantum Information Theory

von-Neumann Lecture Series by **Prof. Marius Junge** (University of Illinois, USA) held at TUM Mathematics, Boltzmannstr. 3.

‚**Operator-Algebraic Methods in Quantum Information Theory‘**

Lecture series from **May 15 to June 20**.

Tue 16:00 – 18:00 (room 03.06.011)

Wed 16:00 – 18:00 (room 02.10.011)

Location: Boltzmannstr.3, Garching

In this lecture series, we will illuminate some recent connection between operator algebra theory and quantum information theory. **The first part of the lecture** will focus on estimates of entropy and mutual information, including some recent results on decay to equilibrium. **The second part** will be concerned with the operator algebraic background for various type of Bell inequalities.

Mon May 7th, 2018 - 4:15 pm

Lecture by**Martin Plenio** on

'Diamond Quantum Devices: From Quantum Simulation to Medical Imaging'

Lecture by

'Diamond Quantum Devices: From Quantum Simulation to Medical Imaging'

TUM Campus, **Chemistry Building**, Lichtenbergstrasse 4, 6th floor (yellow section), **Seminar Room CH63.214** at 4.15 pm.

**Prof. Martin Plenio** (University of Ulm) will talk on

*‚Diamond Quantum Devices: From Quantum Simulation to Medical Imaging‘*

Perfect diamond is transparent for visible light but there are famous diamonds, such as the famous Oppenheim Blue or the Pink Panther worth tens of millions of dollar, which have intense colour. An important source of colour in diamond are lattice defects which emit and absorb light at optical frequencies and may indeed possess a non-vanishing ground state electronic spin.

I will explore the physics of one of these defects, the nitrogen vacancy center, and show how we can manipulate its electronic spin and make use of this capability to create quantum simulators, quantum sensors and perhaps surprisingly applications in medical imaging that may, we hope, find applications for example in cancer research and treatment.

Fri May 4th, 2018 - 10:30 am

Seminar by Julian Roos on

'Non-Markovianity Measures in the Many-Body Context'

Seminar by Julian Roos on

'Non-Markovianity Measures in the Many-Body Context'

MPQ, **Seminar Room B0.41 in Library** at 10.30 am.

**Julian Roos** will talk on

*‚Non-Markovianity Measures in the Many-Body Context‘*

The ability to coherently control the dynamics of an ever-increasing number of particles pushed development of quantum technologies during the past decade. In order to achieve scalability, environment-induced decoherence effects need to be identified, understood and minimised such that the required thresholds for error correction are achieved. Also, people now control and modify the environment itself to design noise. All of this triggered renewed interest in fundamental studies of open quantum systems (OQS) amongst which are multiple studies on the existence of two different dynamical regimes: Markovian dynamics, underlying, e.g., the well known Lindblad master equations and non-Markovian dynamics, which are usually associated with recoherence and information backflow from the environment to the system.

I will introduce you to several measures that are widely used in the field to quantify the ‘amount‘ of non-Markovianity that is present in the reduced dynamics of an OQS and provide some examples of their use in the context of time evolution of matrix product states. Here, the OQS consists, e.g., of two spins in the center of a spin chain and thus any system-bath weak-coupling assumptions (used in the derivation of the Lindblad form) are clearly invalid. Still there seem to exist special cases where the underlying dynamics are Markovian.

Fri April 20th, 2018 - 10:30 am

Two-Part Inaugural Seminar by Maximilian Buser on

'Open Quantum Systems with Initial System-Environment Correlations' and 'Quasi-One-Dimensional Systems with Artificial Gauge Fields: Interactions and Finite Temperatures'

Two-Part Inaugural Seminar by Maximilian Buser on

'Open Quantum Systems with Initial System-Environment Correlations' and 'Quasi-One-Dimensional Systems with Artificial Gauge Fields: Interactions and Finite Temperatures'

MPQ, **Seminar Room B0.41 in Library** at 10.30 am.

**Maximilian Buser** will talk on **two topics**

*‚Open Quantum Systems with Initial System-Environment Correlations‘*

Open quantum systems exhibiting initial system-environment correlations are notoriously difficult to simulate. — We point out that given a sufficiently long sample of the exact short-time evolution of the open system dynamics, one may employ transfer tensors for the further propagation of the

reduced open system state. This approach is numerically advantageous and allows for the simulation of quantum correlation functions in hardly accessible regimes.

We benchmark this approach against analytically exact solutions and exemplify it with the calculation of emission spectra of multichromophoric systems as well as for the reverse-temperature estimation from simulated spectroscopic data.

*‚Quasi-One-Dimensional Systems with Artificial Gauge Fields: Interactions and Finite Temperatures‘*

Artificial, highly tunable gauge (or ”magnetic”) fields have been successfully implemented in a number of optical lattice experiments with ultracold neutral Bose gases. In this context,

quasi-one-dimensional ladder-like lattices are of significant interest. They are the most simple geometries allowing the exploration of intriguing physical effects related to quantum Hall physics, exotic topological states and superconductivity. While experimental research mainly focused on non-interacting particles, recent results encourage the prospect of future experiments with strongly interacting bosons.

Analytical, mean-field and DMRG-based studies provided extensive theoretical results regarding the ground state properties of such strongly interacting, ladder-like systems. The presence of gauge fields clearly enriches the corresponding phase diagrams. For instance, it gives rise to so-called Meissner and vortex lattice phases as well as to intriguing effects such as chiral current reversals.

The aim of this work (in progress) is to provide theoretical predictions in experimentally much more feasible regimes. Therefore, we plan to investigate the effects of finite temperature states and intend to employ DMRG-based simulation techniques.

April 16th to July 2nd, 2018

von-Neumann Lecture Series by Prof. Daniel Kressner (EPFL Lausanne, CH) on Low-Rank Approximation

von-Neumann Lecture Series by Prof. Daniel Kressner (EPFL Lausanne, CH) on Low-Rank Approximation

von-Neumann Lecture Series in Computer Science by **Prof. Daniel Kressner** (EPFL Lausanne, CH) held at TUM Computer Science (Host Prof. Huckle), Boltzmannstr. 3.

‚**Low-Rank Approximation‘**

Lecture series from **April 16th to July 2nd**.

Mon 14:00 – 17:00 (room 02.08.020)

Location: Boltzmannstr.3, Garching

Low-rank compression is an ubiquitous tool in scientific computing and data analysis. There have been numerous exciting developments in this area during the last decade and the goal of this course is to give an overview of these developments, covering theory, algorithms, and applications of low-rank matrix and tensor compression.

Specifically, the following topics will be covered:

1. Theory

- Low-rank matrix and tensor formats (CP, Tucker, TT, hierarchical Tucker)
- A priori approximation results

2. Algorithms

- Basic operations with low-rank matrices and tensors
- SVD-based compression
- Randomized compression
- Alternating optimization
- Riemannian optimization
- Nuclear norm minimization
- Adaptive cross approximation and variants

3. Applications

- Image processing
- Matrix and tensor completion
- Model reduction
- Solution of large- and extreme-scale linear algebra problems from various applications (dynamics and control, uncertainty quantification, quantum computing, …)
- Tensors in deep learning

Depending on how the course progresses and the interest of the participants, hierarchical low-rank formats (HODLR, HSS, H matrices) may be covered as well.

Hands-on examples using publicly available software (in Matlab, Python, and Julia) will be provided throughout the course.

Fri April 13th, 2018 - 10:30 am

Double-Feature Seminar by Nicola Pancotti and Moritz August on

'Neural Networks Quantum States, String-Bond States and Chiral Topological States'

Double-Feature Seminar by Nicola Pancotti and Moritz August on

'Neural Networks Quantum States, String-Bond States and Chiral Topological States'

MPQ, Lecture Hall (**moved to B0.32**) at 10.30 am.

**Nicola Pancotti **and** Moritz August** will talk on

*‚Neural Networks Quantum States, String-Bond States and Chiral Topological States‘*

Neural Networks Quantum States have been recently introduced as an ansatz for describing the wave function of quantum many-body systems. In this talk we will give an overview of recent works on Neural Networks Quantum States taking the form of Boltzmann machines. We will explain the motivation for considering Boltzmann machines in machine learning and explain how they can be used to study quantum systems. We will then focus on the expressive power of this class of states and discuss their relationship to Tensor Networks.

In particular we will show that restricted Boltzmann machines are String-Bond States with a non-local geometry and low bond dimension and explain how it enables us to define generalizations of restricted Boltzmann machines that combine the entanglement structure of tensor networks with the efficiency of Neural Networks Quantum States. We will then provide evidence that these techniques are able to describe chiral topological states both analytically and numerically.

Finally we will discuss how String-Bond States can also be used in traditional machine-learning applications.

based on: I. Glasser, **N. Pancotti, M. August**, I. Rodriguez, and I. Cirac, *Phys. Rev. X.* **8**, 011006 (2018)

Fri Mar 23rd, 2018 - 10:30 am

Seminar by Stephan Welte on

'Processing of Two Matter Qubits Using Cavity QED'

Seminar by Stephan Welte on

'Processing of Two Matter Qubits Using Cavity QED'

MPQ, Lecture Hall (**moved to B0.32**) at 10.30 am.

**Stephan Welte** will talk on

*‚Processing of Two Matter Qubits Using Cavity QED‘*

In a quantum network, optical resonators provide an ideal platform for the creation of interactions between matter qubits. This is achieved by exchange of photons between the resonator-based network nodes, and in this way enables the distribution of quantum states and the generation of remote entanglement [1].

Here we will show how single photons can also be used to generate local entanglement between matter qubits in the same network node [2]. Such entangled states are indispensable as a resource in a plethora of quantum communication protocols.

We will give an overview of the necessary experimental toolbox for an implementation with neutral atoms. Several entanglement protocols showing the generation of all the Bell states for two atoms will be presented. We will also detail how we experimentally exploit the employed method for quantum computation and quantum communication applications.

[1] S. Ritter et al., *Nature* **484**, 195 (2012)

[2] A. Sørensen and K. Mølmer, *Phys. Rev. Lett.* **90**, 127903 (2003)

Fri Mar 16th, 2018 - 10:30 am

Double-Feature Seminar by Anna-Lena Hashagen and Lukas Knips on

'Information-Disturbance Tradeoffs'

Double-Feature Seminar by Anna-Lena Hashagen and Lukas Knips on

'Information-Disturbance Tradeoffs'

MPQ, Lecture Hall (**moved to B0.32**) at 10.30 am.

**Anna-Lena Hashagen **and **Lukas Knips **will give a double-feature on

*Information-Disturbance Tradeoffs*

**In the first part** of our double feature, we investigate the tradeoff between the quality of an approximate version of a given measurement and the disturbance it induces in the measured quantum system. We prove that if the target measurement is a non-degenerate von Neumann measurement, then the optimal tradeoff can always be achieved within a two-parameter family of quantum devices that is independent of the chosen distance measures.

This form of almost universal optimality holds under mild assumptions on the distance measures such as convexity and basis-independence, which are satisfied for all the usual cases that are based on norms, transport cost functions, relative entropies, fidelities, etc. for both worst-case and average-case analysis. We analyze the case of the cb-norm (or diamond norm) more generally for which we show dimension-independence of the derived optimal tradeoff for general von Neumann measurements.

A SDP solution is provided for general POVMs and shown to exist for arbitrary convex semialgebraic distance measures.

**In the second part**, we evaluate the information-disturbance tradeoff experimentally for the observation of a qubit by implementing the full range of possible measurements and determining the measurement error for a given disturbance. The special case of the worst-case total variational distance and the 1-1 norm distance is considered.

The various measurements are realized by a tunable Mach-Zehnder-Interferometer, which supplies the ancillary degrees of freedom necessary to implement arbitrary POVMs and quantum channels for the measurement of a polarization qubit. We demonstrate the tightness of the bound by saturating it with high significance. Furthermore, we show that the optimal procedure outperforms the optimal cloning protocol, not only on a theoretical level, but clearly resolvable in the laboratory.

For more detail, see the preprint arXiv:1802.09893 .

Fri Feb 23rd, 2018 - 10:30 am

Seminar by Michael Fischer on

'Chains of Nonlinear Tunable Superconducting Resonators'

Seminar by Michael Fischer on

'Chains of Nonlinear Tunable Superconducting Resonators'

MPQ, Lecture Hall (**moved to B0.32**) at 10.30 am.

**Michael Fischer** will talk on

*Chains of Nonlinear and Tunable Superconducting Resonators*

In this talk I will first give a brief introduction and overview of superconducting quantum circuits as a basis for quantum simulation. I will then present a quantum simulation system of the Bose-Hubbard-Hamiltonian in the driven dissipative regime in the realm of circuit QED in more detail.

The system consists of series-connected, capacitively coupled, nonlinear and tunable superconducting resonators. The nonlinearity is achieved by galvanically coupled SQUIDs, placed in the current anti-node of each resonator and can be tuned by external coils and on-chip antennas.

Theoretical models of the Bose-Hubbard system predict bunching and antibunching behavior both in the second order auto- and cross-correlation function of the bosonic modes in the lattice sites. Characterization measurements of our sample show that we can reach the parameter space of interest for these quantum simulation experiments.

Fri Feb 16th, 2018 - 10:30 am

Seminar by Jakob Wierzbowski on

'Long-Lived Quantum Emitters in hBN-WSe_{2} van-der-Waals Heterostructures'

Seminar by Jakob Wierzbowski on

'Long-Lived Quantum Emitters in hBN-WSe

MPQ, Lecture Hall (**moved to B0.32**) at 10.30 am.

**Jakob Wierzbowski** will talk on

*‚Long-Lived Quantum Emitters in hBN-WSe _{2} van-der-Waals Heterostructures‘*

We present a significant linewidth narrowing (12.5 %) of free excitons in hBN encapsulated TMDs and localized (< 350 nm) single-photon emitters with long lifetimes of ~18 ns in hBN/WSe2 heterostructures.

February 12th - 16th 2018

Block Course by Prof. Michael Keyl (FU Berlin) on*Mathematical Aspects of Quantum Field Theory (Part 2)* together with Study Course 'Theoretical and Mathematical Physics' (TMP)* *at LMU and IMPRS 'Quantum Science and Technology' (QST) at MPQ, Theresienstrasse 39, room A449, Mon through Fri 10-12 am plus 2-4 pm.

Block Course by Prof. Michael Keyl (FU Berlin) on

Block Course by Prof. Michael Keyl (FU Berlin) on *Mathematical Aspects of Quantum Field Theory (Part 2)* together with TMP and IMPRS-QST. Held at LMU Physics, Theresienstr. 39, Room A449, Mon through Fri: 10-12 am plus 2-4 pm.

In the beginning of the semester, we studied QFT in the Wightman framework. This included in particular scalar and operator valued distributions, Wightman axioms, Wightman functions and the reconstruction theorem, and the Borchers-Uhlmann algebra with its representations. As an explicit example we studied the free scalar field, its Wick-ordered products, and self-interacting models in 1+1 dimensions. In the latter context questions of renormalization were discussed.

**Now, the second series will be devoted to perturbation theory:** After a short look at the S-matrix, we will use the Epstein-Glaser formalism to construct the perturbation series term by term as a formal power series. This will be carried out in detail with \Phi^4 self interactions as an explicit example. In this context perturbative renormalizability will also be discussed.

For the complete script (parts 1 and 2) follow this link.

*3 ECTS points can be acquired by writing an at least 10 page essay on a topic related to the **course. *

*Quantum fields and Wightman axioms*- Distributions
- Quantum fields
- Wightman distributions and the reconstruction theorem

*The free scalar field*- Representations of the Ponicaré group
- Relations to the Klein-Gordon equation
- Representations of the CCR
- the field and its properties

*Interacting fields or: why is this so difficult?**Scattering theory*(Feb. 2018)*Perturbative theory à la Epstein-Glaser*(Feb. 2018)

- M. Reed and B. Simon,
*Methods of Modern Mathematical Physics,*Vol 2. and 3. - N. N. Bolgolubov, A. A. Logunov, A. I. Osak, I. T. Todorov,
*General Principles of Quantum Field Theory*, Kluwer (1990). - R. Haag,
*Local Quantum Physics*, Springer (1996).

Fri Feb 2nd, 2018 - 10:30 am

Seminar by Prof. Maurice de Gosson on

'Properties of Phase Space Distributions in the Cohen Class'

Seminar by Prof. Maurice de Gosson on

'Properties of Phase Space Distributions in the Cohen Class'

MPQ, Lecture Hall (**moved to B0.32**) at 10.30 am.

**Prof. Maurice de Gosson** will talk on

*‚Properties of Phase Space Distributions in the Cohen Class‘*

Non-standard phase space distributions play an increasingly important role in quantum mechanics, to witness recent work by Koczor, Zeier, and Glaser who highlight the relation of these distributions with tomography.

In this talk we will discuss the properties (marginal conditions, Moyal identity) for a large class of phase space distributions obtained from the usual Wigner distribution by convolution with a Cohen kernel. We will examine in detail two particular cases from this perspective. the Husimi distribution, and the Born-Jordan distribution. The latter arises naturally when one uses the Born and Jordan quantization scheme instead of the traditional Weyl correspondence.

Fri Jan 26th, 2018 - 10:30 am

Seminar by Michael Lohse on

'Exploring 4D-Quantum-Hall Physics with a 2D Topological Charge Pump'

Seminar by Michael Lohse on

'Exploring 4D-Quantum-Hall Physics with a 2D Topological Charge Pump'

MPQ, Lecture Hall (**moved to B0.32**) at 10.30 am.

**Michael Lohse** will talk on

The discovery of topological states of matter has greatly improved our understanding of phase transitions in physical systems. Instead of being described by local order parameters, topological phases are described by global topological invariants and are therefore robust against perturbations. A prominent example is the two-dimensional integer quantum Hall effect: it is characterized by the first Chern number, which manifests in the quantized Hall response that is induced by an external electric field.

Generalizing the quantum Hall effect to four-dimensional systems leads to the appearance of an additional quantized Hall response, but one that is nonlinear and described by a 4D topological invariant—the second Chern number.

Here we report the observation of a bulk response with intrinsic 4D topology and demonstrate its quantization by measuring the associated second Chern number. By implementing a 2D topological charge pump using ultracold bosonic atoms in an angled optical superlattice, we realize a dynamical version of the 4D integer quantum Hall effect. Using a small cloud of atoms as a local probe, we fully characterize the nonlinear response of the system via in situ imaging and site-resolved band mapping. Our findings pave the way to experimentally probing higher-dimensional quantum Hall systems, in which additional strongly correlated topological phases, exotic collective excitations and boundary phenomena such as isolated Weyl fermions are predicted.

Fri Jan 19th, 2018 - 10:30 am

ExQM Planning Seminar for 2018, e.g., ExQM Workshop

ExQM Planning Seminar for 2018, e.g., ExQM Workshop

MPQ, Lecture Hall (**moved to B0.32**) at 10.30 am.

We will plan** activities in 2018 **including

- ExQM Workshop 2018
- video clips of results
- next seminars
- hand-over to next ExQM generation

**Please all attend**.

Fri Jan 12th, 2018 - 9:00 am

Opening Lectures of the*Max-Planck Harvard Research Centre for Quantum Optics* at Deutsches Museum

Opening Lectures of the

The Max-Planck Harvard Research Centre for Quantum Optics (MPHQ) will be opened with a two-day symposion on **Jan. 11th at IAS Garching** and on **Jan. 12th at Deutsches Museum**. It is a joint venture with the Harvard Quantum Optics Center.

See the programmes here:

- 12th Jan. at Deutsches Museum (high-key: including talks by
**nobel laureates**Glauber & Ketterle) - 11th Jan. at IAS Garching

The event is open to the public and the participation of students is highly encouraged! The Max Planck Research Center for Quantum Optics aims for becoming one of the major internationally recognized scientific collaborations of its kind in the field of Quantum Optics.