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On Fri June 10th, 2022 – 4.00 pm via zoom
Roeland Wiersema (U Waterloo, CAN, The Vector Institute) will give an ExQM talk on
`Optimizing Quantum Circuits with Riemannian Gradient Flows´
In this talk, I will introduce a Riemannian optimization scheme over the special unitary group and discuss its implementation on a quantum computer. We illustrate that the resulting Riemannian gradient-flow algorithm has favorable optimization properties for deep circuits and that an approximate version of this algorithm can be performed on near-term hardware.
On Fri June 3rd, 2022 – 11.00 am via zoom
we plan the final ExQM workshop series. A theory part will be held as sattelite workshop to the IAMP-EMS Summer School on Mathematical Physics hosted by TUM see conference page. Aug. 29th – Sept. 2nd 2022 featuring Prof. Reinhard Werner as ExQM speaker.
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.
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´
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)
Online Conference hosted by DPG with contributions by ExQM in Mathematical Physics (MP) Section.
Online Conference hosted by DPG with contributions by ExQM in Atoms, Molecules, Quantum Optics and Photonics (SAMOP) Section .
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.
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.
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.
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.
Maximilian Buser receives his honours document.
Online Conference hosted by the European Quantum Flagship via University of Galway, Ireland with contributions by ExQM.
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.
Online Conference hosted by German Physics Society and University of Jena.
3D Online Conference hosted by the Munich Centre of Quantum Science and Technology (MCQST) and taking place in a virtual model of „Deutsches Museum“.
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.
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.
The 52th SMP focusses on „Channels, Maps and All That„. It specially commemorates the late Andrzej Kossakowski.
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‚
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.
The 24th QIP is hosted and chaired by Profs. Michael Wolf and Robert König.
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.
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.
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.
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‚
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.
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.
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).
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.
MCQST Virtual-Conference Programme
Held via meetanyway due to covid-19.
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).
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 .
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 (T2* ~ 10-100ns) [1], could limit their applicability for distributed quantum technologies.
Unlike single electron and hole spins which are sensitive to the fluctuating nuclear spin environment in III-V materials, singlet-triplet (S-T) qubits in pairs of coupled dots – quantum dot molecules (QDMs) – have extended spin coherence times when operated at a sweet spot for which the S-T splitting is independent of electric and magnetic field fluctuations. Such optically addressable S-T spin qubits promise to extend the obtainable T2* times by several orders of magnitude whilst retaining the advantages outlined above. Previously, experiments using Schottky gated samples have provided important insights into orbital structure, exchange couplings, phonon couplings and spin-dephasing [2, 3]. 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.
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
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.
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.
You may wish to install www.zoom.us for preparation. Stay healthy!
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.
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.
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.
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.
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´
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 .
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.
Moritz August, Anna-Lena Hashagen, Bálint Koczor, Lukas Knips, David Leiner, Stephan Welte, Jakob Wierzbowski receive their honours documents during the ceremony.
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.
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´
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.
On Fri Oct. 25th in MPQ Lecture Hall at 10.30 am
Markus Hasenöhrl (Group Michael Wolf) will talk on
`’Interaction-Free‘ Channel Discrimination´
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.
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.
NB: will be held in Bad Aibling, see Conference Schedule.
MPQ, Lecture Hall at 10.30 am.
We will plan activities in 2019/20 including
Please all attend.
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)
Held at Deutsches Museum, Centre for New Technologies (ZNT), Museumsinsel 1, see map.
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.
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.
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‘
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.
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.
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.
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, 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
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.
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‘
Conference Centre, Gögginger Str. 10, Augsburg
Michael Lohse and Christian Sames (QCCC) receive their honours documents during the ceremony.
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‘
For more detail see also arXiv:1808.02697 and arXiv:1811.05872 .
'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.
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.
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.
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.
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
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.
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.
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.
von-Neumann Lecture Series by Prof. Marius Junge (University of Illinois, USA) held at TUM Mathematics, Boltzmannstr. 3.
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.
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.
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.
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 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
2. Algorithms
3. Applications
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.
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)
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)
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.
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.
MPQ, Lecture Hall (moved to B0.32) at 10.30 am.
Jakob Wierzbowski will talk on
‚Long-Lived Quantum Emitters in hBN-WSe2 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.
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.
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.
MPQ, Lecture Hall (moved to B0.32) at 10.30 am.
Michael Lohse will talk on
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.
MPQ, Lecture Hall (moved to B0.32) at 10.30 am.
We will plan activities in 2018 including
Please all attend.
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:
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.