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Dieter Meschedes Forschungsgruppe
Home AMO-Physikkolloquien
  • Stefan van Waasen

  • Invited speaker: Stefan van Waasen
    Affiliation: FZ Jülich
    Title: ZEA-2 - System House for Research
    Time and room:

  • Andre Eckardt

  • Invited speaker: André Eckardt
    Affiliation: MPI Dresden
    Title: Generalized Bose Condensation in Non-Equilibrium Steady States of Driven-Dissipative Quantum Systems
    Time and room: 17:15, lecture hall IAP
    Abstract: Statistical mechanics provides a powerful framework for predicting the equilibrium properties of matter. The lack of such a universal concept for driven many-body systems makes their theoretical treatment difficult; but, at the same time, it also allows for engineering non-equilibrium states of matter with novel properties beyond the strict constraints of thermodynamics. As an example, I will discuss ordering (Bose condensation) in non-equilibrium steady states of systems in thermal environments that are strongly driven by time-periodic forcing [1,2], temperature gradients [1-3], and pumping [4, 5]. Our work predicts robust excited-state and fragmented condensation [1-5], Bose condensation in environments well-above the equilibrium critical temperature [3], and it explains experiments with photons in structured environments [4,5]. I will also present recent results on the interplay between lasing and equilibrium-like Bose condensation in systems of photons in dye-filled cavities, as they are studied experimentally in Bonn.

    [1] Generalized Bose-Einstein condensation into multiple states in driven-dissipative systems, Daniel Vorberg, Waltraut Wustmann, Roland Ketzmerick, André Eckardt, Phys. Rev. Lett. 111, 240405 (2013), arXiv:1308.2776
    [2] Non-equilibrium steady states of ideal bosonic and fermionic quantum gases
    D. Vorberg, W. Wustmann, H. Schomerus, R. Ketzmerick, A. Eckardt, Phys. Rev. E 92, 062119 (2015)
    [3] High-temperature nonequilibrium Bose condensation induced by a hot needle
    A. Schnell, D. Vorberg, R. Ketzmerick, A. Eckardt, Phys. Rev. Lett. 119, 140602 (2017).
    [4] Pump-power-driven mode switching in a microcavity device and its relation to Bose-Einstein condensation, H. A. M. Leymann, D. Vorberg, T. Lettau, C. Hopfmann, C. Schneider, M. Kamp, S. Höfling, R. Ketzmerick, J. Wiersig, S. Reitzenstein, A. Eckardt, Phys. Rev. X 7, 021045 (2017)
    [5] A unified theory for excited-state, fragmented, and equilibrium-like Bose condensation in pumped photonic many-body systems, D. Vorberg, R. Ketzmerick, and A. Eckardt, Phys. Rev. A 97, 063621 (2018)


  • Alberti

  • Invited speaker: Andrea Alberti
    Affiliation: Institut für Angewandte Physik, Universität Bonn
    Title: Discrete-time quantum machines using neutral atoms in optical lattices
    Time and room: 14:15, lecture hall HISKP
    Abstract: Neutral atoms trapped in optical lattices have been instrumental in the past years to advance our understanding of quantum phases of matter, for the determination of fundamental constants, and for numerous applications in quantum technology, ranging from quantum sensors and time-keeping, up to quantum simulations of complex many-body systems. Neutral atoms in optical lattices also provide a promising platform to store and process quantum information, where large ensembles of identical atoms can be prepared and manipulated with control at the single-particle and single-site level [1].

    In this colloquium, I will present experiments in which the optical lattice potentials are made to depend on the electron spin state of caesium atoms in order to realize discrete-time quantum machines [2]. In a discrete-time quantum machine, the time evolution—instead of being determined by a static Hamiltonian—is governed by a series of discrete operations, which are rapidly applied in sequence. Using the extra degree of freedom provided by the spin, we can transport atoms in space along different spin-dependent quantum paths with subnanometer precision. In this way, we can achieve fast delocalization of matter waves on a time scale of 10 µs, which is two orders of magnitudes faster than the tunnelling time in a shallow optical lattice. Very recently, using optimal quantum control theory, we could speed up the delocalization process up to so-called quantum speed limit of our optical lattice system.

    An example of a discrete-time quantum machine at the single particle level is provided by quantum walks: Depending on its spin state, the atom is moved, at regular time steps, either one site to the left or to the right, delocalizing it over multiple quantum paths. By “reprogramming” the operations defining one step of the quantum walk, we have simulated charged particles in external electric [3] and magnetic fields [4], and studied novel topological phases of periodically driven band insulators [5]. On a more fundamental level, relying on ideal negative measurements, we have tested the “quantum¬ness” of the walk, demonstrating a 6-σ violation of the Leggett-Garg inequality, which rules out any macro-realistic interpretation based on well-defined trajectories [6].

    I will conclude with an outlook towards Hong-Ou-Mandel-like interference experiments, which enable the detection of quantum statistics using a pair of distant atoms [7]. Generalizations to a higher number of identical atoms hold the promise to construct an atom BosonSampling machine with a large number of indistinguishable particles, which can be scaled well above the 50-particle limit of classical simulations based on today’s supercomputers.

    [1] C. Robens, S. Brakhane, W. Alt, F. Kleißler, D. Meschede, G. Moon, G. Ramola, and A. Alberti, ”High numerical aperture (NA = 0.92) objective lens for imaging and addressing of cold atoms,” Opt. Lett. 42, 1043 (2017).
    [2] C. Robens, S. Brakhane, W. Alt, D. Meschede, J. Zopes, and A. Alberti, “Fast, High-Precision Optical Polarization Synthesizer for Ultracold-Atom Experiments,” Phys. Rev. Applied 9, 034016 (2018).
    [3] M. Genske, W. Alt, A. Steffen, A. H. Werner, R. F. Werner, D. Meschede, A. Alberti, “Electric quantum walks with individual atoms,” Phys. Rev. Lett. 110, 190601 (2013); C. Cedzich, T. Rybár, A. H. Werner, A. Alberti, M. Genske and R. F. Werner, “Propagation of quantum walks in electric fields,” Phys. Rev. Lett. 111, 160601 (2013).
    [4] M. Sajid, J. K. Asbóth, D. Meschede, R. Werner, and A. Alberti, ”Creating Floquet Chern insulators with magnetic quantum walks,” arXiv:1808.08923 [quant-ph] (2018).
    [5] T. Groh, S. Brakhane, W. Alt, D. Meschede, J. K. Asbóth, and A. Alberti, “Robustness of topologically protected edge states in quantum walk experiments with neutral atoms,” Phys. Rev. A 94, 013620 (2016).
    [6] C. Robens, W. Alt, D. Meschede, C. Emary, and A. Alberti, “Ideal Negative Measurements in Quantum Walks Disprove Theories Based on Classical Trajectories,” Phys. Rev. X 5, 011003 (2015).
    [7] C. F. Roos, A. Alberti, D. Meschede, P. Hauke, and H. Häffner, “Revealing Quantum Statistics with a Pair of Distant Atoms,” Phys. Rev. Lett. 119, 160401 (2017).

  • Ott

  • Invited speaker: Herwig Ott
    Affiliation: Technische Universität Kaiserslautern
    Title: Rydberg Physics Meets Ultracold Quantum Gases
    Time and room: 17:15, lecture hall IAP
    Abstract: During the last two decades, ultracold quantum gases have become a valuable experimental platform for many-body physics, and a series of groundbreaking studies with bosonic and fermionic quantum gases has been carried out. At the same time, cooling and trapping of ultracold atoms has revolutionized the field of Rydberg physics, a discipline, which has its origin in atomic physics. Today, both research directions are closely linked to each other.

    In my talk, I will show how the two formerly disjunct areas of physics can benefit from each other. In particular, I will show that so-called Rydberg molecules can be employed to tune the interaction in an ultracold quantum gas via an optical Feshbach resonance.

  • Reichel

  • Invited speaker: Jakob Reichel
    Affiliation: Laboratoire Kastler Brossel, ENS Paris
    Title: Creating Multiparticle Enganglement with Optical Fiber Microcavities
    Time and room: 17 h c.t., lecture hall IAP
    Abstract: An exciting and fast-growing research field has emerged at the interface of fundamental physics and technology, where the nonclassical features of quantum mechanics are employed to engineer powerful, radically new functionalities. Multiparticle entanglement is a key resource in these quantum technologies. I will describe how high-finesse optical cavities can be used to produce and detect such entanglement in ensembles of ultracold atoms and other quantum emitters, and show examples from recent experiments in our group with fiber microcavities on atom chips. One application is quantum metrology, and I will show progress towards a spin-squeezed atomic clock on a chip.