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Quantum technologies

Dieter Meschede's research group
Home AMO physics colloquia
  • Sebastian Diehl

  • Invited speaker: Prof. Sebastian Diehl
    Affiliation: Universität Innsbruck
    Title: Bose Condensation Phenomena in Driven Open Quantum Systems
    Time and room: 17:15 lecture hall IAP
    Abstract: Quantum optics and many-body physics increasingly merge together in ultracold atomic gases and certain classes of solid state systems. This gives rise to new non-equilibrium scenarios in stationary state, where coherent and dissipative dynamics appear on an equal footing.
    Here we report on dynamical critical phenomena of the non-equilibrium Bose condensation transition in many-body ensembles subject to particle loss and pumping. Using two complementary renormalization group approaches, in three spatial dimensions we establish an effective thermalization mechanism of the low frequency dynamics. Still, the microscopic drive conditions are witnessed even on the largest scales via a new, independent universal critical exponent. Such systems thus define an out-of equilibrium universality class beyond the classification scheme of equilibrium dynamical transitions.
    Furthermore, we address the fate of non-equilibrium Bose condensation in two dimensions. We show that such driven Bose systems cannot exhibit algebraic superfluid order, unless being strongly anisotropic. Our result implies, in particular, that Bose condensation in currently investigated exciton-polariton systems must be an intermediate scale crossover phenomenon, while the true long distance correlations fall off exponentially. We obtain these results through a mapping of the long-wavelength condensate dynamics onto the anisotropic Kardar-Parisi-Zhang equation.

  • Martin Zwierlein

  • Invited speaker: Prof. Martin Zwierlein
    Affiliation: MIT, Cambridge, MA
    Title: Solitonic Waves In A Fermionic Superfluid
    Time and room: 17:15 lecture hall IAP
    Solitons - solitary waves that maintain their shape as they propagate — occur as water waves in narrow canals, as light pulses in optical fibres and as quantum mechanical matter waves in superfluids and superconductors. Their highly nonlinear and localized nature makes them very sensitive probes of the medium in which they propagate. We create long-lived solitary waves in a strongly interacting superfluid of fermionic atoms and directly observe their motion [1]. As the interactions are tuned from the regime of Bose–Einstein condensation of tightly bound molecules towards the Bardeen–Cooper–Schrieffer limit of long-range Cooper pairs, the waves' effective mass increases dramatically, to more than 200 times their bare mass. This mass enhancement is more than 50 times larger than the theoretically predicted value for planar solitons. I will present new experiments that reveal the microscopic nature of the observed solitary waves. Our work provides a benchmark for theories of non-equilibrium
    dynamics of strongly interacting fermions.

    [1] Tarik Yefsah, Ariel T. Sommer, Mark J.H. Ku, Lawrence W. Cheuk, Wenjie Ji,
    Waseem S. Bakr, Martin W. Zwierlein, Heavy Solitons in a Fermionic Superfluid, Nature 499, 426-430 (2013)

  • Giovanna Morigi

  • Invited speaker: Prof. Giovanna Morigi
    Affiliation: Universität Saarbrücken
    Title: Quantum structures of photons and atoms
    Time and room: 17:15 lecture hall IAP

    In this talk I will discuss several examples of selforganization of atoms inside a single-mode resonator. When a laser transversally pumps the atom, photon scattering into the resonator depends on the atoms density distribution within the cavity, and in turn determines the strength of the mechanical forces of light on the atoms, hence the atomic density. The dynamics is thus nonlinear and can lead to ordered atomic structures when the laser intensity exceeds a threshold determined, amongst other, by the rate of cavity losses. I will first discuss the dynamics of selforganization in the semiclassical regime, in which the atomic motion is cooled by the photon scattering processes which pump the cavity. I will then consider the quantum regime, where the atoms are ultracold bosons. The atoms are confined by an external optical lattice, whose period is incommensurate with the cavity mode wave length, and are driven by a transverse laser, which is resonant with the cavity mode. While for pointlike atoms photon scattering into the cavity is suppressed, for sufficiently strong lasers quantum fluctuations can support the build-up of an intracavity field, which in turn amplifies quantum fluctuations. In this
    parameter regime the atoms form clusters which are phase locked, thereby maximizing the intracavity photon number. I will argue that this system constitutes a novel setting where quantum fluctuations give rise to effects usually associated with disorder.


  • Antonio Negretti

  • Invited speaker: Dr. Antonio Negretti
    Affiliation: Universität Hamburg
    Title: Optimal Control of a Quantum Many-Body System
    Time and room: 17:15 lecture hall IAP

    This presentation concerns open-loop optimal control and its application to experiments. Optimal control theory is a well-established research field and it has been already successfully applied to classical engineering. In order to accomplish entangled-based technology, beyond first-principle experiments, an accurate control of the quantum system dynamics is of paramount importance. To this end, optimal control offers a number of methods in order to engineer tailored dynamics of quantum systems.
    After illustrating the underlying idea of optimal control and sketching how most of the popular optimization algorithms work, we shall introduce a recently developed quantum optimization algorithm that enables us to specifically design experiments of quantum physics in realistic settings. Importantly, such a method allows optimizing efficiently the dynamics of quantum many-body systems. As a specific example, we illustrate how the method successfully works in optimizing the dynamics of a Bose-Einstein condensate with the aim of realizing a Ramsey-like interferometer with motional states of the condensate.

  • Marcello Dalmonte

  • Invited speaker: Dr. Marcello Dalmonte
    Affiliation: Institute for Theoretical Physics, Universität Innsbruck
    Title: A case for quantum Ice and frustrated magnetism with Rydberg-dressed atoms in optical lattices
    Time and room: 17:15 lecture hall IAP
    Abstract: The concept of gauge symmetry permeates through many branches of our understanding of physical phenomena, ranging from fundamental interactions in particle physics, to frustrated magnetism and spin models. In this talk, we will discuss how some simple instances of gauge theories relevant for the study of condensed matter problems can be realized in ultra-cold atom gases in optical lattices. In particular, the case of quantum Ice, a paradigmatic model for frustrated magnetism, will be discussed in some detail, starting from a review of its classical counterpart, then illustrating how its involved quantum dynamics can be generated with bosonic atoms weakly admixed with Rydberg states. Finally, a brief overview on the implementation of gauge symmetries in synthetic systems and quantum simulators will be presented.