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Quantum technologies

Dieter Meschede's research group
Home AMO physics colloquia
Colloquia
  • Giovanni Barontini

    (19/04/16)
  • Invited speaker: Dr. Giovanni Barontini
    Affiliation:  University of Birmingham
    Title: Strong Coupling Regime In Fibre Microcavities: From EIT To Multi-Particle Entanglement

    Time and room:  17:15 h, lecture hall IAP

    I will report on recent results obtained with cold atoms in fibre microcavities at Imperial College London and at the Laboratoire Kastler-Brossel in Paris. In the London experiment, we show how the Hamiltonian eigenstates of the system can be revealed through spectroscopic measurements despite the fast decoherence rate of the microcavity. We observe an avoided crossing in the dressed cavity spectrum, usually taken as evidence of strong coupling, notwithstanding the complete overdamping of Rabi oscillations in our experiment. We interpret this as dipole-induced transparency of the cavity, relying on destructive quantum interference to uncover the normal modes which might be expected to lie obscured [1]. In the Paris experiment instead, the atom-cavity coupling rate greatly exceeds every loss rate allowing to reach the single-atom strong coupling regime and to perform almost non-destructive measurements. Building on this we have developed a method based on the quantum Zeno dynamics to create symmetric entangled states in ensembles of several tens of atoms. We characterize the resulting states by performing quantum tomography, yielding a time-resolved account of the entanglement generation. In addition, we study the dependence of quantum states on measurement strength and quantify the depth of entanglement. Our results show that quantum Zeno dynamics can be used as a versatile tool for fast and deterministic entanglement generation [2].
    [1] Y.-H. Lien, G. Barontini, M. Scheucher, J. Goldwin, and E. A. Hinds, in preparation
    [2] G. Barontini, L. Hohmann, F. Haas, J. Estéve, and J. Reichel, Science 349, 1317 (2015)

  • Vera Guarrera

    (18/04/16)
  • Invited speaker: Dr. Vera Guarrera
    Affiliation: National Physical Laboratory

    Special Colloquium
    Title: Towards Guided Ultracold Atom Interferometers: NPL's Approach

    Time and room:  17:15 h, lecture hall IAP

    Atom interferometry constitutes nowadays one of the most promising and realistic forefronts in the accomplishment of quantum-based technologies. Parallel to the evolution of free-falling atom interferometers, new concepts with trapped and guided atoms have been developed in the last few years. Due to the large control over the atomic wavefunction and to the long interrogation times, an outstanding combination of high sensitivity and spatial resolution is expected to be achieved in compact and, ideally, portable devices. In this talk I will present our all-optical approach towards the realization of BEC-based guided interferometers. In particular I will discuss the recent results on the realization of a continuous Bragg splitter generated by the interference of two atomic waveguides. Progress towards the implementation on a miniaturized light chip will be finally reported.

  • Henning Moritz

    (09/02/16)
  • Invited speaker: Prof. Henning Moritz
    Affiliation: Institut für Laser-Physik
    Title: Superfluidity and coherence in fermionic quantum gases

    Time and room:  17:15 h, lecture hall IAP

    Frictionless flow is one of the most striking macroscopic phenomena arising from quantum physics. Its appearance is remarkably widespread, ranging from superconductivity in solids to superfluidity in liquids and dilute gases. In this colloquium, I will present experiments in which we take the chance to study the stability and coherence of superfluids in experimental model systems, ultracold Fermi gases. In the first part, I will focus on the stability against external perturbations. The corresponding quantity, the critical velocity, is typically highest in the strongly interacting regime. I will show how we can determine the critical velocity in this regime and study its evolution in the crossover from Bose-Einstein condensation to Bardeen-Cooper-Schrieffer superfluidity.

    Superfluidity and coherence are intimately connected. In 3D systems, bosonic atoms or Cooper pairs condense into a macroscopic wavefunction exhibiting true long range coherence. Meanwhile, 2D superfluids show a strikingly different behavior: True long-range coherence is precluded by thermal fluctuations, nevertheless Berezinskii-Kosterlitz-Thouless (BKT) theory predicts that 2D systems can still become superfluid. However, the first order correlation function g1(r) decays algebraically with distance with a temperature-dependent scaling exponent tau. I will present local coherence measurements of a strongly interacting 2D gases. They allow us to observe this algebraic, scale-free decay. We can determine the scaling exponent and the superfluid density as a function of phase space density.

  • Piet. O. Schmidt

    (01/12/15)
  • Invited speaker: Prof. Piet O. Schmidt
    Affiliation: QUEST Institut, PTB Braunschweig and Leibniz Universität Hannover
    Title: Quantum Logic Spectroscopy of trapped Ions

    Time and room:  17:15 h, lecture hall IAP
    Abstract: Precision spectroscopy is a driving force for the development of our physical understanding. However, only few atomic and molecular systems of interest have been accessible for precision spectroscopy in the past, since they miss a suitable transition for laser cooling and internal state detection. This restriction can be overcome in trapped ions through quantum logic spectroscopy. Coherent laser manipulation originally developed in the context of quantum information processing with trapped ions allow the combination of the special spectroscopic properties of one ion species (spectroscopy ion) with the excellent control over another species (logic or cooling ion). In my talk I will show that quantum logic spectroscopy enables the development of accurate optical clocks based on aluminium and highly-charged ions as well as precision spectroscopy of broad and non-closed transitions in calcium isotopes. Finally, I present non-destructive internal state detection and spectroscopy of molecular ions using quantum logic. This represents a first step towards extending the exquisite control achieved over selected atomic species to much more complex molecular ions. Applications of quantum logic spectroscopy ranging from the measurement of atomic, molecular and nuclear properties over optical clocks for relativistic geodesy to the search for a variation of fundamental constants will be discussed.

     

  • Nikolay Vitanov

    (24/11/15)
  • Invited speaker: Prof. Nikolay Vitanov
    Affiliation: Universität Sofia
    Title: High-fidelity quantum control and quantum information processing with composite pulses

    Time and room:  17:15 h, lecture hall IAP
    Abstract: The technique of composite pulses has been used for a long time in nuclear magnetic resonance and, since recently, in quantum optics and quantum information. This technique replaces the single pulse used traditionally for driving a two-state quantum transition by a sequence of pulses with suitably chosen phases, which are used as control parameters for shaping the excitation profile in a desired manner. Composite pulses produce unitary operations, which combine very high fidelity with robustness to parameter variations. We have developed a pool of composite pulses by using a novel SU(2) approach to design recipes for construction of single-qubit operations, including broadband, narrowband and passband pulses, universal composite pulses, composite adiabatic passage and composite STIRAP, some of which have already been demonstrated in experiments with doped solids. We have also designed efficient and robust composite techniques for construction of highly entangled states, e.g. Dicke and NOON states, and multi-qubit gates, e.g. C-phase, Toffoli, and generally CN-phase gates.