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Quantentechnologie

Dieter Meschedes Forschungsgruppe
Home AMO-Physikkolloquien
Colloquia
  • 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.

  • Isabelle Staude

    (17/11/15)
  • Invited speaker: Dr. Isabelle Staude
    Affiliation: Universität Jena
    Title: Tailoring Light Fields with Resonant Dielectric Nanosurfaces

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


     

    High-refractive-index dielectric nanoresonators and their assemblies show complex and sometimes unexpected interactions with light, including optically-induced magnetic response, directional scattering, Fano resonances, and strong near-field enhancements. Using the capabilities of modern nanotechnology, these interactions can be tuned by the size, shape, material composition, and arrangement of the nanoresonators. In addition, dielectric nanoresonators exhibit very low absorption losses at optical frequencies. Based on these unique optical properties, high-index dielectric nanoresonators represent versatile building blocks of resonant nanosurfaces with tailored linear and nonlinear optical properties. This talk will review our recent advances in light wave control with dielectric nanosurfaces using silicon nanodisks as nanoresonators. It will focus on nanosurfaces designed to impose a spatially variant phase shift onto an incident light field, thereby providing control over its wave front. Based on the simultaneous excitation of electric and magnetic dipole resonances, the nanoresonators can be tailored to emulate the behavior of the forward-propagating elementary wavelets known from Huygens’ principle. This concept allows for the realization of nanosurfaces with near-unity transmittance efficiency, full phase coverage, and a polarization insensitive response. Various examples of wavefront control will be discussed, including beam shaping and holographic imaging, both of which we have experimentally demonstrated with high efficiency at telecom frequencies.

  • Jens Eisert

    (03/11/15)
  • Invited speaker: Prof. Jens Eisert
    Affiliation: Freie Universität Berlin
    Title: Taming The Non-Equilibrium

    Time and room:  17:15 h, lecture hall IAP
    Abstract: Complex quantum systems out of equilibrium are at the basis of a number of the most intriguing puzzles in physics. This talk will be concerned with recent progress on understanding how quantum many-body systems out of equilibrium eventually come to rest and thermalise. The first part of the talk will highlight theoretical progress on this question, taking in several ways a quantum information view - employing ideas of Lieb-Robinson bounds, quantum central limit theorems and of concentration of measure. These findings will be complemented by experimental work with ultra-cold atoms in optical lattices, in setups constituting dynamical "quantum simulators", allowing to probe physical questions that are not only out of reach for state-of-the-art numerical techniques based on matrix-product states, but also relate to classically computationally hard problems.