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Dieter Meschedes Forschungsgruppe
Home AMO-Physikkolloquien
  • Franz Gießibl

  • Invited speaker: Franz J. Gießibl
    Affiliation: Universität Regensburg
    Title: Progress in Studying Matter on the Atomic Scale
    Time and room: 17:15, lecture hall IAP
    Abstract: The scanning tunneling microscope (STM), invented in 1981 by Binnig and Rohrer, has provided direct access to the world of atoms. STM relies on vacuum tunneling with an exponential increase of a tunneling current between two biased conductive electrodes at a factor of ten per Å. If a tip has one atom that sticks out one Å more than all the others, this front atom carries ten times more current than the other atoms. The monotonic decrease of current with distance facilitates distance feedback and allows to scan the tip across a sample with atomic precision. In 1986, Binnig, Gerber and Quate introduced atomic force microscopy (AFM), a method that also images insulators by relying on forces. Unlike the current, the force between tip and sample is non-monotonic and includes long- and short range components. AFM has been inferior in resolution to STM for a long time. Today, it exceeds STM in spatial resolution. That progress was enabled by advances in measuring small forces and by the isolation of chemical bonding forces from strong background forces.

    Figure: AFM image of an Fe3 and Fe2 cluster on a Cu(111) surface.


  • Nicolas Sangouard

  • Invited speaker: Nicolas Sangouard
    Affiliation: University of Basel
    Title: Towards entanglement at macroscopic scales
    Time and room: 9 h c.t., Conference room 1, Physikalisches Institut

    Abstract: Although the birth of quantum theory brought with it numerous objections, it is nowadays well accepted as a theory describing the behavior of microscopic systems. Two photons for example, can be entangled so that they lose their individual properties and are described as a whole only. In principle, but so far without any demonstration, quantum theory applies at any scale, and even macroscopic objects can exhibit the properties of quantum particles, such as entanglement.
    During this talk, I will present several attempts to detect quantum correlations in macroscopic systems. This will include proposals and corresponding experimental results in atomic and photonic systems. In particular, I will present in detail the first results of a project aiming to lay the basis for a new class of experiments interfacing quantum systems and biological detectors. This proposal could lead to the first experiment where entanglement is detected with the human eye. The success of this endeavor relies on coherent amplification techniques that can be used to upgrade the human eye up to the point where they can reveal the quantum nature of few photon entangled states. Beyond their fundamental interest, our amplification techniques might find applications in quantum technologies e.g. to upgrade widely available but rudimentary detectors to communicate securely and to generate certified randomness. 
  • Christof Weitenberg

  • Invited speaker: Christof Weitenberg
    Affiliation: Universität Hamburg
    Title: Observation of a dynamical phase transition in the non-equilibrium dynamics of ultracold quantum gases in driven optical lattices
    Time and room: 17:15, lecture hall IAP
    Abstract: Ultracold atoms are a versatile system to emulate solid-state physics including the fascinating phenomena of gauge fields and topological band structures. By circular driving of a hexagonal optical lattice, we engineer the Berry curvature of the Bloch bands and realize a Haldane-like model. We have developed a fully momentum-resolved state tomography of the Bloch states, which allows measuring the distribution of Berry curvature and obtaining the Chern number [1].
    Furthermore, we study the time-evolution of the many-body wavefunction after a sudden quench of the lattice parameters and observe the appearance, movement, and annihilation of dynamical vortices in reciprocal space. We identify them as the Fisher zeros in the Loschmidt amplitude and define them as a dynamical equivalent of an order parameter, which suddenly changes its value at critical evolution times [2]. Our measurements constitute the first observation of a so-called dynamical phase transition and address the intriguing question of the relation between this phenomenon and the equilibrium phase transition in the system.

    [1] Flaeschner et al., Science 352, 1091 (2016). [2] Flaeschner et al., arXiv:1608.05616 (2016)

  • Enrique Solano

  • Invited speaker: Enrique Solano
    Affiliation: University of the Basque Country, Bilbao
    Title: Digital and Analog Quantum Simulations in Superconducting Circuits
    Time and room: 17:15, lecture hall IAP
    Abstract: I will introduce the concept of quantum simulation as a means of reproducing physical and unphysical models onto another quantum system, which is tpyically more controllable. I will explain the role played by analog and digital quantum simulators in different quantum platforms, while providing specific examples in superconducting circuits. Furthermore, I will discuss the possible merge of digital and analog concepts to reach quantum supremacy in this promising quantum technology.

  • Fabian Grusdt

  • Invited speaker:  Fabian Grusdt
    Affiliation: Harvard University
    Title: Quantum impurities in strongly correlated phases of ultracold atoms
    Time and room: 17:15, lecture hall IAP
    Abstract: The detection and control on the level of individual lattice sites achieved by experiments with ultracold atoms and photons allows for new measurements which are difficult to realize using traditional solid-state setups. This makes such systems ideal candidates to explore exotic phases of matter, including the fractional quantum Hall effect or high-temperature superconductors. An overview will be given of the recent progress towards reaching this goal with cold atom quantum simulators. The main theme of this talk are mobile quantum impurities inside the strongly correlated quantum systems described above. On the one hand, impurities can be used as coherent probes of the many-body system. Specifically I will show how topological invariants can be measured in a fractional quantum Hall setup in this way. In other cases, impurities are at the heart of the many-body problem itself: For example, the high-temperature superconducting phase is obtained by doping mobile holes into a Mott insulator with anti-ferromagnetic spin correlations. A simple physical theory of a single hole in such systems will be presented.

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