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Dieter Meschedes Forschungsgruppe
Home AMO-Physikkolloquien
  • Matthias Keller

  • Invited speaker: Matthias Keller
    Affiliation: University of Sussex
    Title: Interfacing Ions and Photons
    Time and room: 17:15, lecture hall IAP
    Abstract: The complementary benefits of trapped ions and photons as carriers of quantum information make it appealing to combine them in a joint system. Ions provide low decoherence rates, long storage times and high readout efficiency, while photons travel over long distances. To interface the quantum states of ions and photons efficiently, we use calcium ions coupled to an optical high-finesse cavity via a Raman transition.
    To achieve strong ion-cavity coupling we employ fibre tip cavities integrated into the electrodes of an endcap style ion trap. With a cavity length of 380 mm the resulting ion-cavity coupling strength is 17 MHz with a cavity line width of 10 MHz. We trap single calcium ions with a life time of several hours and have optimised the ion-cavity overlap to observe the interaction of the cavity with the ion.
    In another experiment, we combine a conventional cavity with a linear ion trap to facilitate the investigation of the interaction of multiple ions with a single cavity mode. We have demonstrated the localisation of several ions in a collinear cavity-trap system and have demonstrated the emission of polarised single photons from this system.
    To enable the use of fibre cavities in applications such as single photon sources and nodes in quantum networks the coupling between the cavity and the fibre must be significantly improved. We have developed a system to integrate mode matching optics into a fibre system and have demonstrated a mode matching between cavity and fibre on the order of 90%.

  • Wilhelm Zwerger

  • Invited speaker: Wilhelm Zwerger
    Affiliation: TU München
    Title: Have Ultracold Atoms Led to Progress in Theoretical Physics?
    Time and room: 10:15, lecture hall HISKP
    Abstract: Ultracold atoms have opened new directions in physics, both by realizing paradigmatic models in many-body and statistical physics and by allowing to explore phenomena which have never been accessible before. In this talk, I will discuss to which extent ultracold atoms have also triggered new developments in theoretical physics.
    It is shown that for the quantum many-body problem with zero range interactions which appears naturally in the context of ultracold atoms, there are a number of exact relations which connect the short distance behavior of the one- and two-body density matrix with thermodynamic properties and also the high-frequency or large momentum behavior of correlation functions. These relations follow from a Wilson operator product expansion and thus apply to arbitrary states of the many-body system. Two examples for their concrete application are given: the violation of scale invariance in RF-spectroscopy of two-dimensional Fermi gases and deep inelastic scattering on strongly interacting Bose gases at large momentum.

  • 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)