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Quantum technologies

Dieter Meschede's research group
Home AMO physics colloquia
Colloquia
  • Stefan Kaiser

    (25/10/16)
  • Invited speaker: Stefan Kaiser
    Affiliation: MPI FKF Stuttgart
    Title:  Ultrafast Optical Control of Complex Quantum Materials 

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

    It is well known fact that various phase transitions in condensed matter can by triggered by external parameters such as temperature, pressure, electric field or magnetic field. Finding systems that show phase transitions triggered by external stimulation of light became a particular interesting field of research.
    Advanced nonlinear optical methods such as ultra-broad band pump-probe spectroscopy open new ways of controlling ultrafast dynamics in complex solid-state materials on unprecedented timescales. In quantum materials, finding new ways of manipulating the complex interplay of electronic phases or effectively tuning electronic interactions opens new avenues in controlling physical properties and designing new functionalities.
    I will show how we investigate different scenarios like the balancing between competing phases triggered by ultrashort light pulses or possibilities of dynamical stabilization of new states of matter in periodically driven light fields. In particular I will discuss the remarkable possibilities to induce superconductivity in high temperature cuprate superconductors by melting competing “stripe”-order [1] or even promoting it to temperatures far above Tc; for some underdoped materials even up to room temperature for a few picoseconds [2,3]. Possible light-induced superconductivity in the doped fullerides K3C60 [4] will serve as important example that inducing such intriguing effects is a more general effect and not restricted to the rather specialized class of cuprate systems.
    [1] D. Fausti et al. Science 331, 189 (2011).
    [2] S. Kaiser et al. Phys. Rev. B 89, 184515 (2014).
    [3] W. Hu et al. Nature Materials 13, 705 (2014).
    [4] M. Mitrano et al. Nature 530, 461 (2016).
     

  • Janos Asbóth

    (22/09/16)
  • Invited speaker: Dr. Janos K. Asbóth
    Affiliation: Wigner Research Centre for Physics of the Hungarian Academy of Sciences, Budapest
    Title: The Hofstadter butterfly takes flight in quantum walks
    Time and room: 17:15 h, lecture hall IAP

    The Hofstadter butterfly [1] is the intricate self-similar structure of subgaps that the single energy band of a charged particle hopping on a 2-dimensional lattice develops as a magnetic field perpendicular to the lattice is turned on. It has inspired physicists for 40 years (e.g., an important role in the exploration of the quantum Hall effect), and now enjoys a renewed interest as experiments might finally be close to observing it. In case the charged particle has several internal states, the spectrum as a function of magnetic field is a multiband Hofstadter butterfly, where each energy band develops a set of subgaps. We show that besides developing subgaps, the bands of topologically nontrivial (e.g., Chern) insulators must also flow in energy as the magnetic field is tuned, because eigenstates flow across topological gaps at a steady rate. We thus connect the global topology of multiband Hofstadter butterflies, i.e., the pattern in which bands flow into each other, with the topological invariants of the underlying lattice Hamiltonians. Our results also apply to quantum walks, and other periodically driven systems, where we obtain a simple formula for the Rudner topological invariant [2], which has potential to be directly measured.

    [1]: D. R. Hofstadter, Phys. Rev. B 14, 2239–2249 (1976)
    [2]: M. S. Rudner et al, Phys. Rev. X 3, 031005 (2013)
  • Dieter Suter

    (21/06/16)
  • Invited speaker: Prof. Dieter Suter
    Affiliation: TU Dortmund
    Title: Quantum Computers: Promise, Problems and Possible Solutions

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

    The "digital revolution" that is transforming our lives and our economy is based on the ubiquity of information-processing devices whose processing power increased exponentially, following Moore's law. As this trend is approaching fundamental physical limits, new directions are explored for even more powerful computational devices based on quantum mechanical systems. Such devices can solve problems that will remain out of reach for conventional computers. The main difficulty for their implementation is the fragility of information stored in coherent superpositions of quantum mechanical eigenstates. This talk will highlight some aspects of the potential offered by quantum computers, as well as the difficulties that must be overcome to realise this potential. Our current work concentrates on finding solutions for some of these problems.

  • Philipp Strack

    (14/06/16)
  • Invited speaker: Dr. Philipp Strack
    Affiliation: Universität zu Köln
    Title: Interacting and Emergent Photons in Quantum Optics and Condensed Matter

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

    Photons, that is, the quanta of light fields, typically interact only weakly. In our research at the interface of quantum optics and condensed matter [1], we explore theoretically the behavior of quantum materials in which quantum light couples strongly and coherently to non-linear matter components. The matter, in turn, can mediate photon-photon interactions. Examples include dense optical media in optical cavities or fractionalized spins in quantum magnets, which couple to emergent gauge fields. In this talk, will describe our work on many-body physics across these areas and try to elucidate the differences and commonalities between the photons of quantum optics versus the emergent photons in strongly correlated condensed matter materials.

    [1] http://www.thp.uni-koeln.de/~strack/
     

  • Sven Höfling

    (13/06/16)
  • Invited speaker: Prof. Sven Höfling
    Affiliation: Universität Würzburg; University of St Andrews, UK

    Special Colloquium
    Title: Polariton Lasers

    Time and room:  16:15 h, lecture hall IAP

    More than a century after the introduction of incandescent lighting and half a century after the realization of semiconductor lasers, semiconductor light sources are continuing to revolutionize applications and having a paramount impact on our everyday life. The creativity of quantum and photonics engineers and material scientists results in semiconductor light emitting diodes and semiconductor lasers with unprecedented characteristics, including ever better efficiency or brightness and ultra-wide wavelength coverage. In this talk, after a summary of general semiconductor laser research undertaken in our group, I will focus on the description of a novel kind of coherent light emitter based on a semiconductor microcavity with embedded quantum wells. In contrast to conventional lasers, this sort of device, termed polariton laser, relies not on stimulated emission of photons but on stimulated scattering of bosonic quasiparticles, the polaritons. These devices have lower thresholds than conventional lasers, and I will describe the physics underlying these devices and routes towards possible practical applications.