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Quantum technologies

Dieter Meschede's research group
Home AMO physics colloquia
  • Chris Westbrook

  • Invited speaker: Dr. Chris Westbrook
    Affiliation: Laboratoire Charles Fabry de l'Institut D'Optique
    Title: An Atomic Hong Ou Mandel Experiment

    Time and room: special colloquium, 17:15 h, lecture hall IAP
    Abstract: The quantum theory has introduced physicists to two major counter-intuitive concepts. On the one hand, there is wave-particle duality, which means that objects normally described as particles can also behave as waves, while entities primarily described as waves, such as light, can behave as particles. This revolutionary idea nevertheless relies on concepts borrowed from classical physics, either waves or particles evolving in an ordinary space-time. On the other hand, entanglement can lead to interferences between the amplitudes of multi-particle states, which happen in Hilbert space and have no classical counter-part. This fundamental feature has of course been strikingly demonstrated by the violation of Bell’s inequalities. There is however, a conceptually simpler situation in which the interference between two-particle amplitudes entails a behavior impossible to describe by any classical model. This is the celebrated Hong Ou and Mandel experiment, in which two photons arriving simultaneously in the input channels of a beam-splitter always emerge together in one of the output channels. This effect has been extensively used to characterize the quality of non-classical light sources. In Palaiseau our group has realized a close analog to the Hong Ou Mandel experiment using atoms. I will discuss the experiment and comment on prospects for extending our methods to other, traditionally optical experiments such as the violation of Bell's inequalities with atoms.

  • Jacques Carolan

  • Invited speaker: Dr. Jacques Carolan
    Affiliation: University of Bristol
    Title: Verifying the unverifiable: certifying quantum complexity in linear optical experiments
    Time and room: 17:15 h, lecture hall IAP
    Abstract: Photons propagating through integrated linear optical circuits have emerged as a promising candidate for quantum technologies due to their outstanding low-noise properties and prospects for scalability. I will give a brief overview of photonic quantum computing then motivate a class on non-universal quantum simulator, a boson sampler, which promises to achieve quantum supremacy in the much nearer term. The complexity of this type algorithm means its solution is formally unverifiable and the task of establishing correct operation becomes one of gathering sufficiently convincing circumstantial evidence. I will present scalable methods to experimentally establish correct operation for this class of sampling algorithm, which we implement with two different types of optical circuits for 3, 4, and 5 photons, on Hilbert spaces of up to 50,000 dimensions. Finally, I will present new work on reconfigurable linear optical circuits, whose versatility can be applied to a wide variety of quantum experiments.  These results have wider applications in the verification of large scale quantum systems and photonic quantum computing beyond boson sampling.

  • Julia Stähler

  • Invited speaker: Dr. Julia Stähler
    Affiliation: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin
    Title: Ultrafast Electron Dynamics at Oxide Surfaces: How Metallic is a Semiconductor?
    Time and room: 17:15 h, lecture hall IAP
    Abstract: Light absorption in a semiconductor creates non-equilibrium conditions that relax by a multitude of pathways involving electron dynamics on femto- and picosecond timescales. These can be monitored in real time using time-resolved one- and two-photon photoelectron spectroscopy of the occupied and unoccupied electronic structure, respectively. On ZnO(10-10), hydrogen adsorption causes the formation of a charge accumulation layer through downward surface band bending. Despite this metallicity, highly stable sub surface-bound excitons form within only 200 fs after above band gap photoexcitation. Strong excitation close to the Mott limit enhances the screening of the Coulomb interaction (CIA) and reduces the exciton formation probability [1]. On the other hand, in the case of the strongly correlated electron material VO2, strong photoexcitation even leads to an instantaneous collapse of the band gap, followed by hot carrier relaxation within 200 fs. In conjunction with many body perturbation theory, these results show that the photoinduced semiconductor-to-metal transition is caused by photohole doping at the top of the VO2 valence band: The significantly enhanced screening of the CIA through low-energy intraband transitions causes the drastic band gap renormalization [2].

    [1] J.-C. Deinert et al., Phys Rev Lett 113, 057602 (2014)

    [2] D. Wegkamp, M. Herzog et al., Phys Rev Lett 113, 216401 (2014)

  • Alexander Szameit

  • Invited speaker: Prof. Alexander Szameit
    Affiliation: Universität Jena
    Title: Integrated Optical Circuits For Classical And Quantum Light
    Time and room: special colloquium, 13:15 h, lecture hall HISKP
    Abstract: The implementation of wave guiding structures on a chip constitutes a superior possibility to specifically tailor the dynamics of light with in a very small volume. Integrated optical circuits allow the realization of propagation phenomena of classical and quantum light that are not observable in free space. Such structures are in particular useful as model system for quantum mechanical processes that are inaccessible in the laboratory otherwise.
    In my presentation I will present the basics of coupled optical waveguide structures and discuss several applications for emulating quantum mechanical processes, such as topological insulation, non-Hermitian PT-symmetric systems and even supersymmetric photonic structures.


  • Uwe Bovensiepen

  • Invited speaker: Prof. Uwe Bovensiepen
    Affiliation: Universität Duisburg-Essen
    Title: Non-equilibrium Electronic States and Their Dynamics in Solid Materials Excited and Analyzed by Femtosecond Laser Pulses
    Time and room: 17:15 lecture hall IAP
    Abstract: Optical excitations in solid materials decay typically on femto- to picosecond time scales due to interactions which lead to a redistribution of the excess energy among the electronic, the lattice, and the spin subsystem, before final dissipation. We perform pump-probe experiments in order to analyze these excitations and the action they generate through their relaxation directly in the time domain. In this talk time- and angle-resolved photoemission results which probe the excited state with energy and momentum sensitivity on complex materials like high temperature superconductors will be discussed. Furthermore, we investigate spin currents generated in the optically excited state and their influence on the transient magnetization of optically excited ferromagnets. We employ pump-probe experiments where we detect the complex magneto-optical Kerr effect to obtain an effective depth sensitivity in to order to monitor these currents. In epitaxial Co films on a metallic substrate spin currents generate a magnetization depth profile with a depletion of spin polarization at the interface. This local depletion competes with thermalization of the electron and spin system which suppresses the magnetization predominantly close to the surface.