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Dieter Meschedes Forschungsgruppe
Home AMO-Physikkolloquien
  • Sandro Wimberger

  • Invited speaker: Sandro Wimberger
    Affiliation: Università di Parma 
    Title: Discrete-time walks of a Bose-Einstein condensate in momentum space
    Time and room: 09:15, Seminar room I, HISKP 1.021
    Abstract: Each step in a discrete-time quantum walk is typically understood to have two basic components: a „coin-toss“ which produces a random superposition of two states, and a displacement which moves each component of the superposition by different amounts. Here we report on the experimental realization of a walk in momentum space with a spinor Bose-Einstein condensate (BEC) subject to a quantum ratchet realized with a pulsed, off-resonant optical lattice. By an appropriate choice of the lattice detuning, we show how the atomic momentum can be entangled with the internal spin states of the atoms. For the coin-toss, we propose to use a microwave pulse to mix these internal states. We present first experimental results of such a quantum walk based on a new type of ratchet, and through a series of simulations, demonstrate how our system can allow for the investigation of possible biases and classical-to-quantum dynamics in the presence of natural and engineered noise. Moreover, the same setup offers the possibility to realize classical random walks by applying a random sequence of intensities and phases of the time-dependent lattice chosen according to a given probability distribution. This distribution converts on average into the final momentum distribution of the atoms. In particular, it is shown that a power-law distribution for the intensities results in a classical Lévy walk in momentum space. Finally, we propose another implementation of a BEC quantum walk in reciprocal or quasimomentum space with exciting possibilities to investigate the effects of long-range quantum correlations induced by atom-atom interactions.

  • Günter Huber

  • Invited speaker: Günter Huber
    Affiliation: Universität Hamburg
    Title: Semiconductor Laser Pumped Rare Earth Ion Doped Solid-state Lasers
    in the Visible and Near Infrared Spectral Region
    Time and room: 17:15, lecture hall IAP
    Abstract: The talk reviews the basic concepts of advanced highly efficient rare earth ion doped
    solid-state lasers based on laser ions such as Yb3+, Tm3+, Er3+, Pr3+, and Tb3+ which
    have opened new prospects for laser applications at various wavelengths and power
    regimes. The main emphasis is placed on the interplay between materials aspects and
    most relevant spectroscopic as well as laser related properties in the search for new
    solid-state laser systems.
    For the near infrared spectral region Yb3+-doped laser crystals feature very high
    efficiencies and reduced heat generation due to small Stokes-losses between pump and
    laser photons. In particular, Yb3+:Lu2O3 possesses high thermal conductivity and have
    been operated at record slope efficiencies of 80% in continuous wave operation and at
    more than 100 W of average power in the mode-locked sub-ps operation regime. Laser
    diode pumped, highly efficient 2-μm Tm3+- and 3-μm Er3+-lasers with special interest
    for medical applications are based on interionic interactions of Tm3+ and Er3+ laser ions,
    Breakthroughs regarding efficient visible coherent light generation have been achieved
    with Pr3+- and Tb3+-lasers operating in the green, orange, and red spectral region under
    blue semiconductor laser pumping. Here both, the development of blue semiconductor
    pump lasers and the use of suitable short wavelength hosts with minimized excited state
    absorption of the laser ions contributed to major achievements.
    The functionality of laser crystals can be further increased by direct micro-structuring of
    bulk crystals with ultrafast laser pulses yielding for instance efficient waveguide lasers
    with diffraction limited, fundamental modes in the near infrared and visible spectral
    region. This simple direct light writing technique is also suitable for the fabrication of
    more complex structures for integrated optics in single crystalline dielectrics.

  • Klas Lindfors

  • Invited speaker: Klas Lindfors
    Affiliation: Universität zu Köln
    Title: Plasmonic Nanoantennas For Wireless Signal Transmission
    Time and room: 17:15, lecture hall IAP
    Abstract: Optical nanoantennas and nanoantenna arrays are highly innovative approaches for optical signal transmission. Transmitting the signal via a free-space link (power law signal decay) instead of plasmonic waveguides (exponential signal decay) allows realizing low-loss optical communications links without sacrificing deep sub-wavelength field confinement at the transmitting and receiving points. This is a promising route for reconciling the size mismatch between diffraction-limited integrated photonics and integrated electronics. In my talk I will present results from our work on realizing plasmonic nanoantenna devices to control the transmission of light [1,2] and on integrating single quantum emitters into nanoantennas [3,4].
    [1] D. Dregely et al., Nat. Commun. 5, 4354 (2014).
    [2] K. Lindfors et al., ACS Photonics 3, 286 (2016).
    [3] M. Pfeiffer et al., Nano Lett. 14, 197 (2014).
    [4] H. Zhang et al., Appl. Phys. Lett. 106, 101110 (2015).

  • Christoph Stampfer

  • Invited speaker: Christoph Stampfer
    Affiliation: RWTH Aachen and Forschungszentrum Jülich
    Title: Quantum Point Contacts in Graphene
    Time and room: 17:15, lecture hall IAP
    Abstract: Quantum point contacts are cornerstones of mesoscopic physics and central building blocks for quantum electronics. Although the Fermi wavelength in high-quality bulk graphene can be tuned up to hundreds of nanometers, the observation of quantum confinement of Dirac electrons in nanostructured graphene systems has proven surprisingly challenging. Here I show ballistic transport and quantized conductance of size-confined Dirac fermions in lithographically-defined graphene constrictions. The fabricated graphene constrictions are encapsulated in hexagonal boron nitride sheets allowing for high carrier mobilities. The constrictions have widths ranging from around 200 to 800 nm. At high charge carrier densities, the observed conductance agrees excellently with the Landauer theory of ballistic transport without any adjustable parameter. Experimental data and simulations for the evolution of the conductance with magnetic field unambiguously confirm the identification of size quantization in the constriction. Close to the charge neutrality point, bias voltage spectroscopy reveals a renormalized Fermi velocity of ~1.5x106 m/s in our graphene constrictions. Moreover, at low carrier density transport measurements allow probing the density of localized states at edges, thus offering a unique handle on edge physics in graphene devices.

  • Martin Schulze

  • Invited speaker: Martin Schultze
    Affiliation: MPI für Quantenoptik, Garching
    Title: Attosecond Spectroscopy of Multi-Electron Dynamics in Atoms and Solids
    Time and room: 17:15, lecture hall IAP
    Abstract: Light-matter interaction starts with light-field driven electron dynamics. Attosecond spectroscopy can achieve a temporal resolution way above optical frequencies and thus allows to investigate the energy exchange dynamics between electric fields and matter with unprecedented detail.

    I will discuss how such experiments reveal the influence of electronic correlations on the photoelectric effect and show how solid state attosecond spectroscopy provides us with a time-domain understanding of multi electron dynamics also in solids. These studies observe lasting and transient optical excitations across the band gap of semiconductors and dielectrics with sub-femtosecond response time, the resulting band-structure modifications and the energy exchange dynamics between light-field and solid.

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