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Quantum technologies

Dieter Meschede's research group
Home AMO physics colloquia
  • Prof. Ferdinand Schmidt-Kaler

  • Invited speaker: Prof. Ferdinand Schmidt-Kaler
    Affiliation: Universität Mainz
    Title: Quantentechnologien mit kalten Ionen: Quantencomputer und Quantenrepeater
    Time and room: 17:15, lecture hall IAP
    Abstract: In den 30er Jahren des letzten Jahrhunderts fanden E. Schrödinger und W. Heisenberg die Gesetze der Quantenmechanik. Seitdem überraschen uns die Eigenschaften der Quantenwelt, die jedem klassischen Verständnis widersprechen. Inzwischen ist es ein Brennpunkt der aktuellen Forschung, Quanteneigenschaften gezielt zu nutzen um moderne Anwendungen zu ermöglichen. Dazu werden Quantenbits – quantenmechanische Überlagerungen zweier logischer Zustände – in Atome, Ionen oder Photonen eingeschrieben, verarbeitet, weitergeleitet und ausgelesen.

    Ich stelle am Beispiel von kalten, gefangenen Ionenkristallen dar, wie man Quantenbits für einen zukünftigen Quantencomputer nutzten kann [1]. Dazu werden die Ionen einzeln mit Laserpulsen manipuliert und quantenlogische Gatteroperationen realisiert [2,3]. Für die Skalierbarkeit hin zu einer großen Zahl von Quantenbits werden die Ionen in einer speziellen Paulfalle gehalten und bewegt. Ionenkristalle können auch genutzt werden, um magnetische Wechselwirkungen in einem Vielteilchensystem zu simulieren, wie etwa Ferro- bzw. Antiferromagnetismus oder auch Phasenübergänge in frustrierten Systemen [4]. Ein weiteres Anwendungsgebiet der Quanteninformationsverarbeitung ist das Verteilen geheimer Schlüssel für die Kommunikation. Hier forschen wir an einem Quantenrepeater, der es ermöglichen soll über weite Entfernungen Verschränkung zwischen Quantenbits aufzubauen und abhörsichere Kommunikation oder Teleportation zu ermöglichen.

    [1] J. I. Cirac und P. Zoller, Phys. Rev. Lett. 74, 4091(1995).
    [2] F. Schmidt-Kaler et al., Nature 422, 408 (2003).
    [3] Poschinger et al, PRL 105, 263602 (2010).
    [4] Bermudez et al, arXiv:1108.1024, angenommen (2011)

  • Prof. Gerd Röpke

  • Invited speaker: Prof. Gerd Röpke
    Affiliation: Universität Rostock
    Title: Radiation and Line Spectra from Dense Plasmas
    Time and room: 17:15, lecture hall IAP
    Abstract: Emission and absorption of electromagnetic radiation is a fundamental process in plasma physics. It is of relevance, e.g., for heating and cooling, diagnostics of plasmas and the generation of light. Besides continuum radiation (bremsstrahlung), the line spectrum is of
    interest. The shape of spectral lines is determined by microscopic processes in the plasma.

    A quantum statistical approach to the optical properties of dense plasmas is presented that allows a systematic treatment of many-particle effects. In particular, profiles of spectral lines are obtained. Various examples are given: Hydrogen-like radiators, few-electron radiators, inner-shell transitions, in particular K_alpha radiation.

    Radiation from a strongly correlated system is not emitted by a single ion but by the whole plasma. A fundamental theory of radiation has to be formulated starting from many-body quantum electrodynamics.


  • Prof. Carsten Rockstuhl

  • Invited speaker: Prof. Carsten Rockstuhl
    Affiliation: Universität Jena
    Title: Amorphous Nanophotonics
    Time and room: 17:15, lecture hall IAP
    Abstract: Most nanooptical systems are fabricated with deterministic top-down technologies. This leads in many cases to periodically arranged nanostructures which have a limited spatial extension along the third dimension. The perfect control over all geometrical features with such techniques is often an asset; but it can be also detrimental since, e.g., it is difficult to reach bulk optical nanomaterials, and the periodic arrangement often causes undesired effects, e.g., strong spatial dispersion in metamaterials denies the unambiguous introduction of effective material parameters. Such limitations are about to be lifted by relying on bottom-up nanofabrication technologies. There, self-organization methods and techniques from the field of colloidal nanochemistry are used to build complex functional elements from an ensemble of simple building blocks, i.e. in most cases metallic nanospheres.
    This talk gives an introduction to the topic of amorphous nanophotonics from the point of view of somebody that discusses such systems on theoretical grounds. Emphasis is put on a description of challenges and an outline of promises associated to amorphous nanophotonics; but limitations that have been encountered are equally critically assessed.

    Selected references:
    [1] S. Mühlig, A. Cunningham, S. Scheeler, C. Pacholski, T. Burgi, C. Rockstuhl, and F. Lederer, “Self-Assembled Plasmonic Core-Shell Clusters with an Isotropic Magnetic Dipole Response in the Visible” ACS Nano Vol. 5 6586, (2011)
    [2] A. Cunningham, S. Mühlig, C. Rockstuhl, and T. Bürgi, “Coupling of plasmon resonances in tunable layered arrays of gold nanoparticles” Journal of Physical Chemistry C Vol. 115 8955, (2011)
    [3] C. Rockstuhl, C. Menzel, S. Mühlig, J. Petschulat, C. Helgert, C. Etrich, A. Chipouline, T. Pertsch, and F. Lederer, “Scattering properties of metaatoms” Physical Review B Vol. 83 245119, (2011)

  • Prof. Philipp Treutlein

  • Invited speaker: Prof. Philipp Treutlein
    Affiliation: Universität Basel
    Title: Quantum Metrology With Ultracold Atoms On A Chip
    Time and room: 17:15, lecture hall IAP
    Abstract: Atom chips provide a versatile quantum laboratory for experiments with ultracold atomic gases. Electromagnetic fields from microstructured wires and electrodes are used to cool, trap, and coherently manipulate the quantum state of the atoms. This enables chip-based atomic clocks and interferometers, which combine high precision with a compact and robust setup. In such an interferometer, we generate spin-squeezed states of the atoms through the "one-axis twisting model" first described by Kitagawa and Ueda in 1993. These multi-particle entangled states are a useful resource for quantum metrology, as they allow one to overcome the standard quantum limit of interferometric measurement.

    High-resolution imaging of electromagnetic fields is a metrology task for which atom chips are particularly well-suited. We have used ultracold atoms for microwave field imaging near on-chip waveguides. This novel technique is of interest for testing and optimizing integrated microwave circuits, which are at the heart of modern communication technology.

    M. F. Riedel, P. Böhi, Yun Li, T. W. Hänsch, A. Sinatra, and P. Treutlein, Nature 464, 1170 (2010).
    R. Schmied and P. Treutlein, New J. Phys. 13, 065019 (2011).
    P. Böhi, M. F. Riedel, T. W. Hänsch, and P. Treutlein, Appl. Phys. Lett. 97, 051101 (2010).

  • Prof. Serge Haroche

  • Invited speaker: Prof. Serge Haroche
    Affiliation: Collège de France und Ecole Normale Supérieure, Paris
    Title: Real Time Quantum Feedback Control Of Non-Classical Field States In A Cavity
    Time and room: 13:15, lecture hall IAP
    Abstract: In our Cavity Quantum Electrodynamics experiments, we use a beam of Rydberg atoms to manipulate and probe non-destructively microwave photons trapped in a very high Q superconducting cavity. In recent years, we have realized ideal quantum non-demolition (QND) measurements of photon numbers, observed the radiation quantum jumps due to cavity relaxation and prepared non-classical fields such as Fock and Schrödinger cat states. Combining QND photon counting with a homodyne mixing method, we have reconstructed the Wigner functions of these non-classical states and, by taking snapshots of these functions at increasing times, obtained movies of the decoherence process.

    The next step in this research program was to demonstrate ways to protect these non-classical states against decoherence. One natural method is to implement a procedure analogous to the feedback strategies used to stabilize complex systems in classical physics. A probe measures the system state, a controller determines the action leading it towards the chosen operating point and an actuator realizes this action. Juggling, for instance, relies on feedback loops from the eye to the hands through the brain. Transferring the feedback concept to the quantum world faces a fundamental difficulty: quantum measurement changes randomly the state of the system and this change must be taken into account in the feedback procedure. In our first demonstration experiment, we have chosen as an operating point a fixed photon number in our cavity. The “eye” in this “photon juggling game” is the beam of Rydberg atoms which performs a weak QND measurement extracting a partial information about the photon number. The “brain” is a computer using this information to estimate in real time the state of the field in the cavity and the “hand” is a classical microwave source feeding the cavity. These operations are performed in successive loops bringing the field towards the desired photon number. Subsequent quantum jumps of the field are detected by the computer and their effect is reversed by the feedback procedure. In this way, a pre-determined photon number is maintained in a steady state. We hope to be able to extend soon these experiments to the protection of other non-classical states such as Schrödinger cats of radiation.