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Quantum technologies

Dieter Meschede's research group
Home AMO physics colloquia
  • Markus Lippitz

  • Invited speaker: Prof. Dr. Markus Lippitz
    Affiliation: Max-Planck-Institut für Festkörperforschung, Stuttgart
    Title: Nonlinear Spectroscopy Of A Single Nanoobject
    Time and room: 17:15  lecture hall IAP
    Abstract: Nanoobjects with a size between 1 and 100 nanometers show fascinating properties that deviate strongly from those of bulk solids. The plasmon resonance of metal nanoparticles or the electron confinement in quantum dots are prominent examples. However, even in the best preparation methods, nanoobjects differ from each other in size, shape, or local environment. Experiments on the single particle level allow the experimenter to circumvent the ensemble heterogeneity. In this presentation I will demonstrate nonlinear optical spectroscopy of a single nanoobject.

    Single nanoobjects, especially at room temperature, show only a weak interaction with light, as only a low number of electrons is involved. Nonlinear optical signals which are already weak for a bulk solid become difficult to detect. I will show how an optical nanoantenna is able to enhance the signal of a single nanoobject so that nonlinear spectroscopy becomes possible. We investigate the mechanical breathing mode of a single gold nanodisc by antenna-enhanced transient absorption spectroscopy [1].
    Very large optical nonlinearities can be found on optical two-level systems such as semiconductor quantum dots. Their quantum-optical properties find use as single photon source or quantum bit. However, to be really used, a quantum bit needs to be connected to some kind of circuit. I will give an overview of our work on coherent reading and writing of quantum bits [2] and their coupling to plasmonic nanocircuits [3].

    [1] T. Schumacher et al. , “Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle”, Nature Commun. 2 (2011) 333.

    [2] C. Wolpert et al., “Transient reflection: A versatile technique for ultrafast spectroscopy of a single quantum dot in complex environments”, Nano Letters 12 (2012) 453.

    [3] M. Pfeiffer et al. , “Enhancing the Optical Excitation Efficiency of a Single Self- Assembled Quantum Dot with a Plasmonic Nanoantenna”, Nano Letters 10 (2010) 4555.

  • Adam Cohen

  • Invited speaker: Prof. Dr. Adam Cohen
    Affiliation: Harvard University
    Title: Controlling Light-Matter Interactions: From Superchiral Light To Magnetochemistry
    Time and room: 10:15  lecture hall IAP
    Abstract: Through careful engineering of light-matter interactions, one can achieve surprising levels of control over molecular states, even under ambient conditions.  I will give two examples in which we achieve large enhancements, under ambient conditions, in what are normally very small optical effects.  First I will describe the theory and experiment underlying "superchiral" light, i.e. light that can preferentially excite a chiral molecule of one handedness relative to the mirror image molecule, with selectivity far greater than that of circularly polarized plane waves.  Then I will describe experiments in magnetochemistry, where minuscule magnetic fields have large effects on photochemical reactions under ambient conditions.

    [1] Y. Tang and A. E. Cohen “Enhanced enantioselectivity in excitation of chiral molecules by superchiral light” Science, 332, 333-336 (2011);
    [2] Y. Tang, A. E. Cohen “Optical chirality and its interaction with matter” Phys. Rev. Lett., 104, 163901 (2010);
    [3] H. Lee, N. Yang, A. E. Cohen “Mapping nanomagnetic fields using a radical pair reaction” Nano Letters, 11, 5367-5372 (2011)

  • Prof. Jonathan Keeling

  • Invited speaker: Prof. Jonathan Keeling
    Affiliation: St. Andrews University
    Title: Non-equilibrium Coherence in Light-matter Systems: Condensation, Lasing and the Superradiance Transiton
    Time and room: 17:15, lecture hall IAP
    Abstract: I will discuss theoretical questions prompted by recent experiments on cold atoms in optical cavities [1], and on exciton-polaritons in microcavities [2], discussing the relation between the emergence of coherent light in these systems and ideas of superradiance, Bose-Einstein condensation and lasing. All of these phenomena involve the appearance of coherent fields, but the details of the transition, and the nature of the coherent system differ. In particular, I will discuss the ingredients needed that allow the microcavity polariton system to show coherence while remaining in the strong coupling regime [1], and I will discuss how recent experiments on cold atoms [1] succeed in undergoing the superradiance transition, despite the "no-go theorem" [3] for this transition in equilibrium. Both the cold atom and the exciton polariton systems are intrinsically non-equilibrium, with pumping and decay, and I will explore some of the consequences this has for properties of the coherent state of these systems.

    [1] K. Baumann, C. Guerlin, F. Brennecke and T. Esslinger, Nature 464, 1301 (2010); J. Keeling, M. J. Bhaseen and B. D. Simons Phys. Rev. Lett 105 043001 (2010); [2] J. Kasprzak et al, Nature 443 409 (2006). J. Keeling and N.G. Berloff Contemporary Physics 52 131 (2011); [3] K. Rzazewski, K. Wodkiewicz and W. Zakowicz, Phys. Rev. Lett. 35 432 (1975)

  • 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.