Coherence of a Bose-Einstein Condensed Light Field (Prof. M. Weitz)

  • Date: Jul 12, 2016
  • Time: 02:30 PM - 03:30 PM (Local Time Germany)
  • Speaker: Prof. Dr. Martin Weitz, Institut für Angewandte Physik, Universität Bonn
  • Room: Herbert Walther Lecture Hall
  • Host: MPQ
Bose-Einstein condensation has been observed with cold atomic gases, quasiparticles in solid state systems as polaritons, and more recently with photons in a dye-filled optical microcavity.

I will here describe measurements of our Bonn group determining both the first and the second order coherence of a photon Bose-Einstein condensate. The optical condensate is generated in a wavelength-sized optical cavity, where the small mirror spacing imprints a low-frequency cutoff with a spectrum of photon energies restricted to well above the thermal energy. Thermal equilibrium of the photon gas is achieved by repeated absorption re-emission cycles in dye molecules. In this system the photo-excitable dye molecules act as a reservoir for the condensate particles, which allows to reach a regime with large "grand canonical" number fluctuations, of order of the total particle number. We observe photon bunching, as known from thermal (lamp-type) sources. To study the first order coherence of the condensate, we have examined the temporal interference signal of the photon Bose-Einstein condensate with a narrowband laser source acting as a phase reference. Our data reveals a separation of characteristic timescales for first and second-order coherence properties. From a thermodynamic viewpoint, the observed regime with well-defined phase of the grand canonical statistics condensate means that an order parameter exists despite the large statistical number fluctuations. Grand canonical statistics optical sources in the condensed phase hold prospects for both fundamental studies and optical imaging technology.

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