Coherence of a Bose-Einstein Condensed Light Field (Prof. M. Weitz)
- Datum: 12.07.2016
- Uhrzeit: 14:30 - 15:30
- Vortragende(r): Prof. Dr. Martin Weitz, Institut für Angewandte Physik, Universität Bonn
- Raum: Herbert Walther Lecture Hall
- Gastgeber: MPQ
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.