# Mainz 2017 – wissenschaftliches Programm

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# Q: Fachverband Quantenoptik und Photonik

## Q 38: Quantum Gases: Bosons V

### Q 38.2: Vortrag

### Mittwoch, 8. März 2017, 14:45–15:00, P 204

**Spatial first-order correlations of a trapped two-dimensional photon gas** — •Tobias Damm, David Dung, Frank Vewinger, Julian Schmitt, and Martin Weitz — Institut für Angewandte Physik, Universität Bonn, Wegelerstr. 8, D-53115 Bonn

Bose-Einstein condensation was observed for ultracold atoms, polaritons, and more recently with photons in a dye-filled optical microcavity. In the latter experiment, the used short cavity provides a low frequency cutoff with a quadratic dispersion relation. Photons thermalize by repeated absorption and emission processes on dye molecules.

Here we report measurements of the first-order spatial coherence of the thermalized photon gas trapped in the dye microcavity, both in the classical Boltzmann regime as well as in the condensed phase. The dye microcavity emission is analyzed with a Michelson interferometer utilizing a cat eye retroreflector replacing one of the plane reflecting mirrors. Below condensation threshold correlations are determined by the thermal de Broglie length. We observe the expected 1/√*T* dependence with temperature, which verifies the thermal character. The onset of Bose-Einstein condensation agrees with the assumption that quantum statistical effects emerge when the thermal de Broglie wavepackets overlap, a property so for verified only for material gases. Above this critical phase-space-density we observe long range order, with the correlation length eventually exceeding the size of the condensate.