# Talk: Martin Weitz – November 2 2017

Aula Conversi, Dipartimento di Fisica (I piano edificio Marconi)
3pm-4pm (info)
Martin Weitz (Universität Bonn)

Bose-Einstein condensation has been observed with cold atomic gases,
exciton-polaritons, and more recently with photons in a dye-filled optical
microcavity. I will here describe recent measurements of our Bonn group
determining the heat capacity of a two-dimensional photon gas in the regime
around the Bose-Einstein phase transition. Moreover, the entropy of the
optical quantum gas has been determined.

The photon Bose-Einstein 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 is achieved by repeated absorption
re-emission processes on the dye molecules. To determine calorimetric
properties of the optical quantum gas, we analyze spectra of the dye
microcavity emission at different levels of the phase space density, from
which we first determine the internal energy per photon, and after
differentiation with respect to the ratio of temperature and critical
temperature the heat capacity can be determined. At the phase transition, the
observed specific heat shows a cusp-like singularity, illustrating critical
behavior, analogous to the $$\lambda$$-transition of liquid helium. From the
optical spectra we have also determined the entropy per photon of the trapped
photon gas.