Interaction and polarization measurements of photon Bose-Einstein condensate

Abstract

Bose-Einstein condensation (BEC) is a state of matter where bosons macroscopically occupy the ground state of a system. In 2010 the group of Martin Weitz achieved a two-dimensional Bose-Einstein condensation of photons by using a dye-filled micro- cavity. In 2015 our research group also achieved a photon BEC using a similar setup. Although one expects the photons to be non-interacting, the radius of the condensate has been shown to grow as a function of photon density. This implies repulse photon-photon interactions. This leads us to question what the strength of the interactions is. To investigate the interaction strength we take single shot images of the photon gas, while varying the photon density in the cavity. From each image we determine the number of condensate photons and the radius of the condensate. Subsequently we de- termine the interaction strength to be g = (6.6 +/- 0.7)E-3 and g = (8 +/- 1)E-2 using a dye concentration of 1.5 mM and 6.0 mM respectively. This implies that the interaction strength increases for increasing dye concentration. The source of the interactions is not yet understood and should be investigated further. We also determine the polarization state of the photon gas. In order to do so we build an experimental setup which can measure all four Stokes parameters simulta- neously. Since the phase transition into Bose-Einstein condensation is an example of spontaneous symmetry breaking, one would expect the condensate to have a differ- ent polarization each time it is created. We show that this is not the case and that every newly created BEC has the same polarization state, i.e. linearly polarized in the horizontal direction. We show this symmetry breaking is not affected by the dye concentration, photon density inside the cavity or the cavity mirrors. It is however caused by the intra-cavity polarization of the pump beam. We show that we can rotate the direction of the polarization by rotating the intra-cavity polarization of the pump beam.