Coherent Fraction of a Uniform Two-dimensional Bose Gas in Thermal Equilibrium

Strong coupling of cavity photons and quantum-well excitons gives rise to new bosonic quasiparticles called exciton-polaritons. We have measured the distribution function of a homogenous polariton gas and show that it is well described by a Bose-Einstein distribution, indicating that the polariton gas is in thermal equilibrium. In addition, we measure the coherent fraction of the polariton gas by creating the interference pattern of the light emitted from the gas overlapped with its mirror image. The visibility of the interference fringes gives a direct measurement of the coherent fraction of the polariton gas as a function of the total polariton density. This allows comparison of our results with theoretical predictions of the coherent fraction of an interacting 2D Bose gas. This work connects to several fundamental questions in quantum mechanics, such as how coherence can occur spontaneously in Bose-Einstein condensates and how coherence is lost in standard quantum systems.