  Exciton-polaritons in semiconductor microcavities have been used to
demonstrate quantum effects such as Bose-Einstein condensation, superfluity,
and quantized vortices. However, in these experiments, the polaritons have not
reached thermal equilibrium when they undergo the transition to a coherent
state. This has prevented the verification of one of the canonical predictions
for condensation, namely the phase diagram. In this work, we have created a
polariton gas in a semiconductor microcavity in which the quasiparticles have a
lifetime much longer than their thermalization time. This allows them to reach
thermal equilibrium in a laser-generated confining trap. Their energy
distributions are well fit by equilibrium Bose-Einstein distributions over a
broad range of densities and temperatures from very low densities all the way
up to the threshold for Bose-Einstein condensation. The good fits of the
Bose-Einstein distribution over a broad range of density and temperature imply
that the particles obey the predicted power law for the phase boundary of
Bose-Einstein condensation.

Quantum Gases;Atomic Physics;Quantum Physics