Bose condensation of exciton polaritons

Because of their unique property of bringing pure quantum effects into the real world scale phase transitions towards condensed phases have always fascinated scientists. Bose-Einstein condensation (BEC) appearing upon cooling a gas of bosons below a critical temperature has been given a striking demonstration in dilute atomic gases at temperatures below 200nK. By confining photons in a semiconductor microcavity and strongly coupling them to electronic excitations one may create polaritons. These bosonic quasi-particles are billion times lighter than rubidium atoms thus theoretically allowing a BEC at standard cryogenic temperatures. Here we present experiments giving compelling evidence for a BEC of polaritons. Above a critical density we observe massive occupation of the ground state developing from a thermalized distribution of the polariton population at (16-20) K. The spontaneopus onset of a coherent state is manifested by the increase of temporal coherence build-up of spatial long-range order and the reduction of the thermal noise observed in second order coherence experiments. All of these indicate spontaneous onset of a macroscopic quantum phase.