Algebraic Order And The Berezinskii-Kosterlitz-Thouless Transition In An Exciton-Polariton Gas

We observe quasi-long-range coherence in a two-dimensional condensate of exciton-polaritons. Our measurements confirm that the spatial correlation algebraically decays with a slow power law, whose exponent quantitatively behaves as predicted by the Berezinskii-Kosterlitz-Thouless theory. The exciton-polaritons are created by nonresonant optical pumping of a microcavity sample with embedded GaAs quantum wells at liquid helium temperature. Michelson interference is used to measure the coherence of the photons emitted by decaying exciton-polaritons.