  We observe quasi-long range coherence in a two-dimensional condensate of
exciton-polaritons. Our measurements are the first to 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 non-resonant optical pumping of a
micro-cavity 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.
