  The dynamics of spatial coherence and momentum distribution of polaritons in
the regime of Bose-Einstein condensation are investigated in a GaAs microcavity
with embedded quantum wells under nonresonant excitation with picosecond laser
pulses. It is shown that the onset of the condensate first order sparial
coherence is accompanied by narrowing of the polariton momentum distribution.
At the same time, at sufficiently high excitation densities, there is
significant qualitative discrepancy between the dynamic behavior of the width
of the polariton momentum distribution determined from direct measurements and
that calculated from the coherence spatial distribution. This discrepancy is
observed at the fast initial stage of the polariton system kinetics and,
apparently, results from the strong spatial nonuniformity of the phase of the
condensate wave function, which equilibrates on a much longer time scale.

Quantum Gases;Mesoscale and Nanoscale Physics;Quantum Physics