  The wavefunction of a collection of identical quantum particles of integer
spin (bosons) is even under exchange of the coordinates of any two of them.
This symmetrization rule of quantum mechanics leads to stimulation of the
scattering rate into final states whose population is larger than one. For
massless photons, it is routinely observed in the laser. For massive bosons,
most previous efforts concentrated in realizing the largely populated
equilibrium Bose-Einstein condensate, and observing stimulated emission into
it. Here we show that we do not need to produce such a condensate to observe
final state stimulation. Instead, we take advantage of the highly
non-equilibrium exciton reservoir and of the large polariton populations
produced by external laser action on a semiconductor microcavity, and measure
final state stimulation of the scattering rate of these excitons into the
polaritons.
