  The demonstration of Bose-Einstein condensation in atomic gases at
micro-Kelvin temperatures is a striking landmark while its evidence for
semiconductor excitons still is a long-awaited milestone. This situation was
not foreseen because excitons are light-mass boson-like particles with a
condensation expected to occur around a few Kelvins. An explanation can be
found in the underlying fermionic nature of excitons which rules their
condensation. Precisely, it was recently predicted that, at accessible
experimental conditions, the exciton condensate shall be "gray" with a dominant
dark part coherently coupled to a weak bright component through fermion
exchanges. This counter-intuitive quantum condensation, since excitons are
mostly known for their optical activity, directly follows from the excitons
internal structure which has an optically inactive, i.e., dark, ground state.
Here, we report compelling evidence for such a "gray" condensate. We use an
all-optical approach in order to produce microscopic traps which confine a
dense exciton gas that yet exhibits an anomalously weak photo-emission at
sub-Kelvin temperatures. This first fingerprint for a "gray" condensate is then
confirmed by the macroscopic spatial coherence and the linear polarization of
the weak excitonic photoluminescence emitted from the trap, as theoretically
predicted.
