  Microcavity exciton polariton systems can have a wide range of macroscopic
quantum effects that may be turned into better photonic technologies. Polariton
Bose-Einstein condensation and photon lasing have been widely accepted in the
limits of low and high carrier densities, but identification of the expected
Bardeen-Cooper-Schrieffer (BCS) state at intermediate densities remains
elusive, as the optical-gain mechanism cannot be directly inferred from
existing experiments. Here, using a microcavity with strong polarization
selectivity, we gain direct experimental access to the reservoir absorption in
the presence of polariton condensation and lasing, which reveals a fermionic
gain mechanism underlying the polariton laser. A microscopic many-particle
theory shows that this polariton lasing state is consistent with an
open-dissipative-pumped system analog of a polaritonic BCS state.
