Stimulated light backscattering from exciton Bose-condensate

The new effect — light backstattering from exciton Bose-condensate is considered. The effect is connected with photoinduced coherent recombination of two excitons in the condensate with production of two photons with opposite momenta. The effect of two-exciton coherent recombination leads also to the appearence of the second order coherence in exciton luminescence connected with squeezing between photon states with opposite momenta. The estimations given for Cu$_2$O and GaAs excitons show that the effect of stimulated light backscattering can be detected experimentally. Moreover, in the system of 2D excitons in coupled quantum wells the effect of stimulated anomalous light transmission must also take place. Analogous effects can take place also in systems of Bose-condensed atoms in excited (but metastable) states.