Quantum phases of light coupled to a periodic Bose-Einstein condensate in a cavity

We study a composite photon-Bose condensed atoms (BEC) system in an optical lattice confined in a cavity. We show that the system is able to produce an effective photon-photon repulsion in the presence of an external pump. We predict for the first time zero-temperature quantum phases of light coupled to BEC in the photon blockade regime in the presence of an external pump.