Signatures of the electromagnetic field quantization in the nonlinear optical response of excitons

We present a fully quantum mechanical theory for an interacting system of p hotons and Coulomb correlated electrons and holes in a semiconductor, using a pertubation series in the exciting laser field. The theory provides micr oscopic descriptions of nonlinear optical processes in semiconductors relat ed to the electromagnetic field quantization. As an application of the theo ry we show that it is possible to transfer the exciton-exciton Coulomb corr elation to photons, thus producing pairs of near-gap photons with a high de gree of quantum entanglement. The photon pairs emerge from the spontaneous optical decay of biexcitons into two polaritons. The pair intensity correla tions, calculated in the low density limit for a CuCl slab and for semicond uctor microcavities, exhibit quantum features which can be observed by coin cidence detection.