Generation of two identical photons from a quantum dot in a microcavity
  We propose and characterize a two-photon emitter in a highly polarised, monochromatic and directional beam, realized by means of a quantum dot embedded in a linearly polarized cavity. In our scheme, the cavity frequency is tuned to half the frequency of the biexciton (two excitons with opposite spins) and largely detuned from the excitons thanks to the large biexciton binding energy. We show how the emission can be Purcell enhanced by several orders of magnitude into the two-photon channel for available experimental systems. 
E. del Valle, A. Gonzalez-Tudela, E. Cancellieri, F. P. Laussy and C.   Tejedor
cond-mat.mes-hall quant-ph