Quantum theory of an exciton condensate subject to radiative decay

Semiconductor excitons obey Bose-Einstein statistics in the low-density lim it. In this paper, a two-level quantum model for an exciton condensate subj ect to radiative decay is presented. The condensate state has a well-define d exciton number and a slow phase diffusion rate analogous to a coherent st ate created by a (photon) laser. Since excitons recombine radiatively, the statistics of the device can be observed by photodetection. Our analytical and numerical results at steady state reveal that the change in population statistics of the ground state is gradual across the threshold region, abov e which a condensate forms. When radiative losses are present, the condensa te mode is "quasicoherent;" in the limit of no losses a coherent state is a pproached. Far below threshold the ground state is well described by a ther mal distribution. The higher level, that is the "pump," remains thermally d istributed at all times. [S0163-1829(99)13243-0].