  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.
