Entangling photons by means of the nonlinear response of quantum wells   to an ultrashort pulse
  We show that many-body correlations among excitons originating from the Pauli exclusion principle in a quantum well embedded inside a microcavity provide a possibility to produce pairs of entangled photons by ultrashort laser pulses with a yield of $\sim 10^{-2}$. The quantum-field theoretical two-particle density matrix in second quantization is used to calculate entanglement for arbitrary emission angles. %At time scales where the heavy-light hole splitting is resolved the resonances corresponding to different two-exciton %states develop, which allow for a simple kinematic theory relating the %states of the outgoing photons with the respective two-exciton states. Largest response can be expected at symmetric emission angles for resonances with the heavy-heavy and light-light two-exciton states with remarkably nontrivial dependence of entanglement on the emission angles and on the ellipticity parameters of the excitation. We show that the angle dependence can be tailored by means of the microcavity. Interestingly, the emitted entangled 2-photon states are always in a triplet state. 
Mikhail Erementchouk, Michael N. Leuenberger