Nonlocal Rydberg enhancement for four-wave-mixing biphoton generation

Tunable Rydberg interactions in cold atoms can be used to generate strongly correlated photon pairs through a spontaneous four-wave-mixing process in which all driving fields are allowed to work on resonance. The scheme relies on electromagnetically induced transparency and an imbalanced dark state with most atoms trapped in a ground state and a weakly populated Rydberg state. We show in particular that the combination of these two effects results in negligible linear absorption and Raman gain at the Stokes and anti-Stokes frequencies of generated photon pairs, while the nonlocal (Rydberg) nonlinearities contribute significantly to the enhancement of photon-pair generation rates and cross correlations at very low pump strengths and optical depths.