Biphoton generation enhanced by nonlocal nonlinearity via Rydberg interactions.

Strongly correlated Stokes and anti-Stokes photon pairs (biphotons) exhibiting very large generation rates and spectral brightnesses could be attained at extremely low pump powers and optical depths. This is realized via spontaneous four-wave mixing in cold atoms with enhanced nonlocal (Rydberg) optical nonlinearities and prepared into a dark state with a large population imbalance. The scheme works with all light fields on resonance yet with negligible linear absorption and Raman gain.