Four-wave mixing in ultracold atoms using intermediate Rydberg states

Ultracold Rb atoms were used to demonstrate nondegenerate four-wave mixing through a Rydberg state. Continuous 5S-5P-nD two-photon excitation to the Rydberg state was combined with an nD-6P tuned laser in a phase-matched geometry. The angular dependence, spatial profile, and dependence on detuning were investigated, showing good agreement with theory. Under optimum conditions 50% of the radiation was emitted into the phase-matched direction. The directional emission is promising for studying quantum effects in blockaded atom clouds.