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 $5S\text{\ensuremath{-}}5P\text{\ensuremath{-}}nD$ two-photon excitation to the Rydberg state was combined with an $nD\text{\ensuremath{-}}6P$ 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.