Four-Wave Mixing and Coherent Processes in Ultracold Atoms Using Intermediate Rydberg States.

Continuous 5S-5P-$n$D two-photon excitation to the Rydberg state was combined with an $n$D-6P tuned laser to explore coherent processes using intermediate Rydberg states. In a phase-matched geometry, four-wave mixing was demonstrated in good agreement with theory. The directional emission was optimized to 50{\%} for off-resonant Rydberg excitation. Further coherent schemes have been explored using small excitation volumes, showing promise for studying quantum effects in blockaded atom clouds.