Enhancement of Rydberg atom interactions using ac Stark shifts*

A microwave dressing field was used to induce resonant energy transfer in translationally cold Rydberg atoms. The $^{85}$Rb Rydberg atoms were obtained by laser excitation of cold atoms in a magneto-optical trap. When the amplitude of a $1.356$ GHz dressing field was scanned, the two-atom dipole-dipole process $43d_{5/2} + 43d_{5/2} \rightarrow 45p_{3/2} + 41f_{5/2,7/2}$ was enhanced due to the induced degeneracy of the initial and final states. The resulting spectrum had a series of resonant field amplitudes corresponding to different magnetic sublevels possible for the states involved. The scanned field amplitude was calibrated using microwave spectroscopy of the $43d_{5/2}-41f$ transition under the influence of non-resonant dressing fields. The calibrated resonant field amplitudes agree well with ac Stark shift calculations performed using the Floquet approach. This method for enhancing interactions is complementary to dc electric field induced resonant energy transfer, but benefits from the ability to shift energy levels in either direction by choice of frequency.