Enhancement of Rydberg atom interactions using ac Stark shifts
  The ac Stark effect was used to induce resonant energy transfer between translationally cold Rydberg atoms. The Rb Rydberg atoms were obtained by laser excitation of cold atoms from a magneto-optical trap. Using two-photon microwave spectroscopy of the 49s_{1/2}-50s_{1/2} transition, the field strength of a 28.5 GHz dressing field was calibrated. When this dressing field was set at specific field strengths, the two-atom dipole-dipole process 43d_{5/2} + 43d_{5/2} -> 45p_{3/2} + 41f was dramatically enhanced, due to induced degeneracy of the initial and final states. A series of observed resonant field strengths correspond to different magnetic sublevel possibilities for the initial and final states. These agree well with calculated resonance fields based on a perturbative ac Stark shift formula. This method for enhancing interactions is complementary to dc electric-field-induced resonant energy transfer, but has more flexibility due to the possibility of varying the applied frequency. 
P. Bohlouli-Zanjani, J. A. Petrus, J. D. D. Martin