Microwave dressing of Rydberg dark states
  We study electromagnetically induced transparency (EIT) in the 5s$\rightarrow$5p$\rightarrow$46s ladder system of a cold $^{87}$Rb gas. We show that the resonant microwave coupling between the 46s and 45p states leads to an Autler-Townes splitting of the EIT resonance. This splitting can be employed to vary the group index by $\pm 10^5$ allowing independent control of the propagation of dark state polaritons. We also demonstrate that microwave dressing leads to enhanced interaction effects. In particular, we present evidence for a $1/R^3$ energy shift between Rydberg states resonantly coupled by the microwave field and the ensuing breakdown of the pair-wise interaction approximation. 
M. Tanasittikosol, J. D. Pritchard, D. Maxwell, A. Gauguet, K. J.   Weatherill, R. M. Potvliege, and C. S. Adams
physics.atom-ph quant-ph