Microwave dressing of Rydberg dark states

We study electromagnetically induced transparency (EIT) in the 5s -> 5p -> 46s ladder system of a cold Rb-87 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 +/- 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 pairwise interaction approximation.