Microwave dressing of Rydberg dark states

We study electromagnetically induced transparency (EIT) in the 5s$\to$5p$\to$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 $1\mathrm{/}{R}^3$ energy shift between Rydberg states resonantly coupled by the microwave field and the ensuing breakdown of the pair-wise interaction approximation.