Microwave dressing of Rydberg dark states

We study electromagnetically induced transparency (EIT) in the 5s→5p→46s ladder system of a cold ⁸⁷Rb 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 ±10⁵ 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/R³ energy shift between Rydberg states resonantly coupled by the microwave field and the ensuing breakdown of the pairwise interaction approximation.