We present an experimental demonstration of converting a microwave field to an optical field via frequency mixing in a cloud of cold ^{87}Rb atoms, where the microwave field strongly couples to an electric dipole transition between Rydberg states. We show that the conversion allows the phase information of the microwave field to be coherently transferred to the optical field. With the current energy level scheme and experimental geometry, we achieve a photon-conversion efficiency of ∼0.3% at low microwave intensities and a broad conversion bandwidth of more than 4 MHz. Theoretical simulations agree well with the experimental data, and they indicate that near-unit efficiency is possible in future experiments.
We study electromagnetically induced transparency (EIT) in the 5s$\rightarrow
5p\rightarrow$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 pair-wise interaction approximation.