Efficient microwave-to-optical conversion using Rydberg atoms.

We demonstrate microwave-to-optical conversion using six-wave mixing in cold Rb-87 atoms where the microwave field couples to two Rydberg states and propagates collinearly with the converted optical field. Our experiment is performed with a free-space microwave field, and we achieve a conversion efficiency of about 5% for the microwave photons entering the conversion medium. In addition, we theoretically investigate all-resonant six-wave mixing and outline a realistic experimental scheme for reaching an efficiency close to 70%.