  Sub-wavelength arrays of atoms exhibit remarkable optical properties,
analogous to those of phased array antennas, such as collimated directional
emission or nearly perfect reflection of light near the collective resonance
frequency. We propose to use a single-sheet sub-wavelength array of atoms as a
switchable mirror to achieve a coherent interface between propagating optical
photons and microwave photons in a superconducting coplanar waveguide
resonator. In the proposed setup, the atomic array is located near the surface
of the integrated superconducting chip containing the microwave cavity and
optical waveguide. A driving laser couples the excited atomic state to Rydberg
states with strong microwave transition. Then the presence or absence of a
microwave photon in the superconducting cavity makes the atomic array
transparent or reflective to the incoming optical pulses of proper frequency
and finite bandwidth.
