  We use a microwave field to control the quantum state of optical photons
stored in a cold atomic cloud. The photons are stored in highly excited
collective states (Rydberg polaritons) enabling both fast qubit rotations and
control of photon-photon interactions. Through the collective read-out of these
pseudo-spin rotations it is shown that the microwave field modifies the
long-range interactions between polaritons. This technique provides a powerful
interface between the microwave and optical domains, with applications in
quantum simulations of spin liquids, quantum metrology and quantum networks.
