The exchange of photons between single Rydberg atoms and a single mode of a superconducting cavity with a quality factor $Q=8\mathrm{}\times {10}^8$ at 2 K was observed. Signals could still be detected with an average number of only 0.06 atom simultaneously in the cavity. With one Rydberg atom the linewidth of the maser transition at about 21 GHz was power broadened and at higher densities asymmetry of the transition was observed, which is ascribed to an ac Stark effect.

• Received 20 August 1984

DOI:https://doi.org/10.1103/PhysRevLett.54.551