We have built the first quantum oscillator working on two-photon stimulated emission of radiation. It oscillates continuously on a degenerate two-photon transition between the levels 40S and 39S of rubidium atoms. The oscillation is sustained in a Q≃${10}^8$ niobium superconducting cavity at 68.415 87 GHz. At threshold, this new maser system operates with only a few atoms and a few tens of photons at a time in the cavity.

• Received 28 July 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.1899