Key points are not available for this paper at this time.
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 Q10^8 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.
Brune et al. (Mon,) studied this question.