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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{}10⁸ 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.
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Brune et al. (1987) studied this question.
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