A maser action using a current-biased mesoscopic Josephson junction within a high-Q micra-cavity is discussed. A Josephson junction behaves like an artificial atom as far as the interaction with the photon field is concerned. Under a certain bias current, the Josephson atom is excited or pumped by macroscopic resonant tunneling. When the pumping overcomes the loss of photons from the cavity, the maser action is possible. Quantum statistical properties of photons are also investigated numerically, based on a Jayns-Cummings model with pumping and cavity damping. We found that a mesoscopic Josephson junction placed in the high-Q microcavity acts as a thresholdless maser with sub-Poissonian photon statistics.
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