Persistent single-photon production by tunable on-chip micromaser with a   superconducting quantum circuit
  We propose a tunable on-chip micromaser using a superconducting quantum circuit (SQC). By taking advantage of externally controllable state transitions, a state population inversion can be achieved and preserved for the two working levels of the SQC and, when needed, the SQC can generate a single photon. We can regularly repeat these processes in each cycle when the previously generated photon in the cavity is decaying, so that a periodic sequence of single photons can be produced persistently. This provides a controllable way for implementing a persistent single-photon source on a microelectronic chip. 
J. Q. You, Yu-xi Liu, C. P. Sun and Franco Nori