Two-photon emission from thin superconducting rings irradiated by   coherent microwave fields
  Superconducting current instability in thin rings, irradiated by a coherent microwave electromagnetic field, is studied. On the regular circular motion of the Cooper pairs in the supercurrent states the field imposes coherent oscillations of the condensate as a whole. The oscillating condensate should emit photons with the energies determined mainly by the discrete values of the superconducting ring energy. For the first time the probability of the supercurrent decay accompanied by the two- photon emission, is derived. The ring thickness is less than both the field skin-depth and the London penetration depth. Numerical results for the energy-angular distributions of the photons, as well as the waiting time of the stimulated two-photon transition depending on both the ring sizes and fluxoid number in the initial current state, are presented. 
A.I. Agafonov
cond-mat.supr-con quant-ph