Receipt date:It is shown that currents in thin-film superconducting rings irradiated by coherent microwave fields, can discretely decay with photons emission. These quantized jumps of the supercurrent correspond to destruction one or several magnetic flux quanta trapped in the ring. The ring thickness should be less than the skin-depth of the low-frequency field. The probability of the microwave field-induced single-photon decay of the supercurrent states in these rings is obtained. The angle distributions of the photons emitted by superconducting rings, and the current state lifetimes, depending on the ring sizes and the fluxoid numbers in the initial states, are studied. PACS: 74.25.N-, 74.78.-w, 74.90.+n 1. INTRODUCTION Currents that can exist in multiply connected superconductors in the absence of external magnetic fields, are, in principle, metastable. The current can be varied by the quantum jumps, at which the number of the magnetic-flux quanta, trapped in the superconductor, changes by one or several unites 1,2 . For the spontaneous current decay this quantum jump requires a collective transition of all the Cooper pairs involved in the supercurrents. The probability of such macroscopic fluctuations which cover the whole system with nearly 21 10 particles per cubic centimeter, vanishes that explains the persistence of the superfluid current in the ring.Besides the macroscopic fluctuation approach 3 , the quantized decay of the supercurrent states in the multi-connected structures, in particular, in rings can be due to the radiative decay of these states with the photons emission. However, instability of these current states in the superconducting rings with respect to the electromagnetic vacuum, as far as we know, has not been studied.
