Superconducting maser

A new approach to designing active superconducting elements is developed. The mixed state in the superconducting films is influenced by an alternative magnetic field directed perpendicular to the film surface. The field slowly varies in time, and sets up the vortex structure in the film. The interaction of the vortices with planar pinning centers leads to a metastable mixed state, which can bring into the ground state. Such a transition synchronized by an external resonant circuit causes the generation of the electromagnetic radiation. Coherent microwave radiation has been directly detected from superconducting thin films thus excited. Moreover, it is found that the radiation depresses the noise radiation, so that low-frequency noise is transformed into the coherent radiation. Harmonic mixing and amplification of the electromagnetic radiation are also detected. The system resembles a maser in which the energy two-level structure is formed by Abricosov vortices.