MICROWAVE-INDUCED QUANTUM TRANSITIONS IN A SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE

In this paper we consider the interaction of a quantum-mechanical supe rconducting quantum interference device (SQUID) with a classical monoc hromatic electromagnetic (EM) field. We solve the time-dependent Schro dinger equation for the SQUID ring in this EM field. We demonstrate th at nonperturbative transitions between the low-lying states of the rin g, due to coherent multiphoton absorption processes, dominate, even wh en the single-photon energy is small compared with the energy separati on between these states. We present experimental data on small-capacit ance SQUID rings, interacting with microwave fields, which point to th e existence of such nonperturbative transitions.