We address parametric amplifiers and kinetic inductance detectors, using concepts of the microscopic theory of superconductivity, and focusing on the interaction of microwave radiation with the superconducting condensate. This interaction was identified in recent experiments as the source of the apparent dissipation in microwave superconducting microresonators at low temperatures. Since the evaluation of the performance of practical devices based only on changes in kinetic inductance is not sufficiently informative about the underlying physical processes, we design an experiment with a tunnel measurement of a microwave-driven superconducting wire, in which the tunneling process is not affected by the microwaves. We conclude that such an experiment is feasible with current technology, but is unfortunately difficult to incorporate into standard superconducting resonators optimized for performance in applications. Nevertheless, given the limits of the commonly used phenomenological theories, such an experiment will provide the groundwork for further optimization of the performance.

We describe theoretically the depairing effect of a microwave field on diffusive s-wave superconductors. The ground state of the superconductor is altered qualitatively in analogy to the depairing due to a dc current. In contrast to dc depairing, the density of states acquires, for microwaves with frequency ω0, steps at multiples of the photon energy Δ±n ω0 and shows an exponential-like tail in the subgap regime. We show that this ac depairing explains the measured frequency shift of a superconducting resonator with microwave power at low temperatures.