Subthreshold response of rf SQUIDs to a general external perturbation

Abstract

We consider a rf SQUID subjected to an external magnetic flux that includes dc, periodic, and stochastic components. Using a previously developed linearization technique, the response of the SQUID (below its hysteresis threshold) to this force is determined as a function of the internal parameters of the device. The measurement process, wherein the flux sensed by the SQUID is coupled to a tank circuit, is also considered. The results obtained, using the linearization technique referred to above, are compared with previously reported work on this subject. It is seen that, while not exact, this technique allows one to compute, analytically, the response (measured as an autocorrelation function or its associated spectral density) very accurately below the SQUID hysteresis threshold.