  In this Chapter we investigate with the methods of signal detection the
response of a Josephson junction to a perturbation to decide if the
perturbation contains a coherent oscillation embedded in the background noise.
When a Josephson Junction is irradiated by an external noisy source, it
eventually leaves the static state and reaches a steady voltage state. The
appearance of a voltage step allows to measure the time spent in the metastable
state before the transition to the running state, thus defining an escape time.
The distribution of the escape times depends upon the characteristics of the
noise and the Josephson junction. Moreover, the properties of the distribution
depends on the features of the signal (amplitude, frequency and phase), which
can be therefore inferred through the appropriate signal processing methods.
Signal detection with JJ is interesting for practical purposes, inasmuch as the
superconductive elements can be (in principle) cooled to the absolute zero and
therefore can add (in practice) as little intrinsic noise as refrigeration
allows. It is relevant that the escape times bear a hallmark of the noise
itself. The spectrum of the fluctuations due to the intrinsic classical (owed
to thermal or environmental disturbances) or quantum (due to the tunnel across
the barrier) sources are different. Therefore, a careful analysis of the escape
times could also assist to discriminate the nature of the noise.
