Maximum attainable sensitivity of dc squids built with Josephson tunnel junctions

The performance of dc squids built with Josephson tunnel junctions is analyzed for the ideal equivalent circuit with the junctions unshunted and for the fundamental relation according to which a variation of the magnetic flux produces a change in the critical current and consequently a variation of the high-frequency component of the voltage signal, the low-frequency component of the voltage signal is passed by a narrow-band filter. The dynamic characteristics of a low-inductance interferometer, analogous to those of a single Josephson tunnel junction, are calculated first for a device without noise. They are then calculated for a device with fluctuation noise, the effect of the latter depending largely on the ratio of fluctuation intensity to rate of current buildup. A lower bound is established for the energy sensitivity of the device, this lower bound is found to be much higher with high-frequency pumping near the upper frequency limit and not much different than that for a conventional squid with shunted Josephson tunnel junctions.