Low Noise DC SQUID Amplifiers and Related Systems

Thin film DC SQUID's have been made with integral double transformers. This coupling method allows efficient matching of a low noise 7 pH SQUID to a 1 (mu)H source. The coupling efficiency is in good agreement with estimates derived from the geometry. The best example was measured to have a minimum intrinsic energy sensitivity S(,E) of 1.3 x 10('-32) Joules/Hz, and a coupled energy sensitivity of 3.3 x 10('-32) Joules/Hz. The measured values of S(,E) are greater, by factors of 2 to 15, than the values predicted by a computer simulation of thermal noise. Quasi-bistable behavior was observed in earlier versions of this design. Some of this behavior is explained by transmission line resonances in the loop. This effect was eliminated in the later versions. A model for transmission line resonances was developed for a single Josephson junction shunted by a transmission line. An example of this device had at least seven different bistable regions at the predicted locations.