Low frequency noise in small-area tunnel junction dc SQUIDs

The noise in a small-area tunnel junction dc SQUID fabricated with a 2.5 μm Pb-alloy technology has been measured in the frequency interval 0.5 Hz <= f <= 4 kHz using a commercial rf SQUID as a low noise preamplifier. The minimum intrinsic energy sensitivity, epsilon, above approx. equal to 1 kHz is approx. equal to 8h, in reasonable agreement with the value of 5h previously measured at 70 kHz with an FET readout scheme. At lower frequencies epsilon α f-0.9 at 1.6 K, and epsilon α f-0.75 at 4.2 K. The presence of the increased noise at lower frequencies can be understood in terms of a thermal fluctuation model in which equilibrium temperature fluctuations modulate junction critical currents. (orig.)