Low frequency noise in flux-locked DC SQUIDS

The authors present the results of measurements of the noise of open-loop and flux-locked DC SQUIDs (superconducting quantum interference devices) at frequencies from 100 kHz to 10^-4 Hz. The SQUIDs studied in this work were low-1/f-noise devices. Measurements were made in DC magnetic fields of 2×10^-3 G using conventional shielding and also in an ultralow magnetic field environment of 10^-7 G using expanded superconducting foil shields and a feedback-controlled slow cooling technique. The open-loop results are consistent with the earlier work of C.D. Tesche (1985). Some of the runs in the moderate-background-field environment (those with reduced shielding) showed a metastable structure in which switching between the metastable states was stimulated by electromagnetic interference. A description of the apparatus used for obtaining the various noise measurements is provided