High sensitivity double relaxation oscillation superconducting quantum interference devices

Double relaxation oscillation superconducting quantum interference devices (SQUIDs) (DROSs) have been fabricated with estimated relaxation frequencies up to 14 GHz. Both the intrinsic flux noise and the performance in a flux locked loop with direct voltage readout have been studied. In flux locked loop, a noise level of 0.55 μφ0/√Hz corresponding to an energy sensitivity of 34 h has been obtained for a DROS with a SQUID inductance of 29 pH. The intrinsic sensitivity improves with increasing relaxation frequency, leveling off to a value of 13 h at relaxation frequencies higher than about 3 GHz