SQUID Operation at a Constant Voltage

We have studied the operation of a superconducting quantum interference device (SQUID) at a constant voltage. A SQUID was coupled directly to the low noise preamplifier without any flux modulation. The deviation of the SQUID voltage from the reference voltage is amplified and produces a feedback current into the SQUID. We evaluated a flux change in the SQUID as a change of bias current that compensated the operating voltage through a dynamic resistance of the SQUID. For a single YBa₂Cu₃O₇-d SQUID, we obtained the flux-current characteristics with the current modulation of 8 muA at 10 mu _v and the maximum conversion efficiency of flux to current of 24 mu A / Phi_o. A noise level in the flux and current feedback mode with both a feedback current for bias and a feedback flux was 33 muA / Phi_o at 1 kHz, and slightly larger than that in the flux feedback mode with conventional flux locked loop (FLL) operation at a constant bias current. For a magnetometer with parallel SQUID arrays, current detection of an output at a constant voltage and noise measurements in the flux and current feedback mode were also performed.