Continuous feedback operation of a two-stage dc SQUID system

We report on the continuous feedback operation of a two-stage de Supercondu cting Quantum Interference Device (SQUID) system in which one SQUID works a s a low-noise preamplifier to read out a second low-noise sensor SQUID. We first flux-locked the readout SQUID and measured the characteristics of the sensor SQUID, including its current-voltage characteristics, current-flux characteristics, and noise spectrum. We then investigated how the two SQUID s could work together by simultaneously flux-locking both the sensor and th e readout SQUIDs. With this two-SQUID) flux-locking scheme, we recovered th e noise spectrum of the sensor SQUID to within 4 %. We discuss how to under stand and optimize the two-stage SQUID feedback system.