A voltage biased superconducting quantum interference device bootstrap circuit

Abstract

We present a dc superconducting quantum interference device (SQUID) readoutcircuit operating in the voltage bias mode and called a SQUID bootstrap circuit(SBC). The SBC is an alternative implementation of two existing methods forsuppression of room-temperature amplifier noise: additional voltage feedback andcurrent feedback. Two circuit branches are connected in parallel. In the dc SQUIDbranch, an inductively coupled coil connected in series provides the bias currentfeedback for enhancing the flux-to-current coefficient. The circuit branch parallelto the dc SQUID branch contains an inductively coupled voltage feedback coilwith a shunt resistor in series for suppressing the preamplifier noise current byincreasing the dynamic resistance. We show that the SBC effectively reduces thepreamplifier noise to below the SQUID intrinsic noise. For a helium-cooled planarSQUID magnetometer with a SQUID inductance of 350 pH, a flux noise of about 3 μΦ0 Hz − 1/2 and a magnetic field resolution of less than 3 fT Hz − 1/2 were obtained. The SBC leads to a convenient direct readout electronics for a dc SQUIDwith a wider adjustment tolerance than other feedback schemes.