Study on bias reversal readout working at suppressing low frequency noise of dc SQUID with different βc

How to reduce of low frequency noise of low Tc DC superconducting quantum interference device (DC SQUID) has been an important problem and bias reversal readout scheme is an effective method to tackle it. However, the effect varies with the change of the parameter of sensors. Wang et al. found that DC SQUID with different ranges of βc matches different readout circuits to gain the lowest white noise. It is regrettable that this study did not involve the discussion of low frequency. In this work, we compare the noise of SQUID with a large range of βc measured by static bias and bias reversal. The preamplifier used is six parallel connected bipolar transistors (6 × SSM2220). Finally, we found that the bias reversal scheme is suitable to match SQUIDs with βc  <  1.