Automatic adjustment of bias current for direct current superconducting quantum interference device

A new method of adjusting the bias current of dc superconducting quantum interference device (SQUID) is described. It is shown that the signal‐to‐noise ratio of a SQUID magnetometer connected in a flux‐locked loop configuration is proportional to the second harmonic of the output signal from the SQUID. A circuit configuration that can automatically optimize a SQUID’s bias current by measuring this second harmonic and adjusting the bias current accordingly is proposed.