On the ac bias readout schemes for high-Tc superconducting quantum interference devices

The ac bias method is used in the readout system of de-superconducting quantum interference devices (SQUIDs) to suppress the low-frequency noise that originates in the fluctuation of parameters of two junctions in the SQUID. From the analysis of various schemes of the ac bias method, we propose a simple form which expresses the alternation of the operation variables, i.e., the bias current, modulation flux, and bias flux. We considered the modification of the previous schemes in the context of this form and have found a possible method to directly detect the noise component due to the parameter fluctuation. In the modified ac bias scheme, the low-frequency noise of the parameter fluctuation can either be detected or suppressed by manipulating a single variable. We fabricated the electronics to verify this function, and confirmed the versatility of this method from the measurements of the high-T-C SQUID.