Low‐frequency noise reduction in SQUID measurements using a laser‐driven superconducting switch. Part A: Direct input circuit switching

We describe a technique for substantially decreasing the noise in SQUID measurement applications which are intrinsically low frequency or dc and thus fall below the 1/f knee in the SQUID noise energy spectrum. The technique involves reversing the SQUID input leads at a chopping frequency of ∼100 Hz using laser‐driven switches which form a double‐pole–double‐throw network. For sufficiently low noise switching, the demodulate output no longer contains the SQUID low‐frequency noise but rather noise at the SQUID white‐noise floor. We present data demonstrating the low noise performance of a single laser switch, and include the electronics design required for the full implementation of the scheme.