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.