Low-frequency noise reduction in SQUID measurements using a laser-driven superconducting switch. Part B: Modulated inductance 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. Direct switching of the SQUID input leads at a frequency above the 1/f knee, a technique detailed in the preceding article [Anderson et al., Rev. Sci. Instrum. 60, xxx (1989)], extends white-noise floor to much lower frequencies with an accompanying loss of slew rate. For those applications where the loss of slew rate is unacceptable, we present in this article an alternate chopping technique which can preserve the original SQUID slew rate with a loss in the low-frequency signal-to-noise energy ratio of about a factor of four.