Dependence of SQUID Intrinsic Flux Noise on Stewart–McCumber Parameter <inline-formula> <tex-math notation="LaTeX">$\beta_{c}$</tex-math> </inline-formula> of Josephson Junction

Abstract

It is known that three superconducting quantum interference device (SQUID) parameters, namely, the intrinsic flux noise δΦ s , the flux-to-voltage transfer coefficient ∂V/∂Φ, and the dynamic resistance R d , increase with increasing the Stewart- McCumber parameter β c of Josephson junction. It is very difficult to obtain the low values of δΦ s for a strongly damped SQUID due to its small ∂V/∂Φ. In this paper, we employ a two-stage scheme (TSS) to measure δΦ s directly. Here, we use single chip readout electronics and a weakly damped SQUID with a β c of flux noise of the readout-SQUID system is about 4.8 μΦ 0 /√Hz 1.2, both acting as a readout-SQUID system (a preamplifier). The with a corner frequency of about 10 Hz. The front-end SQUID is coupled to the readout SQUID via the input coil with a mutual inductance of M i . In our experiment, the flux gain for a readout SQUID, i.e., G Φ = M i × (∂I/∂Φ) front-end is adjusted to be 20. Using the TSS, we measured δΦ s with different β c changing from 0.3 to 13.5 and found δΦ s increasing from 1.0 to 5.0 μΦ 0 /√Hz. We also discussed the relations between L s , β c , and δΦ s quantitatively. Finally, we proposed designing a suitable β c to match the preamplifier in a direct readout scheme to simplify a SQUID measurement system.