An approach to optimization of the superconducting quantum interference device bootstrapcircuit

Abstract

Recently, we demonstrated and analysed the superconducting quantum interference device(SQUID) bootstrap circuit (SBC). It is a direct readout scheme for dc SQUID in thevoltage bias mode, permitting one to suppress the preamplifier noise. The SBCenables us to control the two key parameters of a voltage-biased SQUID: theflux-to-current transfer coefficient and the dynamic resistance. The flux-to-current,I–Φ, characteristics of SBC are made asymmetric by introducing the additional current feedback.Depending upon the choice of the working point, this feedback can be positive (working pointW2 on thesteeper I–Φ slope) ornegative (W1 on the less steep slope). The dynamic resistance is controlled by the additional voltagefeedback. In our publications to date we presented only the SBC operation atW2, while in this paper we demonstrate operation atW1 and show that also in this regime the preamplifier noise suppression is possible. We used aliquid-helium-cooled Nb SQUID with a loop inductance of 350 pH and attained white flux noise of 2.5 µΦ0 Hz − 1/2 both atW2 andat W1. In the latter case, the linear flux range exceeded one half-flux quantumΦ0. This large linear range should lead to a significantly improved stability and slew rate ofthe system and also make the tolerable spread in circuit parameters much wider than in allSQUID direct readout schemes known to date. Consequently, operation in this regimeopens a new path to possible SBC optimization.