Noise in the rf SQUID

Abstract

This paper presents the results of an extensive study of the sensitivity-limiting noise sources in rf-biased SQUID flux detectors. Our investigation has included both experiments with point contact and thin-film SQUIDs and digital computer simulations. The paper begins with a discussion of rf SQUID operation, with special emphasis on those aspects that determine flux detector sensitivity. We then discuss the origin and consequences of intrinsic device noise in rf SQUID systems. We describe a straightforward technique for determining intrinsic noise amplitude directly from SQUID rf I–V characteristics. Deviations in the measured intrinsic noise of many devices from the Kurkijarvi-Webb prediction are attributed to departures from the ideal Josephson sinusoidal current-phase relation in the SQUID weak link. Kurkijarvf's SQUID tank circuit noise analysis is reviewed and we show how SQUID sensitivity can be maximized by an optimal coupling between SQUID ring and tank circuit. This optimal coupling depends upon circuit parameters, ring inductance, and weak-link properties. The analysis reveals that variations in the current-phase relation can result in a difference of as much as a factor of ten in the attainable sensitivity of two otherwise identical SQUID systems. We also suggest efforts in circuit design that are most likely to lead to further improvements in flux detector sensitivity. Finally, other factors, including external magnetic noise and underdamping of the SQUID ring, are discussed.