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Japanese Journal of Applied Physics

Asymmetric Bias Injection Technique for Drung-Type Superconducting Quantum Interference Devices

Gen Uehara, Naoki Matsuda, Kunio Kazami, Youichi Takada Kado

Published Dec 1, 1993• 17 citations
Publisher

Abstract

An asymmetric bias current injection method is developed to reduce bias-caused noise in Drung-type SQUID (superconducting quantum interference devices) without adding extra coils. A measuring scheme for ∂Φ/∂I b is tested, and it is demonstrated that the reduction is from 100 to 150 times that attained by the conventional SQUID biasing method. It is revealed that the direction of APF (additional positive feedback) coil winding is important in attaining this reduction.

DOI
10.1143/jjap.32.l1735

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An asymmetric bias current injection method is developed to reduce bias-caused noise in Drung-type SQUID (superconducting quantum interference devices) without adding extra coils. A measuring scheme for ∂Φ/∂I b is tested, and it is demonstrated that the reduction is from 100 to 150 times that attained by the conventional SQUID biasing method. It is revealed that the direction of APF (additional positive feedback) coil winding is important in attaining this reduction.
A simple approach to understanding and analyzing direct readout schemes for superconducting quantum interference devices (SQUIDs) is presented. It is shown that the existing methods for suppression of room temperature amplifier noise are based on voltage feedback and current feedback in the SQUID, that were introduced in the original schemes of additional positive feedback and bias current feedback. It is also shown that the way the SQUID is biased (at constant current or voltage) does not affect the noise suppression.
The relationship between superconducting quantum interference device (SQUID) voltage swing and additional positive feedback (APF) parameters has been investigated. Voltage swing is depressed as APF gain increases. In a symmetric-bias-injection SQUID, the voltage depression can be explained by the voltage divider action of APF resistance and the SQUID dynamic resistance. On the other hand, the voltage depression in an asymmetric-bias-injection SQUID is expressed as the effect of the voltage divider action multiplied by the effect of thermal noise of APF resistance. These two effects should be considered when designing SQUIDs with an APF.
We investigated the flux noise characteristics of a superconducting quantum interference device (SQUID) with additional positive feedback (APF) by comparing calculated results with experimental results. Equations for the minimum flux noise of a SQUID with APF and the optimum condition of APF gain (βa) are deduced. We examined the relationship of flux noise versus βaand experimentally confirmed that it has a minimum point. The deduced equations agree well with experimental results. From the viewpoint of total flux noise of SQUID, it is useful to employ the APF technique for a low-intrinsic-noise SQUID, but not for a high-intrinsic-noise SQUID.
A low-noise superconducting quantum interference device (dc-SQUID) with simplified design and readout electronics is highly desirable for various applications. We demonstrate that introducing an asymmetric shunt resistor significantly enhances the noise performance while maintaining the inherent simplicity and tunability of the SQUID design. By optimizing the asymmetry factor, our asymmetric shunted second-order gradient SQUID current sensor reaches a low white flux noise of 0.65 μΦ0/√Hz using a direct readout strategy, representing a threefold improvement over its symmetric counterpart. This noise level surpasses that of the SQUID with the additional positive feedback method and is comparable to the series SQUID arrays, making the asymmetric shunted SQUID suitable for meeting the readout requirements of various scientific instruments.
We experimentally studied two important parameters of helium-cooled superconducting quantum interference devices (SQUIDs) in the voltage bias mode: the dynamic resistance Rd and the flux-to-current transfer coefficient ∂i/∂Φ, with different junction shunt resistors RJ. We investigated a voltage-biased SQUID using the direct readout current-to-voltage converter scheme involving an operational amplifier. At higher RJ, the flux-to-voltage conversion coefficient ∂V/∂Φ becomes sufficiently large to effectively suppress the room-temperature amplifier's noise without any need for additional feedback circuits. The McCumber parameter limits the rise of ∂V/∂Φ. We discuss the performance of voltage-biased SQUIDs at different effective McCumber parameters.
Direct current-superconducting quantum interference devices (SQUIDs) characterized by asymmetric Josephson junctions have never been used for applications. In order to demonstrate their potential advantages, a throughout numerical analysis of different asymmetric configurations has been carried out. A damping resistance has been included in the SQUID circuit and the thermal noise associated with junction and damping resistances has been considered in the numerical model. Magnetic modulations and flux noise spectral densities have been computed as a function of many parameters (bias current, asymmetry, SQUID inductance, and damping resistance) and the performance of symmetric and asymmetric devices have been compared. The results show that, by properly optimizing the SQUID design, asymmetric SQUIDs are characterized by higher magnetic flux to voltage transfer coefficient and lower flux noise. As a result, asymmetric configurations can be very useful in all the applications where high sensitivity is required.