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Review of Scientific Instruments

A dc SQUID for use below 1 K

Z. K. Geng, P. W. Adams, William O. Hamilton, W. W. Johnson, N. Solomonson

Published May 1, 1993• 10 citations
Publisher

Abstract

For detection of weak signals, and for use on ultralow temperature experiments, it would be useful to have a robust superconducting quantum interference device (SQUID) amplifier that works below 1 K. We report on a test of a commercial SQUID amplifier that has lower noise than others we have tested, and works well while attached to a 50-mK chamber. The SQUID noise was improved only modestly when cooled from 4 K to 50 mK. The best energy resolution measured was 2200 ℏ.

DOI
10.1063/1.1144086

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For detection of weak signals, and for use on ultralow temperature experiments, it would be useful to have a robust superconducting quantum interference device (SQUID) amplifier that works below 1 K. We report on a test of a commercial SQUID amplifier that has lower noise than others we have tested, and works well while attached to a 50-mK chamber. The SQUID noise was improved only modestly when cooled from 4 K to 50 mK. The best energy resolution measured was 2200 ℏ.
A dc superconducting quantum interference device (SQUID) with a resonant microstrip input is operated as an amplifier at temperatures down to 20 mK. A second SQUID is used as a postamplifier. Below about 100 mK, the noise temperature is 52±20 mK at 538 MHz, estimated from measurements of signal-to-noise ratio, and 47±10 mK at 519 MHz, estimated from the noise generated by a resonant circuit coupled to the input. The quantum-limited noise temperatures are 26 and 25 mK, respectively. The measured noise temperature is limited by hot electrons generated by the bias current.
The noise performance of dc-SQUIDs (Superconducting Quantum Interference Devices), which nowadays can reach values close to the quantum limit, makes them very attractive as amplifiers for experiments using low temperatures and demanding for extremely high sensitivity. After reviewing the working principle of the dc-SQUID, we describe a thin-film device with a new structure, which allowed us to reach an energy sensitivity of 5.5 h- at 0.9 K and a good coupling with a large input coil. At last we discuss some fundamental physics experiments which take advantage from the SQUID use.
The excess low-frequency flux noise in dc superconducting quantum interference devices (SQUIDs) operated at ultra-low temperatures was studied. A large number of single SQUIDs as well as SQUID arrays from 16 wafers fabricated over a period of six years were characterized at 4.2 K and1/fon the SQUID design or fabrication parameters. In contrast, below 4.2 K the low-frequency noise changed moderately or increased strongly depending on whether the bottom Nb or the insulation layer were fabricated in our newer sputter system instead of the older one. The corresponding excess noise levels ε1/fat1/f∝ f-αwith α typically around 0.6 for good devices. For devices with strong low-frequency excess noise, α increases up to about 0.9. The best energy resolution ε achieved so far with a 50 pH test SQUID operated at 16 mK is 0.62 h at 100 kHz, increasing to 1.64 h at 1 kHz and 14 h at 10 Hz.
We have fabricated and tested thin film, niobium edge junction, double transformer, dc superconducting quantum interference devices (SQUID’s) that were stable under room-temperature storage and thermal cycling and that had very good noise performance. The input inductance, approximately 1.7 μH, was large enough to facilitate good matching to many experiments. When the SQUID was operated as a small-signal amplifier, the minimum detectable energy per unit bandwidth (Se) was 5×10−33 J/Hz at 100 kHz, referred to the SQUID loop (uncoupled). The minimum detectable energy per unit bandwidth was 1.8×10−31 J/Hz at 100 kHz, referred to the input coil. The SQUID’s had good characteristics for flux-locked operation since the small signal Se was low over a substantial range of bias current and magnetic flux. For operation in a flux-locked feedback circuit, Se was 6×10−32 J/Hz at 1 kHz.
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Superconducting quantum interference device (SQUID) is a sensitive detector of magnetic flux signals. Up to now, the main application of SQUIDs has been measurements of magnetic flux signals in the frequency range from near DC to several MHz. Recently, cryogenic low-noise radio-frequency (RF) amplifiers based on DC SQUID are under development aiming to detect RF signals with sensitivity approaching quantum limit. In this paper, we review the recent progress of cryogenic low-noise RF amplifiers based on SQUID technology.