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Journal de Physique IV (Proceedings)

Use of low-noise dc-SQUIDs in fundamental physics experiments

P. Carelli, Maria Gabriella Castellano, Roberto Leoni, G. Torrioli

Published Jun 1, 1998• 0 citations
Publisher

Abstract

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.

DOI
10.1051/jp4:1998347

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Applied Physics Letters•Jan 5, 1998•41 citations
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.
We present the results of an investigation of thin-film dc super conducting quantum interference device (SQUID’s) with very high energy resolution. The SQUID’s were fabricated using integrated-circuit techniques appropriate for superconducting devices. A weakly coupled SQUID with very low inductance (1 pH) had an intrinsic energy sensitivity 6×10−34 J/Hz = 0.9 h. A well-coupled SQUID (coupling constant α = 0.43) had an energy sensitivity referred to the input coil of 4.7×10−32 J/Hz = 71 h.
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.
The dc Superconducting Quantum Interference Device (dc-SQUID) is one of the most sensitive magnetic field sensors available. In this thesis we concentrate on its application as an amplifier. In this configuration, an input circuit of interest can be connected by means of a coupling coil. The intended application of our developed low-Tc SQUID amplifiers is the readout of the first spherical gravitational wave antenna MiniGRAIL. Using small-signal analysis as well as numerical simulations, we study the achievable signal-to-noise ratio of such sensors in practical measurements. Published theories and studies could be extended. Some interesting points treated are the influence of the often used negative feedback (flux-locked loop), an altered input impedance of the SQUID amplifier and the influence of back-action noise, which directly affects the object of interest. The altered operation of the SQUID due to the presence of the input circuit is studied in numerical experiments. Another important issue, the influence of integrated input coils on the dynamics of SQUIDs, is investigated theoretically and experimentally. The performance of our developed sensors is compared to numerical simulations on detailed models. The results give insights into the behavior, design and usage of dc-SQUIDs with integrated coils. For a SQUID with input inductances of 1.5 μH we reached a good coupled energy sensitivity of about 170 ¯h in a dilution refrigerator. Furthermore, we investigated the hot-electron effect. This effect typically limits the sensitivity of superconducting electronics at sub-Kelvin bath temperatures due to a weakened electron-phonon coupling. We performed heating experiments on thin-film resistors which are similar to the shunt resistors of the Josephson junctions used in our sensors. The suppression of the hot-electron effect via electronic thermal transport to attached cooling fins is investigated both experimentally, theoretically and numerically. The numerical technique turns out to be a useful tool for the thermal design of superconducting electronics.
The energy sensitivity of a direct current (dc) superconducting quantum interference device (SQUID) can be improved if it is operated in a two-stage configuration. Employing this technique, a commercial dc SQUID system was modified and made competitive with other sensors especially designed for very low noise applications. We report the noise measurements performed in the temperature range 4.2 K–25 mK. At 4.2 K, the coupled energy sensitivity obtained with the two-stage dc SQUID was approximately ten times better than with a conventional readout electronics. The noise energy decreases linearly until approximately 300 mK, in good agreement with theoretical previsions. At lower temperature the hot-electron effect produces a saturation and the best energy sensitivity measured with open input coil is 35 ℏ.
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.
The dc-superconducting quantum interference device (SQUID) is a low-noise converter from magnetic flux to voltage which can have, in principle, an energy sensitivity near the quantum limit of ℏ/2. A critical parameter for the ideal performance is the device inductance, which must be kept as small as possible. Minimizing the SQUID inductance, however, is a major concern for a practical device; this requirement implies a small SQUID ring and hence magnetic coupling with an external signal is more difficult to achieve. Here we present an original scheme (called multiwasher) to circumvent this problem, and its implementation in an all-refractory thin-film device. Our scheme not only provides good magnetic coupling with a large input coil (0.5 μH) and very low SQUID inductance, but also shielding from outside uniform fields, such as those generated by ambient disturbances. The measured coupled spectral energy sensitivity in the white region at about 1 kHz is 28ℏ at 4.2 K and 5.5ℏ in a pumped helium bath at 0.9 K. The flux noise spectral density at 0.1 Hz and 0.9 K is Φn=1×10−6 Φ0/Hz.