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Soviet Journal of Low Temperature Physics

RF SQUID of 10−28 J/Hz sensitivity at nitrogen temperature

V. I. Shnyrkov, Georgy M. Tsoi, A. G. Kozyr’, V. N. Glyantsev

Published Jul 1, 1988• 3 citations
Publisher

Abstract

Superconducting quantum interferometers with a spectral noise density 8·10−29 J/Hz and 2·10−28 J/Hz in the frequency range f > 30 Hz at 77 K are made of polycrystalline bulk ceramics YBaCuO. The measurements are made on SQUID’s with the quantization circuit inductance L ≥ LF(Φ0/2π)1/kBT. It is found that the 1/f-type noises and their boundary frequency depend on the quality of the initial material.

DOI
10.1063/10.0031984

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Jul 2, 1994•0 citations
Superconducting quantum interferometers with a spectral noise density 8·10−29 J/Hz and 2·10−28 J/Hz in the frequency range f > 30 Hz at 77 K are made of polycrystalline bulk ceramics YBaCuO. The measurements are made on SQUID’s with the quantization circuit inductance L ≥ LF(Φ0/2π)1/kBT. It is found that the 1/f-type noises and their boundary frequency depend on the quality of the initial material.
We have performed a study of the low-frequency noise in high-temperature radio frequency (rf) superconducting quantum interference devices (SQUIDs) cooled in static magnetic fields. At 77 K, the noise of different thin-film YB2C3O7−x washer rf SQUIDs was measured in static magnetic fields up to 55 μT. The SQUID inductances ranged from 150 to 300 pH. The measured low-frequency flux noise of the SQUIDs SΦ1/2 at 1 Hz was between 70 and 350 μΦ0/√Hz, and in terms of magnetic field noise SB1/2 between 120 and 1100 fT/√Hz. The low-frequency noise of all devices at 1 Hz showed no significant increase for all applied static fields.
The operation of YBa2Cu3O7 rf superconducting quantum interference devices (SQUIDs) using a dielectric SrTiO3 resonator as tank circuit was tested. At 77 K, we attained for such resonators unloaded quality factors Q0 up to 7000 at resonant frequencies near 1 GHz, when measuring without electromagnetic shielding. For a washer SQUID with an area of 4×4 mm2 and an inductance of ≂100 pH, which was properly coupled to the resonator, we measured an approximately 60 μV peak-to-peak SQUID signal and a white flux noise spectral density of SΦ1/2=1×10−5Φ0/√Hz, corresponding to a field sensitivity of BN=105 fT/√Hz. With an additional single-layer flux transformer with a diameter of the pickup loop of 22 mm in flip-chip arrangement, the field sensitivity was improved to 30 fT/√Hz.
Applications of Superconducting Quantum Interference Devices (SQUIDs) usually require high sensitivity at relatively low frequencies, often down to 1 Hz or lower. Excess noise, typically with a spectral density scaling inversely as the frequency, can substantially reduce the sensitivity of SQUIDs at low frequencies. We have studied 1/f noise in niobium rf-SQUIDs in some detail. To reduce any noise contribution of the readout electronics, a cryogenic preamplifier was used. When one measures the signal voltage of the SQUID directly, a pronounced 1/f noise was observed in all samples, and was nearly independent of the bias frequency used. The crossover between 1/f and white noise moved towards higher frequencies as the bias frequency was increased, because of a lower white noise contribution. The 1/f noise scaled approximately as the inductance of the SQUID. When operated in a flux locked loop, however, no 1/f noise could be observed above 0.5 Hz. Operating a rf SQUID in a flux locked loop can thus substantially reduce 1/f noise in rf SQUIDs.