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Journal of Low Temperature Physics

Dependence on the pump frequency of the sensitivity of the rf SQUID

Reino I. Ker�nen, Juhani Kurkij�rvi

Published Nov 1, 1986• 3 citations
Publisher
DOI
10.1007/bf00683977

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Input noise in the hysteretic rf SQUID: Theory and experiment

J. N. Hollenhorst, R. P. Giffard

The spectral density of the low-frequency open-circuit intrinsic input noise of the hysteretic rf-biased SQUID is calculated. The noise level, which affects the optimized power sensitivity of the SQUID, is found to be strongly model dependent and thus also constitutes an interesting probe of device physics. The results of the calculation are significantly different from previous predictions. A direct measurement of the total input noise of a typical rf-biased SQUID system is reported. The results, which are consistent our calculation, correspond to a system noise temperature of 3.1±0.4 mK at a frequency of 1 kHz. This value is consistent with the Manley-Rowe equations and with previous experimentally determined upper limits.

Journal of Applied Physics•Mar 1, 1980•19 citations
At a time when the supremacy of SQUIDs as the most sensitive magnetometers is being challenged by the recent progress of optically pumped atomic magnetometers operating at room temperature, looking into new opportunities to further improve SQUIDs sensitivity would probably be most welcomed by the SQUID community. By far the most promising such opportunity is to operate a double (D) rf SQUID (an rf-SQUID in which a dc-SQUID is used instead of a single Josephson junction) in a non-adiabatic regime at frequencies of about a few GHz. At these relatively high frequencies required to assure an ultra-high SQUID sensitivity the SQUID-design complexity and the cost of the electronics involved (current generator and SQUID amplifier) are still kept at reasonable levels. The advantages of using this concept, as presented in my contribution in more detail now, are overwhelming. Ironically, although this idea in its preliminary shape has been around for some time now [1], the opportunity has never been explored experimentally! I am just wondering what else needs to be done to get this message across the SQUID community and have this concept finally tested experimentally?
We present a simple model for the thermal magnetic noise in a rf SQUID coupled to a ferromagnetic core via a superconducting dc flux transformer. We show that, under conditions that are almost always fulfilled in practice, the coupling to the core can be described in terms of a SQUID frequency-dependent complex inductance Ls (ω). This inductance reduces to an ordinary, frequency-independent real value above a roll-off frequency ωc . To this inductance, a Nyquist flux noise generator is associated, with spectral density S′2φi(ω) =−4kBT Im[Ls(ω)]/ ω, where kB is the Boltzmann constant, T the temperature, and Im[Ls (ω)] the imaginary part of Ls (ω). If ωc ≪ωrf , with ωrf the frequency of the rf pump, then the response of the SQUID to low-frequency signals is not modified by the coupling to the core, while the above Nyquist noise acts as an additive input noise. We experimentally demonstrate the model for a SQUID coupled to a toroidal core.
The spectral density of the low-frequency open-circuit intrinsic input noise of the hysteretic rf-biased SQUID is calculated. The noise level, which affects the optimized power sensitivity of the SQUID, is found to be strongly model dependent and thus also constitutes an interesting probe of device physics. The results of the calculation are significantly different from previous predictions. A direct measurement of the total input noise of a typical rf-biased SQUID system is reported. The results, which are consistent our calculation, correspond to a system noise temperature of 3.1±0.4 mK at a frequency of 1 kHz. This value is consistent with the Manley-Rowe equations and with previous experimentally determined upper limits.