Coupling considerations for SQUID devices

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Abstract
Most applications of superconducting quantum interference devices (SQUID’s) involve use of a coupling coil. There are two classes of such applications: magnetometry (magnetic field, field gradient) and voltage or current measurements. We have analyzed the factors which determine the ultimate sensitivity, and present a guide for optimization of signal/noise in each of the two cases. It is found that a single figure of merit, involving both the equivalent input noise of the SQUID and the effectiveness of the coupling to the coil, is applicable to either type of measurement. We propose that this figure of merit be used in evaluating the performance of new SQUID designs.