Ultra-Low Dynamic Current Measurements with an RF SQUID

Abstract

RF SQUIDs are generally used in magnetic flux measurements. However, to measure the current, a circuit similar to that shown in Fig. 1a may be used; here the unknown current i flows in the input coil L _s , coupled with the SQUID cavity of inductance L by means of the mutual inductance M. The latter is given by K(L _s L)^1/2. A current i ₀ defined by Φ₀/M flowing in the input coil causes a variation of one flux quantum into the SQUID cavity. If the minimum detectable flux variation ΔΦ/Φ ₀ is on the order of ~10^-4/Hz^1/2, the minimum detectable current is ≥ 10^-11 A in the 1-Hz bandwidth using accepted values for the parameters of commercial SQUIDs.* The actual limiting factors for current measurements are the geometrical size of the SQUID cavity, which cannot be enlarged without degradation of the signal-to-noise ratio [¹], and the minimum preamplifier noise of the electronic detecting circuitry.