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Ultra-low-noise magnetic sensing with long Josephson tunnel junctions

We study how the magnetic field dependence of the Eck step voltage in long Josephson tunnel junctions (LJTJs) can allow for ultra-low-noise magnetic sensing. The field to be measured is applied perpendicular to a continuous superconducting pickup loop. Wherever the loop has a narrow constriction, the density of the flux-restoring circulating currents will become relatively high and will locally create a magnetic field large enough to bring a biased LJTJ into the flux-flow state, i.e., at a finite voltage proportional to the field strength. This method allows the realization of a novel family of robust and general-purpose superconducting devices which, despite their simplicity, function as ultra-low-noise, wide-band and high-dynamics magnetometers. The performance of low-Tc sensor prototypes, including a highly linear voltage responsivity and an intrinsic voltage spectral density S V 1 / 2 ?> in the pV Hz−1/2 range, promises to be competitive with that of the best superconducting quantum interference devices.