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

Abstract

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 in the pV Hz−1/2 range, promises to be competitive with that of the best superconducting quantum interference devices.