Long Josephson Junction in Ultralow-Noise Magnetometer Configuration

The magnetic field dependence of the Eck step voltage in long Josephson tunnel junctions (LJTJs) can be exploited for ultralow-noise magnetic sensing. The magnetic flux to be measured is captured by a non-interrupted superconducting pickup loop in which a narrow constriction, forming the LJTJ wiring layer, produces a local high density circulating currents which, in turn, induces a magnetic field large enough to set a biased LJTJ at a finite voltage proportional to the field strength (flux-flow state). On this basis, an innovative and ultralow-noise superconducting device has been realized having a highly linear voltage responsivity and an intrinsic voltage spectral density down to few pV/ Hz. Being that the intrinsic voltage fluctuations of these all-Niobium sensors are very low (at T = 4.2 K), a double transformer superconducting quantum interference device (SQUID) amplifier has been used to determine the amplitude of its voltage spectral density obtaining a value as low as 10 pV/Hz. In addition, the 1/f noise does not affect the sensor performance down to a few hertz. The corresponding magnetic field noise, equal to 10 fT/Hz, together with highly linear and broadband voltage responsivity over a wide magnetic flux range, features performances that are comparable with those of a SQUID magnetometers having a comparable flux capture area.