Thin-film DC SQUID gradiometer

A thin-film dc SQUID gradiometer has been fabricated on a single planar substrate. The superconducting pick-up loops consist of a lead strip in the form of a 48 × 16 mm rectangle with a niobium strip bisecting the rectangle. A tunnel junction dc SQUID is symmetrically located on the niobium strip. If there is a spatial gradient in the magnetic field applied to the gradiometer so that the magnetic fluxes threading the two pick-up loops differ, a supercurrent is induced in the niobium strip that is detected by the SQUID. The noise power spectrum of the SQUID is white down to a frequency of about 5 × 10^-2Hz with a rms flux noise of 8 × 10^-5φoHz^-1/2, corresponding to a gradient sensitivity of 2 × 10^-10G cm^-1Hz^-1/2.