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 x 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 x 0.01 Hz with a rms flux noise of 8 x 0.00001 phi sub Hz/sup1/2/, corresponding to a gradient sensitivity of 2 x 0.000001 G cm/1 Hz/sup-1/2/.