A Planar Gradiometer With Discrete SQUID Current Sensor and Long-Baseline Pickup Coil

We have developed and fabricated a LTS SQUID gradiometer using Direct writing lithography technology. A niobiumth in-film first-order pickup coil is connected to the input coil of a SQUID current sensor via superconducting niobium wire bonds. Both, the fine-scale Josephson junctions and the large-scale pick up coil, are fabricated by this technology. The SQUID is designed as a first-order gradiometer with a dual square washer configuration which was fabricated in a cross-type Josephson junction technology. The inner dimension of each pickup loop is 1.3 cmx1.3 cm with a baseline of 6.2 cm, which results in a magnetic field gradient sensitivity of 0.3 nT/(cmPhi(0)). The white noise of the SQUID gradiometer is measured to be 2.6 mu Phi(0)/Hz(1/2 )corresponding to a magnetic field gradient noise of 99 fT/(mHz(1/2)) in the white noise region. We also investigated the gradiometer performance in different environments. The performance specifications could be improved by integrating SQUID and first-order pickup coil on the same chip in the future