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 niobium thin-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 pickup 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 cm × 1.3 cm with a baseline of 6.2 cm, which results in a magnetic field gradient sensitivity of 0.3 nT/(cm·Φ
₀
). The white noise of the SQUID gradiometer is measured to be 2.6 µΦ
₀
/Hz
^1/2
corresponding to a magnetic field gradient noise of 99 fT/(m·Hz
^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.