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Long baseline LTS SQUID gradiometers with sub-μm sized Josephson junctions

In this paper, we present new integrated on-chip planar-type superconducting quantum interference device (SQUID) first order gradiometers, which are manufactured by a new mix and match fabrication technology combining sub-μm sized Josephson junctions (JJs) with cm-scale pickup loops. The fabrication technology is described and the design is schematically shown. These sensors have high voltage swing and low flux noise and provide thus the best so far reported gradient resolution of about 13 fT / m · Hz . These gradiometers were developed and implemented in instruments for applications in mineral exploration. They allow a better system dynamic range (ratio of maximal to minimal detectable signals) and a more effective use of the chip area. The gradiometer performance in terms of gradient resolution is compared against state of the art planar-type SQUID based gradiometers. Two models predicting a gradient noise limit as a function of the chip area and baseline of the gradiometer are discussed. The small geometric dimensions of the gradiometers are very important for future miniaturized instruments in mineral exploration. The comparison of our results with those models highlights the importance of a smaller JJs size for the reduction of the intrinsic gradiometer noise and geometrical dimensions.