A second derivative gradiometer integrated with a dc superconducting interferometer

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Abstract
A second derivative dc-SQUID (Superconducting Quantum Interference Device) gradiometer built on a single chip is described. The prototype shows a flux noise of 8×10−6 φ0/(Hz)1/2 and an inductance of 240 pH. The measured imbalance for fields orthogonal to the chip plane is 240 ppm, while the imbalance for transverse fields is below the measured error. The sensitivity of 1.8×10−8 T/m2 (Hz)1/2 is not yet sufficient for practical applications, but the natural improvement of physical parameters that will be made to use the device in an array of detectors is discussed as well.