Picoammeters Based on Gradiometric Superconducting Quantum Interference Devices

High-sensitivity ac current sensors based on a superconducting quantum interference device have been designed, fabricated and characterized. In particular, double-washer schemes in either parallel or series configurations have been considered. The advantages and the drawbacks of both configurations have been examined by measuring the main features and parameters, such as the flux-to-voltage characteristic, the magnetic field spectral noise and flux-to-current transfer factor. The devices are designed to have similar flux-to-current transfer factors and are fabricated on the same chip to avoid differences in parameters due to the fabrication process. Both devices exhibited a current sensitivity as low as 1-2 pA per bandwidth unit, allowing for their use in ultrahigh-sensitivity applications.