A femtoamperemeter based on a superconducting quantum interferometer and a bulk transformer

A femtoamperemeter based on a superconducting quantum interference device (SQUID) with a planar on-chip gradiometric transformer and an additional volume cryogenic transformer on a toroidal ferromagnetic core is developed, fabricated, and experimentally studied. The conditions for the optimum matching of the SQUID to the signal source are analyzed. It is demonstrated that the conditions for the optimum matching and high coupling coefficient are satisfied in the transformer with a ferromagnetic core. The excess noise introduced by the core is studied experimentally. It is shown that double shielding using superconducting and ferromagnetic shields makes it possible to lower the cutoff frequency of the excess noise to less than 1 kHz.