Double relaxation oscillation superconducting quantum interference devices with gradiometric layout

Double relaxation oscillation superconducting quantum interference devices (DROSs) with a gradiometric signal SQUID and either a reference SQUID or a reference junction will be presented in this article. The devices are user friendly, particularly those with a reference junction. Because of the large flux-to-voltage transfer of ∂V/∂Φ=0.7–1mV/Φ0, the devices can be operated in a flux locked loop based on direct voltage readout without loss of sensitivity. The typical white flux noise of the DROSs amounts to √S𝛷=5–6μΦ0/√Hz, which corresponds to an energy resolution ε=S𝛷/2Lsq≃200h. Coupled to an external planar first-order gradiometer, a white magnetic field sensitivity of √S𝐵<2fT/√Hz was measured inside a magnetically shielded room.