Measured performance parameters of gradiometers with digital output

A planar first-order gradiometer and a DC SQUID (superconducting quantum interference device) comparator have been integrated on the same 6×6 mm² chip in Nb-Nb₂O₅-PbInAu technology and operated in a flux-locked loop. Owing to the digital gradiometer output, a very low flux noise of 7×10^-7 φ₀/√Hz, a coupled noise energy of 34 times Planck´s constant h, and a gradient noise of 3.8 pT/m√Hz have been achieved in the white noise region above about 60 Hz. In spite of the very low flux noise, a maximum slew rate of 1.5×10⁵ φ₀/s at a signal frequency of 4 kHz has been measured. A simple modulation scheme has been developed to cancel Josephson junction critical-current fluctuations, yielding 4.5×10^-6 φ ₀/√Hz at 0.1 Hz