Quantensysteme

A planar Superconducting Quantum Interference Device (SQUID) magnetometer consisting of a parallel gradiometer SQUID with integrated input coils connected to an on-chip pickup loop was designed and fabricated in conventional niobium technology. SQUID bootstrap circuitry (SBC) incorporating suitable current and voltage feedbacks was also integrated in the design. For a SQUID inductance of Ls = 350 pH and a chip size of 5 x 5 mm², the field resolution of the voltage-biased SQUID magnetometer reached < 5 fT/Hz(-1/2) with the bootstrap circuit and an usual preamplifier. We also observed that the effective McCumber parameter beta_c of the junctions is influenced by the bias mode. Indeed, when the nominal junction beta_c was larger than unity, our SQUID magnetometer operated stably in the voltage bias mode. The device exhibited low noise even without SBC.