Switching Probability and Performance in Single-Chip SQUIDs

We measured the switching probability of a two-junction SQUID comparator integrated with a superconducting feedback circuit on one chip for use in biomagnetism. The switching probability, its derivative to input flux, and the flux spectral density measured by varying sinusoidal bias frequency and amplitude agree excellently with the thermal-noise theory. The best sensitivity and cutoff frequency were \(6.2 \times {10^{ - 16{\kern 1pt} }}{\Phi _0}/\sqrt {Hz} \) and 1.1 kHz for a bias frequency at 10 MHz-- sufficient for practical biomagnetic applications.