Thermal‐noise‐limited performance in single‐chip superconducting quantum interference devices

The single‐chip superconducting quantum interference device (SQUID) we made using niobium Josephson junctions integrates a two‐junction SQUID and a superconducting feedback circuit. The switching probability, its derivative to input flux, and the flux spectral density measured by varying the sinusoidal bias frequency and amplitude agree well with thermal noise theory. The best sensitivity and cutoff frequency were 6.2×10−6 Φ0■√Hz and 1.1 kHz for a bias frequency at 10 MHz. The dynamic range was 2.5×105. These data show the SQUID to be suited to practical biomagnetic application.