Asynchronous Digital SQUID Magnetometer With an On-Chip Magnetic Feedback for Improvement of Magnetic Resolution

An asynchronous digital superconducting quantum interference device (SQUID) with complementary output, which needs no periodic high-frequency readout clock input, has been investigated. The magnetic flux resolution of the asynchronous digital SQUID has been improved using an on-chip magnetic feedback. A wide dynamic range asynchronous digital SQUID magnetometer system with improved magnetic resolution has been devised using alternating readout of two digital SQUIDs placed in parallel. The slew rate and the dynamic range of the asynchronous digital SQUID magnetometer were estimated to be 4 × 10¹⁰ Φ₀/s and quite large, respectively. We have designed the asynchronous digital SQUID magnetometer system using 2.5 kA/cm² Nb process. We have experimentally confirmed the magnetic flux resolution of less than flux quantum (Φ₀) and the good linearity of the magnetic response of the asynchronous digital SQUID magnetometer system up to 3000 Φ₀.