Design and Performance of Digital SQUID with Sub-Flux Quantum Resolution

Abstract

Digital SQUIDs based on the single flux quantum (SFQ) logic have attractive characteristics, such as wide dynamic range and high slew rate. However, the resolution of digital SQUIDs is limited to SFQ. To realize sub-flux quantum resolution for these SQUIDs, we have adopted two types of feedback combinations: one with the SFQ input/output for a main loop and the other with a mutually coupled loop. The main loop receives direct Φ_0 feedback and sub-flux quantum feedback Φ_0/n, where $n$ is an integer, from the mutually coupled loop. We confirmed the correct operation of our device at a clock frequency of 1.25 GHz for a digital SQUID with a sub-Φ_0 flux feedback resolution of Φ_0/8 from circuit simulation. The slew rate is found to be 1.56×10^8 Φ_0/s in this simulation (fc=1.25 GHz, n=8). The flux resolution for the digital SQUID shows improvement with increasing n and reached below 40 mΦ_0 with n=2^{10}; however the slew rate is decreased with increasing n and reached 1.22×10^6 Φ_0/s with fc=1.25 GHz and n=2^{10}.