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

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 Φ₀ feedback and sub-flux quantum feedback Φ₀/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-Φ₀ flux feedback resolution of Φ₀/8 from circuit simulation. The slew rate is found to be 1.56×10⁸ Φ₀/s in this simulation (f_c=1.25 GHz, n=8). The flux resolution for the digital SQUID shows improvement with increasing n and reached below 40 m^Φ₀ with n=2¹⁰; however the slew rate is decreased with increasing n and reached 1.22×10⁶ Φ_0/s with f_c=1.25 GHz and n=2¹⁰.