Design and Optimization of Fully Digital SQUID Based on Bi-Directional RSFQ

Abstract

Bi-directional RSFQ benefits from using both positive and negative SFQ pulses to manipulate and transfer digital data. This allows more flexibility in the design of simpler circuits with enhanced performance. On the other hand, using the AC bias current, one can replace on-chip resistive current distributors with inductors. This resembles RQL logic, but in contrast to RQL, it is possible to use the well-established standard RSFQ cells in bi-directional RSFQ. These two advantages (energy-efficient computation and flexibility in design) make bi-directional RSFQ a powerful tool in next-generation supercomputers and also compatible with ultra-low-temperature quantum computers. In this work, to show the power and simplicity of circuits in bi-directional RSFQ, a fully digital SQUID based on bi-directional RSFQ is designed and optimized. The circuit we show here is a delta modulating ADC with digital integrator. The circuit has fewer Josephson junctions than other reported circuits, which makes the proposed circuit more easily realizable in available LTc and HTc technologies.