New AC-Powered SFQ Digital Circuits

Recent progress on reciprocal quantum logic (RQL) has renewed interest in ac powering of superconductor digital circuits, which had been abandoned since the famous IBM project of 1970s. In this paper, we propose and demonstrate new ac-biased single flux quantum (SFQ) circuits, and search for synergy of ac and currently dominating dc biasing schemes. As the first step, we suggest an on-chip ac/dc converter capable of feeding a few dc-biased gates surrounded by their ac-biased counterparts. As the second step, we introduce and present the first successful demonstration of a new ac-powered circuit-an 8192-bit shift register with over 32800 Josephson junctions (JJs) and JJ density of about 6 · 105 JJ per cm2. We suggest a few niche applications for this type of ac-biased circuits, which require high clock rates. For example, these circuits, scalable to millions of JJs per chip, can serve as a convenient benchmark for new SFQ fabrication technology nodes, allowing the operating margins of individual cells to be extracted and thus “visualize” individual fabrication defects and flux trapping events. The circuit can also be developed into a megapixel imaging array for a magnetic field microscope.