Directly coupled adiabatic superconductor logic

Adiabatic superconductor logic (ASL) families are energy-efficient because they can operate with a switching energy much less than the I_cΦ₀ product, where I_c is the critical current for Josephson junctions, Φ₀ is the flux quantum, and I_cΦ₀ is generally on the order of 10^-19 J. A switching energy of 0.03I_cΦ₀ has been demonstrated by adiabatic quantum-flux-parametron (AQFP) logic, which indicates the possibility of extremely energy-efficient digital circuits using ASL. ASL uses signal transformers to propagate and invert the signal current; otherwise, it is difficult to design inverters. However, signal transformers are generally difficult to miniaturize. Therefore, to realize high-density and energy-efficient superconductor circuits, ASL that does not use signal transformers is required. In this paper, we propose ASL without signal transformers, which we call directly coupled quantum-flux-parametron (DQFP). DQFP logic is based on AQFP, but it can invert signal current without using signal transformers. We conducted numerical simulation of a DQFP inverter chain to show that DQFP logic can invert signal current without signal transformers, and that DQFP logic can operate adiabatically. Then, we designed and fabricated basic DQFP circuits, such as an inverter chain and a full adder. All the circuits were found to have wide operating margins. Our results indicate that DQFP logic is suitable for high-density and energy-efficient superconductor circuits.