Ballistic reversible gates matched to bit storage: Plans for an efficient CNOT gate using fluxons

Abstract

New computing technologies are being sought near the end of CMOS transistor scaling, meanwhile superconducting digital, i.e., single-flux quantum (SFQ), logic allows incredibly efficient gates which are relevant to the impending transition. In this work we present a proposed reversible logic, including gate simulations and schematics under the name of Reversible Fluxon Logic (RFL). In the widest sense it is related to SFQ-logic, however it relies on (some approximately) reversible gate dynamics and promises higher efficiency than conventional SFQ which is logically irreversible. Our gates use fluxons, a type of SFQ which has topological-particle characteristics in an undamped Long Josephson junction (LJJ). The collective dynamics of the component Josephson junctions (JJs) enable ballistic fluxon motion within LJJs as well as good energy preservation of the fluxon for JJ-circuit gates. For state changes, the gates induce switching of fluxon polarity during resonant scattering at an interface between different LJJs. Related to the ballistic nature of fluxons in LJJ, the gates are powered, almost ideally, only by data fluxon momentum in stark contrast to conventionally damped logic gates which are powered continuously with a bias. At first the fundamental Identity and NOT gates are introduced. Then 2-bit gates are discussed, including the IDSN gate which actually allows low fluxon-number inputs for more than 4 input states. A digital CNOT, an important milestone for 2-bit reversible superconducting gates, is planned as a central result. It uses a store and launch gate to stop and then later route a fluxon. This use of the store and launch gate allows a clocked CNOT gate and synchronization within. The digital CNOT gate could enable high efficiency relative to conventional irreversible gates and shows the utility of the IDSN as a reversible gate primitive.