Reversible Superconducting Digital Logic for Energy-Efficient Computing
The rapid growth of computation has raised concerns about energy consumption, motivating the development of reversible computing, which operates differently from conventional irreversible logic. Reversible digital logic offers significant energy-efficiency advantages and has potential applications in astronomy detector readout and quantum information science. Reversible Fluxon Logic (RFL) is one of the promising approaches, using unpowered, ballistic flux solitons (fluxons) in long Josephson junctions (LJJs) to encode information. Logical ‘1’ and ‘0’ are represented by the polarity of fluxons pulses, corresponding to clockwise and counterclockwise circulating currents, respectively.
We first demonstrate a low-energy transmission line composed of discrete LJJs, consisting of 80 Josephson junctions with 7.5 μA critical currents and connecting inductors. Measurements confirm ballistic fluxon propagation, with the transmission line operating in the continuous regime and a fluxon rest energy of approximately 47 zJ. To characterize logic gate operations, we then implement a two-polarity detector (TPD) that distinguishes fluxons by polarity, corresponding to the two-bit states in our logic. We observe that one polarity requires a lower bias current due to a ground loop in the otherwise floating LJJ, which can trap an extra fluxon along one path. Energy analysis of screening currents qualitatively agrees with experimental data and provides insight into flux behavior in ballistic reversible circuits. Building on these results, we design a ballistic flip-flop (BFF) gate with a floating fluxons launcher that avoids added ground loop. Simulations demonstrate correct internal phases for gate initialization, accurate digital outputs for both fluxon polarities, and no asymmetry in the output bias. These studies establish our ballistic gate as a robust, energy-efficient logic element, compatible with placement near solid-state qubits at mK temperatures, and demonstrate that ballistic logic is practical, providing a stable foundation for scalable, high-speed, ultra-low-energy computing for next- generation computing systems.