Superconducting Computing with Alternating Logic Elements

Although superconducting single flux quantum (SFQ) technologies offer the potential for low-latency operation with energy dissipation of the order of attojoules per gate, their inherently pulse-driven nature and stateful cells have led to designs in which every logic gate is clocked. This means that clocked buffers must be added to equalize logic path lengths, and every gate becomes a pipeline stage. We propose a different approach, where gates are clock-free and synchronous designs have a conventional look-and-feel. Despite being clock-free, however, the gates are state machines by nature. To properly manage these state machines, the logical clock cycle is composed of two synchronous alternating phases: the first of which implements the desired function, and the second of which returns the state machines to the ground state. Moreover, to address the challenges associated with the asynchronous implementation of Boolean NOT operations in pulse-based systems, values are represented as unordered binary codes - in particular, dual-rail codes. With unordered codes, AND and OR operations are functionally complete. We demonstrate that our new approach, xSFQ, with its dual-rail construction and alternating clock phases, along with "double-pumped" logical latches and a timing optimization through latch decomposition, is capable of implementing arbitrary digital designs without gate-level pipelining and the overheads that come with it. We evaluate energy-delay tradeoffs enabled by this approach through a mix of detailed analog circuit modeling, pulse-level discrete-event simulation, and high-level pipeline efficiency analysis. The resulting systems are shown to deliver energy-delay product (EDP) gains over conventional SFQ even with pipeline hazard ratios (HR) below 1%. For hazard ratios equal to 15% and 20% and a design resembling a RISC-V RV32I core (excluding the cost of interlock logic), xSFQ achieves 22x and 31x EDP savings, respectively.