A case for superconducting accelerators

As scaling of CMOS slows down, there is growing interest in alternative technologies that can improve performance and energy-efficiency. Superconducting circuits based on Josephson Junctions (JJ) is an emerging technology that provides devices which can be switched with pico-second latencies and consumes two orders of magnitude lower switching energy compared to CMOS. While JJ-based circuits can operate at high frequencies and are energy-efficient, the technology faces three critical challenges: limited device density and lack of area-efficient technology for memory structures, low gate fanout, and new failure modes of Flux-Traps that occurs due to the operating environment. Limited memory density restricts the use of superconducting technology in the near term to application domains that have high compute intensity but require negligible amount of memory. In this paper, we study the use of superconducting technology to build an accelerator for SHA-256 engines commonly used in Bitcoin mining. We show that merely porting existing CMOS-based accelerator to superconducting technology provides 10.6X improvement in energy efficiency. Redesigning the accelerator to suit the unique constraints of superconducting technology (such as low fanout) improves the energy efficiency to 12.2X. We also investigate solutions to make the accelerator tolerant of new fault modes and show how this fault-tolerant design can be leveraged to reduce the operating current, thereby improving the overall energy-efficiency to 46X.