Towards practical superconducting accelerators for machine learning using U-SFQ

Most popular superconducting circuits operate on information carried by ps-wide, \(\boldsymbol{\mu}\) V-tall, single flux quantum (SFQ) pulses. These circuits can operate at frequencies of hundreds of GHz with orders of magnitude lower switching energy than complementary-metal-oxide-semiconductors (CMOS). However, under the stringent area constraints of modern superconductor technologies, fully-fledged, CMOS-inspired superconducting architectures cannot be fabricated at large scales. Unary SFQ (U-SFQ) is an alternative computing paradigm that can address these area constraints. In U-SFQ, information is mapped to a combination of streams of SFQ pulses and in the temporal domain. In this work, we extend U-SFQ to introduce novel building blocks such as a multiplier and an accumulator. These blocks reduce area and power consumption by 2 \(\times\) and 4 \(\times\) compared with previously-proposed U-SFQ building blocks, and yield at least 97% area savings compared with binary approaches. Using these multiplier and adder, we propose a U-SFQ Convolutional Neural Network (CNN) hardware accelerator capable of comparable peak performance with state-of-the-art superconducting binary approach (B-SFQ) in 32 \(\times\) less area. CNNs can operate with 5-8 bits of resolution with no significant degradation in classification accuracy. For 5 bits of resolution, our proposed accelerator yields 5 \(\times\) -63 \(\times\) better performance than CMOS and 15 \(\times\) -173 \(\times\) better area efficiency than B-SFQ.