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The control of cryogenic qubits in today's superconducting quantum computerprototypes presents significant scalability challenges due to the massive costsof generating/routing the analog control signals that need to be sent from aclassical controller at room temperature to the quantum chip inside thedilution refrigerator. Thus, researchers in industry and academia have focusedon designing in-fridge classical controllers in order to mitigate thesechallenges. Superconducting Single Flux Quantum (SFQ) is a classical logicfamily proposed for large-scale in-fridge controllers. SFQ logic has thepotential to maximize scalability thanks to its ultra-high speed and very lowpower consumption. However, architecture design for SFQ logic poses challengesdue to its unconventional pulse-driven nature and lack of dense memory andlogic. Thus, research at the architecture level is essential to guidearchitects to design SFQ-based classical controllers for large-scale quantummachines.  In this paper, we present DigiQ, the first system-level design of a NoisyIntermediate Scale Quantum (NISQ)-friendly SFQ-based classical controller. Weperform a design space exploration of SFQ-based controllers and co-design thequantum gate decompositions and SFQ-based implementation of thosedecompositions to find an optimal SFQ-friendly design point that trades areaand power for latency and control while ensuring good quantum algorithmicperformance. Our co-design results in a single instruction, multiple data(SIMD) controller architecture, which has high scalability (>42,000-qubitscales), but imposes new challenges on the calibration of control pulses. Wepresent software-level solutions to address these challenges, which ifunaddressed would degrade quantum circuit fidelity given the imperfections ofqubit hardware.

DigiQ: A Scalable Digital Controller for Quantum Computers Using SFQ Logic