Practical Quantum Computing: The value of local computation

Abstract: As we enter the era of useful quantum computers we need to better understand the limitations of classical support hardware, and develop mitigation techniques to ensure effective qubit utilisation. In this paper we discuss three key bottlenecks in near-term quantum computers: bandwidth restrictions arising from data transfer between central processing units (CPUs) and quantum processing units (QPUs), latency delays in the hardware for round-trip communication, and timing restrictions driven by high error rates. In each case we consider a near-term quantum algorithm to highlight the bottleneck: randomised benchmarking to showcase bandwidth limitations, adaptive noisy, intermediate scale quantum (NISQ)-era algorithms for the latency bottleneck and quantum error correction techniques to highlight the restrictions imposed by qubit error rates. In all three cases we discuss how these bottlenecks arise in the current paradigm of executing all the classical computation on the CPU, and how these can be mitigated by providing access to local classical computational resources in the QPU.