Title: How much quantum computer (emulation) can 2½¢ buy?
[alt.: How much quantum computer (emulation) can two and half cents buy?]

Author: Dan Strano (Unitary Fund)

Abstract:

Upon a systematic comparison of costs for classical emulation with a state-of-the-art quantum computer emulator (Unitary Fund's Qrack library) vs. current-day on-demand pricing for genuine "noisy intermediate-scale quantum" ("NISQ") hardware usage, we find that current-generation NISQ might be economically advantageous even today for "structured" circuits (i.e., not following a strong anti-concentration condition) larger than about 50 qubits by 6 or 7 layers of circuit depth with nearest-neighbor connectivity, comparing by per-shot economic cost for emulation vs. NISQ services. All-to-all connectivity remains difficult to emulate at comparable cost above the limits of simple state vector simulation. At this nearest-neighbor scale or below, Qrack combines various established simulation techniques (like quantum binary decision diagrams and light-cone optimization) to achieve competitive emulation cost at high fidelity. For circuits with a strong anti-concentration condition, the patch and elided circuit approaches of 2019 Arute et al. (as executed by Qrack or other libraries) are found to be passable emulation approaches in themselves to the exact "quantum supremacy" circuits of that report and other related reports, with up to about 50% cross-entropy benchmark fidelity.