NISQ

Preskill's name for quantum devices that are too noisy for long fault-tolerant circuits and large enough that the noise is the main constraint.

NISQ is John Preskill's abbreviation for the noisy intermediate-scale quantum era. In the 2018 essay he means devices whose qubit counts are large enough to be beyond easy classical simulation in some regimes, and whose noise still prevents the long circuits required for fault tolerance. He is naming a period of hardware, not an algorithm and not a complexity class.

VQE is the algorithm in this library most at home in that description: many short shots instead of one long coherent evolution. Living in the NISQ regime does not grant the algorithm a scaling theorem. Decoherence is the physical process Preskill's adjective "noisy" is pointing at. When a vendor or a paper says "NISQ algorithm" without a bound, read it as "designed for short depth," and then look for the actual claim.

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