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What does NISQ mean?
NISQ is the current era of quantum computing, named by John Preskill in 2018: devices with tens to a few hundred noisy qubits and no error correction.
NISQ (Noisy Intermediate-Scale Quantum) devices are too large to simulate exactly on a classical computer but too noisy to run error correction. This is why hybrid (quantum + classical) algorithms with short circuits, such as VQE and QAOA, came to the fore.
Demonstrating a clear, practical advantage on these devices is still a research question. The field’s goal is to move to error-corrected, fault-tolerant quantum computers.
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Related concepts
- VQE (variational quantum eigensolver) · VQE is a hybrid quantum algorithm that approximates the ground-state energy of a molecule by tuning a parameterised quantum circuit in a classical optimisation loop.
- QAOA (quantum approximate optimisation algorithm) · QAOA combines parameterised quantum circuits with classical optimisation to search for approximate solutions to combinatorial optimisation problems such as MaxCut.
- Quantum error correction · Quantum error correction spreads one logical qubit over many physical qubits so that errors caused by noise can be found and fixed without measuring the qubits directly.