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Industry & Era

NISQ

Noisy Intermediate-Scale Quantum — the current era of quantum computing, characterized by machines with 50–1000 qubits but significant error rates.

Definition

NISQ stands for Noisy Intermediate-Scale Quantum. The term, coined by physicist John Preskill in 2018, describes the current generation of quantum computers: machines with enough qubits to be interesting and potentially useful, but with error rates too high and coherence times too short to run full quantum error correction. NISQ devices operate in a regime between simple proof-of-concept experiments and the fully fault-tolerant quantum computers of the future.

In depth

NISQ stands for noisy intermediate-scale quantum, a term John Preskill coined in 2018 for the era in which quantum processors have tens to a few hundred qubits but no error correction. Such devices are too small and noisy to run Shor's algorithm on useful numbers, yet large enough that simulating them classically is difficult in some cases. Preskill's question was whether they could do anything useful before fault-tolerant machines arrive, and the field has spent years exploring it.

NISQ algorithms are designed around shallow circuits and tolerance of noise. Variational algorithms such as VQE and QAOA, error mitigation, and quantum simulation of simple models are typical. Results have been scientifically interesting: quantum supremacy experiments on random circuits, simulations of spin dynamics, and demonstrations of mitigation extending the reach of devices. No NISQ application has yet delivered a clearly practical advantage over classical methods, and some researchers argue that the NISQ approach has reached its limits.

The label is now loosely applied, since hardware has improved. By 2025 devices have hundreds to over a thousand qubits, with two-qubit error rates near 0.1 percent, and early error-corrected demonstrations exist. Many describe the present as a transition to the early fault-tolerant era, with logical qubits in the dozens and algorithms designed for small error-corrected machines. The NISQ era's lasting contribution is the toolkit of error mitigation, benchmarking, and hybrid workflows that continues to be used.

Technical Definition

A NISQ device typically has 50 to a few thousand physical qubits with gate error rates on the order of 0.1–1% per gate. At these error rates, computations are limited to relatively shallow circuits (low depth) before noise dominates. Algorithms designed for the NISQ era — like VQE (Variational Quantum Eigensolver) and QAOA (Quantum Approximate Optimization Algorithm) — are specifically designed to extract useful results within these constraints.

Visual Explanation: An Analogy

The NISQ era is like the early days of classical computing, when computers filled rooms and could only run limited programs before overheating or breaking down — powerful enough to hint at the future, but not yet the transformative technology they'd become.

Key facts

ItemDetail
CoinedJohn Preskill, 2018
Typical devices50 to about a thousand qubits with error rates around 0.1 to 1 percent
Typical algorithmsVQE, QAOA, quantum simulation, and mitigated sampling
Main limitsNoise limits circuit depth to a few hundred layers
Next phaseEarly fault-tolerant machines with logical qubits

Where the idea comes from

Preskill's 2018 paper Quantum Computing in the NISQ Era and Beyond named the era. Google's 2019 supremacy experiment and IBM's 2023 utility experiment are its landmark results.

Real-World Use Cases

  • Running near-term quantum algorithms (VQE, QAOA) for chemistry and optimization
  • Exploring quantum advantage on specific, carefully chosen problems
  • Benchmarking hardware and developing the engineering needed for future fault-tolerant systems

How it differs from related ideas

  • NISQ machines lack error correction, while fault-tolerant machines have it.
  • Noisy simulation of NISQ devices is feasible only in some regimes.
  • The term describes a hardware era, not a specific technology.

Frequently asked questions

What does NISQ mean?

Noisy intermediate-scale quantum: devices with up to a few hundred or thousand qubits and no error correction.

Are NISQ computers useful?

They are useful for research and benchmarking. A clear practical advantage over classical methods has not been demonstrated.

What comes after NISQ?

Early fault-tolerant machines with error-corrected logical qubits, expected from the late 2020s.

Related Terms

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