NISQ Explained: Noisy Intermediate-Scale Quantum Computers
What the letters mean
Noisy means qubits and gates make errors. Intermediate-scale means the machines are big enough to be hard to simulate in some cases, yet too small for full error correction. The term was introduced in 2018 and is still widely used.
Why noise is the main limit
Each gate has a small chance of error, and errors accumulate as a circuit gets deeper. After enough steps, the output is mostly random. That caps the size of programs you can run. Read more about the hardware side in how quantum computers are cooled and what is a qubit.
What NISQ devices are used for
- Research and benchmarking: testing hardware and ideas.
- Variational algorithms: short circuits tuned by a normal computer, aimed at chemistry and optimization.
- Error mitigation: techniques that reduce the effect of noise using extra runs and post-processing, without full error correction.
Has NISQ delivered an advantage?
There have been demonstrations on specially designed tasks, and some have been debated when classical methods caught up. See supremacy and advantage. A clear, useful advantage on a real world problem is still a goal.
What comes after NISQ
The next stage is early fault tolerant machines, using many physical qubits to form reliable logical ones. Timing is uncertain; see the timeline. Breaking cryptography is expected to need that later stage, not NISQ devices, which is why Bitcoin and Solana are not at immediate risk.
Where the term came from
John Preskill, a physicist at Caltech, introduced NISQ in a paper first posted on arXiv in January 2018 and published in the journal Quantum in August 2018. He argued that devices with about 50 to 100 qubits might do some tasks beyond easy classical simulation, but that noise would limit circuit size, and that these devices would be a stepping stone toward fault tolerant machines.
A rough error budget
If each gate is right 99.9 percent of the time, a circuit of 1,000 gates succeeds all the way through roughly 37 percent of the time, since 0.999 to the power 1,000 is about 0.37. At 10,000 gates it is under 0.01 percent. This simple arithmetic is why gate quality matters more than raw qubit counts, and why useful algorithms need error correction. Estimates for breaking real cryptography call for millions of gates or more in a fault tolerant setting.
| Era | Qubits | Errors | Typical use |
|---|---|---|---|
| NISQ | Tens to thousands physical | Uncorrected, mitigated | Research, benchmarks |
| Early fault tolerant | Dozens to hundreds logical | Corrected | Narrow chemistry and physics tasks |
| Large fault tolerant | Thousands of logical | Corrected | Cryptanalysis, big simulations |
What is changing in 2026
Many researchers say the field is leaving pure NISQ. In December 2024 Google reported its Willow chip with error correction that improved as the code grew, an important milestone. In October 2025 Google reported its Quantum Echoes result on Willow as a verifiable advantage for a physics measurement, a company claim still being weighed. Details are in the Willow guide, error correction state of play and logical qubits on trapped ion hardware. Note that the company Quantinuum Ltd is not connected to the QNT memecoin.
Common mistakes
- Treating NISQ as an insult. It is a description of a stage.
- Expecting NISQ machines to break encryption. They cannot.
- Mixing error mitigation with correction. Mitigation reduces the impact on results, but its cost grows quickly with circuit size.
How to check this yourself
When you read about a new machine, look for its two qubit gate error rate, not just its qubit count. See T1 and T2 and the 2018 to 2022 history.
Sources and further reading
- Preskill 2018: Quantum Computing in the NISQ era and beyond (arXiv)
- Preskill 2018 in Quantum, vol. 2, p. 79
- Google: Quantum Echoes on Willow, October 2025
Checked 2026-10-09. Research and standards change often, so check the primary documents. Nothing here is financial advice. The QNT memecoin is independent of Quantinuum Ltd, the real company, and of every lab, company and standards body named on this page.
Frequently asked questions
Who coined NISQ?
Physicist John Preskill introduced the term in 2018.
Can NISQ computers break encryption?
No. Breaking real encryption is expected to need much larger error corrected machines.
Are NISQ computers useful?
They are useful for research and experimentation. Broad practical advantage over classical computers has not been established.
What is error mitigation?
It is a set of techniques that reduces the impact of noise on results without the full overhead of error correction.
What is a logical qubit?
It is a reliable qubit made from many noisy physical qubits using error correction. See logical vs physical qubits.
Is NISQ over?
The term is still used, but leading labs now describe a move toward early fault tolerant machines.
Did Willow show error correction works?
Google reported in December 2024 that errors dropped as its code size increased, which is considered a key milestone, though it is not a full fault tolerant computer.
Why does a 99.9 percent gate still fail often?
Errors multiply over many gates, so long circuits fail unless errors are corrected.
Keep reading
- Quantum Error Correction Explained
Qubits are fragile, so quantum computers need error correction. Learn how logical qubits are built and why this is the key challenge. - Quantum Supremacy vs Quantum Advantage Explained
What the terms quantum supremacy and quantum advantage mean, and why headline claims are often debated. - Types of Quantum Computers: Superconducting, Ion, Photonic and More
A guide to the main ways quantum computers are built, with the strengths and trade-offs of each approach. - Quantum Computing Timeline: Key Milestones From 1981 to Today
A short history of quantum computing, from Feynman's 1981 idea and Shor's algorithm to cloud quantum computers and post-quantum standards.
All Quantum computing guides | Back to top | Search the site
Main pages: Quantum computing explained | Quantum and crypto | Companies | Quantum news | Glossary