NISQ era: what it means

Short answer: NISQ stands for noisy intermediate-scale quantum, a term John Preskill proposed in 2018 for quantum computers that are too small and too noisy to run error-corrected algorithms. Noise in the gates limits how large a circuit can run reliably. NISQ devices are mainly research tools, for example for exploring many-body physics, and they are a step toward more powerful machines.

Where the term comes from

Preskill introduced "NISQ" in a 2018 paper to describe the phase of quantum hardware development that was then beginning. "Intermediate-scale" refers to the size of the devices, and "noisy" refers to the fact that their qubits and gates are imperfect and are not protected by quantum error correction. The term caught on because it names a real constraint: the machines are big enough to be hard to simulate classically in some cases, yet too error-prone to run long algorithms.

What defines this phase

Two limits define NISQ devices. The first is size, the number of qubits available. The second, and the one the 2018 paper stresses, is noise. Noise in quantum gates limits the size of the circuits that can be executed reliably, because errors accumulate with every operation. A circuit that needs many sequential gates will usually produce results swamped by noise before it finishes.

That is why NISQ machines cannot run arbitrary long algorithms. Without error correction, only relatively short computations can be completed before the errors take over. Error mitigation techniques can reduce the effect of noise, but their effectiveness is limited as well.

How NISQ devices compare with classical computers

Preskill argued in 2018 that devices of this kind might be able to perform some tasks that surpass what today's classical digital computers can do. He was careful about the scope of that claim. The comparison is about specific, narrow tasks. It is not about general computing power, and the classical computer remains the dominant tool for almost every computation.

The paper's own summary is measured: a quantum computer at this scale will not change the world right away. It should be seen as a step toward the more powerful quantum technologies of the future, not as the end goal.

What NISQ devices may be used for

The 2018 paper expected NISQ devices to be useful tools for exploring many-body quantum physics. Simulating complex quantum systems is notoriously hard for classical computers, so even a noisy device may give insight into physics that is otherwise difficult to reach. The paper also mentions other possible applications and some algorithms believed to hold promise for this era, but it describes those applications as less clearly defined.

In practice the value of NISQ hardware is mostly research value. It lets scientists test ideas, benchmark hardware and learn what limits real devices.

What comes after NISQ

The NISQ era is a transitional phase. The limits are the current state of the art, not permanent features, and they motivate work on more accurate gates and better hardware. The aim is to move beyond NISQ to machines that are larger, more accurate and able to run longer, more complex algorithms. That is the regime of fault-tolerant quantum computing, which depends on quantum error correction and has not yet been built.

Our other posts on physical and logical qubits and on quantum error correction explain why that step is so demanding.

FAQ

What does NISQ stand for?

Noisy intermediate-scale quantum. John Preskill proposed the term in 2018 for devices that are limited in size and by noise, and that do not use full error correction.

Why is noise the main limit?

Noise in quantum gates limits the size of the circuits that can be executed reliably, because errors build up with each operation.

Can NISQ computers beat classical computers?

The 2018 paper said they might perform some specific tasks that surpass today's classical computers. The scope is narrow and it does not mean general-purpose speed-ups.

What are NISQ devices used for?

Mainly research, such as exploring many-body quantum physics, testing algorithms and benchmarking hardware.

What comes after the NISQ era?

Larger, more accurate machines with quantum error correction, called fault-tolerant quantum computers. They have not been built yet.