Quantum computing

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Circuits of qubits that solve problems differently.

What it is

A quantum computer uses qubits and quantum gates instead of bits and logic gates. A program is a sequence of gates, called a circuit, followed by a measurement.

It does not replace your laptop. It is a specialised machine that is expected to be useful for particular problems, often together with classical supercomputers.

How it works

Gates rotate the state of qubits and create entanglement between them. A Hadamard gate, for example, turns a definite 0 into an equal mix of 0 and 1. Algorithms arrange the gates so that wrong answers cancel and the right answer becomes likely.

Today's machines have tens to hundreds of noisy qubits. The goal is fault-tolerant quantum computing, where many physical qubits protect one reliable logical qubit through error correction.

Try it: apply quantum gates

One qubit starts as 0. Apply gates and see the probabilities. H makes an equal superposition; X flips 0 and 1; Z changes the sign, which only shows after another H.

Probability of 0 100% 100%

Probability of 1 0% 0%

Gates applied: none

Why it matters

Expected uses include simulating molecules and materials, optimisation, and certain machine-learning and cryptographic tasks. Even before large machines exist, hybrid quantum-classical approaches are being tested in industry and research.

Reaching useful scale needs advances in hardware, control electronics, software and skills.

Europe’s position

The EuroHPC Joint Undertaking is installing quantum computers at several European supercomputing sites, and national and Horizon Europe projects develop processors, software and applications.

598 EU-funded projects in quantum computing and simulation, with an EU contribution of EUR 1,188 million (CORDIS; the pillar is suggested automatically and pending review). Quantum simulation is counted in this pillar since taxonomy 2.0.

On this site

Recent EU-funded projects in quantum computing and simulation

Pages of this portal

Glossary

Quantum gate: An operation that changes the state of one or more qubits.
Error correction: Using many physical qubits to protect one logical qubit from errors.
NISQ: Noisy intermediate-scale quantum: today's imperfect machines.
Logical qubit: A more reliable qubit built from many physical qubits with error correction.
Error correction threshold: The error rate per operation below which adding qubits for error correction lowers errors instead of adding to them.
Quantum advantage: A task that a quantum computer does better or faster in practice than the best known classical method.