Quantum Computing

Explainer

Quantum Computing explained

A quantum computer is a machine that uses the rules of quantum physics to process information. Its basic unit is the qubit. A classical bit is either 0 or 1. A qubit can be in a superposition, a state that combines both, and its measured result is probabilistic.

Qubits can also be entangled, which means their outcomes are correlated in ways classical bits cannot be. Quantum algorithms arrange these effects so that wrong answers cancel and right answers become more likely. Qubits are fragile, and noise from the environment (decoherence) causes errors, so practical machines need error correction.

How it differs from the usual approach

A quantum computer is not a faster classical computer. It is only expected to beat classical machines on certain problems, such as simulating quantum systems and factoring large numbers with Shor's algorithm, where the best known classical methods scale much worse. For everyday tasks such as browsing, databases or most AI work there is no expected advantage. The number of qubits alone says little about usefulness. Error rates, connectivity and the ability to correct errors matter just as much.

Where it is used

• Research into simulating molecules and materials.
• Testing algorithms for optimisation and for factoring and search problems.
• Cloud access for researchers and companies to experiment with early hardware.
• Work on quantum error correction and on logical qubits.
• Planning for post-quantum cryptography, since a large machine could break widely used public-key encryption.

What to watch

• Several hardware approaches compete: superconducting circuits, trapped ions, neutral atoms, photons and spins in silicon. None has clearly won.
• The field is shifting from counting physical qubits to demonstrating error-corrected logical qubits with falling error rates.
• In 2019 Google claimed a quantum advantage on a sampling task with 53 qubits. That task had no practical use, and the claim was debated by classical-computing researchers.
• NIST published its first post-quantum cryptography standards in August 2024, so organisations can start moving off encryption that a future large machine could break.

Frequently asked questions