Bits and qubits

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Why a qubit can be more than a 0 or a 1.

What it is

Every digital device stores information in bits. A bit is always in one of two states, 0 or 1, like a switch that is either off or on.

A quantum bit, or qubit, is a physical system that follows the rules of quantum physics: a single atom, an ion, a photon or a tiny superconducting circuit. Until it is measured it can be in a combination of 0 and 1 at the same time. This is called superposition.

How it works

A qubit can be pictured as an arrow on a sphere. The north pole stands for 0 and the south pole for 1. Any other direction is a superposition, and the closer the arrow is to a pole, the more likely that result becomes when you measure.

Measuring is not neutral: it forces the qubit to give a plain 0 or 1, and the superposition is gone. Quantum computers therefore act on many qubits with carefully designed operations and only measure at the end.

Try it: steer a qubit

Drag the arrow on the sphere or use the slider. The nearer the arrow is to the top, the more likely a measurement gives 0.

Angle

Why it matters

Combining many qubits lets a quantum computer represent and process very large numbers of possibilities together. For some problems, such as simulating molecules or searching structured data, this can be dramatically faster than any known classical method.

Qubits are fragile. Heat, vibration and stray fields destroy superposition, which is why quantum hardware is cooled, shielded and still needs error correction.

Europe’s position

European groups work on all the main qubit types: superconducting circuits, trapped ions, neutral atoms, photons and spins in silicon. Public funding runs through the Quantum Flagship, Horizon Europe and national programmes.

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

Glossary

Superposition: A combination of 0 and 1 that exists until the qubit is measured.
Measurement: Reading out a qubit; it returns 0 or 1 and ends the superposition.
Decoherence: The loss of quantum behaviour caused by the environment.
Superconducting qubit: A qubit made from a tiny electrical circuit that loses all resistance when cooled close to absolute zero.
Trapped ion: A charged atom held in place by electric fields and used as a qubit, controlled with lasers.
Neutral atom: An uncharged atom held by focused laser beams, called optical tweezers, and used as a qubit.
Photonic qubit: A qubit carried by a particle of light, for example in its polarisation or in the path it takes.