Quantum physics basics

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Superposition, measurement and interference without the maths.

What it is

Quantum physics describes how very small things behave: atoms, electrons and the particles of light called photons. At this scale nature follows rules that look strange from everyday experience, but they have been confirmed in countless experiments.

Four ideas are enough to follow the rest of this guide: superposition, measurement, interference and decoherence. A fifth, entanglement, has its own topic later on.

How it works

Superposition: a quantum system can be in a combination of several states at once, for example a photon travelling along two paths. Measurement: when you look, you always get one definite result, chosen at random with probabilities set by the superposition, and the superposition is gone.

Interference: the possibilities in a superposition behave like waves. They can add up, making a result more likely, or cancel out, making it less likely or even impossible. Decoherence: contact with the surroundings, such as heat, stray light or vibration, acts like an unwanted measurement and slowly washes out superposition and interference.

Try it: make two paths interfere

A photon is split onto two paths that meet again in front of two detectors, A and B. Delay one path with the slider and watch how likely detector A becomes. Then check which path the photon took.

Delay of the upper path

Check which path the photon takes

Probability of detector A 100% 100%

Why it matters

These rules are what quantum technologies put to work. Quantum computers are designed so that interference strengthens the right answers and cancels the wrong ones, and the sensitivity of superposition to its surroundings is what makes quantum sensors so precise.

Another rule, the no-cloning theorem, says that an unknown quantum state cannot be copied perfectly. It is why quantum key distribution can reveal an eavesdropper, and also why quantum information cannot simply be backed up, which makes error correction harder.

Europe’s position

Quantum physics was largely developed in Europe in the early twentieth century by researchers such as Max Planck, Niels Bohr, Werner Heisenberg and Erwin Schrödinger. The 2022 Nobel Prize in Physics was awarded to Alain Aspect, John Clauser and Anton Zeilinger for experiments with entangled photons.

Universities and research institutes across the EU teach and research quantum physics today, and many of them take part in the EU-funded projects listed on this site.

Glossary

Interference: Possibilities in a superposition adding up or cancelling out, like overlapping waves.
No-cloning theorem: The rule that an unknown quantum state cannot be copied perfectly.
Photon: A single particle of light, the smallest possible amount of light energy.
Coherence time: How long a qubit or other quantum system keeps its superposition before decoherence destroys it.