What it is
Entanglement is a link between quantum particles. When two particles are entangled, they no longer have separate states; the pair has one state that belongs to both.
Measure one particle and you immediately know something about the other: their results are correlated, however far apart they are.
How it works
Entangled pairs are made in the laboratory, for example by sending a laser through a special crystal, or by letting two atoms interact. The pair is then separated and each particle is sent to a different place.
The correlations do not carry a message, so entanglement cannot be used to send information faster than light. What it gives is a resource that is shared between distant places and that no eavesdropper can copy.
Try it: measure an entangled pair
Each press prepares a fresh entangled pair and measures particle A. Watch what happens to B.
A: ? B: ?
No measurements yet.
Why it matters
Entanglement is the key resource behind quantum networks, secure communication and the quantum internet, and it also lets quantum sensors and quantum computers reach performance that separate qubits cannot.
Distributing entanglement over long distances needs quantum repeaters, which is an active research field.
Europe’s position
Europe leads several entanglement-based networks and testbeds, including the Quantum Internet Alliance, which builds a prototype network that links cities with entangled photons.
159 EU-funded projects in quantum communication, with an EU contribution of EUR 416 million (CORDIS; the pillar is suggested automatically and pending review).
On this site
Recent EU-funded projects in quantum communication
- SYMPL: SYMmetric-key cryptography in Post-quantum worLd
- PACMAN: Post-quAntum Cryptography iMplementation with emerging non-volAtile techNologies
- QModem: Developing the world’s first quantum modem
- QTOOL: Single-Photon Light Sources for Quantum Technologies
- HarmoniQCI – Harmonising Quantum Communication
Related documents
- ETSI TS 104 015 V1.1.1 Cyber Security (CYBER); Quantum-Safe Cryptography (QSC); Efficient Quantum-Safe Hybrid Key Exchanges with Hidden Access Policies
- ETSI GR QSC 001 V1.1.1 Quantum-Safe Cryptography (QSC); Quantum-safe algorithmic framework
- ETSI TR 103 965 V1.1.1 CYBER; Quantum-Safe Cryptography (QSC); Impact of Quantum Computing on Cryptographic Security Proofs
Glossary
- Bell pair
- The simplest entangled state of two qubits.
- Quantum repeater
- A device that extends the distance over which entanglement can be shared.
- Correlation
- A relationship between the results of separate measurements.
- Quantum teleportation
- Moving a quantum state to another place using entanglement and an ordinary message; nothing travels faster than light.
Learn more
Further reading
- Understanding Quantum Technologies 2025 (Olivier Ezratty, 8th edition, version 8.1 of 30 October 2025, arXiv:2111.15352 (v6), doi:10.48550/arXiv.2111.15352, CC BY-NC-ND 4.0)See Part 1: Prologue
- Understanding Quantum Technologies 2026 (Olivier Ezratty, 9th edition, version 9.0 of 17 October 2026 (latest edition))See Part 1: Prologue
Olivier Ezratty, Understanding Quantum Technologies (CC BY-NC-ND 4.0). Links only; no text of the book is reproduced on this portal.