What is entanglement?
Entanglement is a link between qubits in which the group has a definite shared state but the individual qubits don't. Measure entangled qubits and their results are correlated in a way that can't be explained by each qubit carrying its own pre-set answer, as long as neither can influence the other faster than light. Experiments have confirmed this. Entanglement can't be used to send a message faster than light, but it's an essential resource in quantum algorithms, error correction and quantum networks.
The simplest example
Two qubits in the state (|00⟩ + |11⟩)/√2, a Bell state, each give 0 or 1 at random, 50% each. But they always agree: if one gives 0, so does the other. That alone could be imitated by a pair of gloves in two boxes. What gloves can't imitate is what happens when the two qubits are measured in different ways: the pattern of agreement across those choices beats a limit that any pre-set answers must obey. That limit is Bell's inequality.
What experiments show
Bell tests have been run many times since the 1970s. In 2015 experiments closed the main loopholes, including one with electron spins 1.3 kilometres apart, and the 2022 Nobel Prize in Physics was awarded for this line of work. The conclusion: nature doesn't work by local hidden answers.
What it can and can't do
| Can | Can't | |
|---|---|---|
| Correlations | Give correlations no classical system can produce | Let one side control the other's result |
| Communication | Make teleportation and some kinds of quantum key distribution possible, always alongside an ordinary channel | Send information faster than light |
| Computing | Let qubits act as one system in algorithms and error correction | Make a computer faster on its own |
Common confusions
- "Measuring one qubit instantly sends a signal to the other." No usable signal travels. Each side sees random results until they compare notes by ordinary means.
- "Entanglement is permanent." It's fragile. Noise in the environment destroys it, which is one of the hardest problems in building quantum computers.
- "Entangled particles are just correlated, like gloves." Bell tests rule that out.
Written by Muhammad Atif · Reviewed 19 October 2026 · How sources are chosen and checked · Report a correction
Sources and further reading
Tags show what kind of source each one is (how sources are used). Checked against these sources on 19 October 2026. Spotted an error? Email hello@plainquantum.com and it will be corrected, with a note.
- BookQuantum Computation and Quantum Information (10th anniversary edition)Michael Nielsen and Isaac Chuang, Cambridge University Press, 2010
- Peer-reviewed paperProposed experiment to test local hidden-variable theories (the CHSH game)Clauser, Horne, Shimony and Holt, Physical Review Letters, 1969
- ExperimentLoophole-free Bell inequality violation using electron spins separated by 1.3 kilometresHensen et al., Nature, 2015
- ReferenceThe Nobel Prize in Physics 2022Nobel Prize Outreach, 2022