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Glossary

Quantum glossary

Bell inequality

Also: bell test, chsh inequality, chsh

A limit on correlations that any theory of pre-set, local “hidden” answers must obey — and that quantum mechanics violates.

The CHSH version bounds a combination of correlations at 2 for any local hidden-variable theory; entangled qubits reach 2√2. Experiments closing the major loopholes earned the 2022 Nobel Prize. The violation rules out local hidden scripts; it does not allow signalling.

|S| ≤ 2 (local), |S| ≤ 2√2 (quantum)

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Related terms

  • Entanglement — A correlation between quantum systems so strong that the whole has a definite state while the parts do not.
  • No-communication theorem — Entanglement cannot be used to send information: nothing done to one half changes the statistics seen at the other.