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Glossary

Quantum glossary

Entanglement

A correlation between quantum systems so strong that the whole has a definite state while the parts do not.

Measuring one entangled qubit tells you about the other, however far away — but the result you get is random, so no message can be sent this way. What makes it quantum is that the correlations hold in every measurement basis at once, which is what Bell tests detect.

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Related terms

  • Bell state — One of four maximally entangled two-qubit states; the simplest and most-used form of entanglement.
  • Bell inequality — A limit on correlations that any theory of pre-set, local “hidden” answers must obey — and that quantum mechanics violates.
  • No-communication theorem — Entanglement cannot be used to send information: nothing done to one half changes the statistics seen at the other.