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Glossary

Quantum glossary

Bell state

Also: bell pair, epr pair, phi plus

One of four maximally entangled two-qubit states; the simplest and most-used form of entanglement.

Made with a Hadamard followed by a CNOT. Measure either qubit and you get a random bit — but the other qubit’s result is then perfectly correlated with it. Neither qubit has a definite state of its own.

|Φ⁺⟩ = (|00⟩ + |11⟩)/√2

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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.
  • Bell inequality — A limit on correlations that any theory of pre-set, local “hidden” answers must obey — and that quantum mechanics violates.
  • CNOT gate — A two-qubit gate that flips the target qubit if and only if the control qubit is |1⟩.