A quantum mechanics concept defining a model element, mathematical object, or experimental method used to predict measurable outcomes. It applies when required assumptions and definitions are specified and yields computable probabilities and expectation values. It does not ensure correctness without validation of approximations, numerical stability, and consistency of units and conventions. It materially affects interpretation of experiments and the reliability of theoretical predictions across quantum systems. The concept is generally stable, though methods and implementations evolve over time.

Principle

If joint measurement statistics on spatially separated subsystems defy decomposition into a mixture of product probability distributions conditioned on shared local variables, they are nonlocally correlated; the organizing idea is that quantum states can encode dependencies not mediated by local classical information.

Demonstration

Two qubits prepared in the singlet state and then measured in suitably chosen bases produce outcome correlations that violate the CHSH inequality, demonstrating correlations stronger than any local hidden-variable model can produce.

Misapplication

Labeling any statistical dependence between outcomes as nonlocal correlation when it arises from a common preparation procedure, direct interaction, or classical communication; confusing entanglement entropy for a direct proof of nonlocality without checking measurement choices.

Consequence

Correct identification of nonlocal correlations implies the impossibility of reproducing the observed statistics with local realistic theories and enables protocols like device-independent cryptography that rely on Bell-inequality violations.

Reversal

Local correlation: outcome statistics that admit a decomposition in terms of local hidden variables or conditional independent distributions, consistent with classical locality constraints and no Bell violation.

Boundary

Applies to joint measurement statistics of quantum systems separated by space-like or effectively independent regions; excludes correlations produced by explicit signaling, classical communication, shared measurement devices, or common-cause classical variables.

Semantic Tension

Tension exists between 'nonlocal correlation' as a formal property of observed statistics (Bell-nonlocality) and looser uses meaning merely 'entanglement' or 'quantum correlation'—entanglement does not always imply Bell nonlocality without specific measurements.

Synthesis

Nonlocal correlation is the experimentally accessible statistical signature that measurement outcomes across separated quantum systems exhibit dependencies incompatible with any classical local-hidden-variable description, typically certified by Bell-type tests and enabling device-independent quantum information tasks.