Quantum nonlocality

Quantum nonlocality is the incompatibility of certain quantum correlations with local hidden-variable explanations, demonstrated by violations of Bell inequalities.

Bell inequality: A constraint on correlations that any theory satisfying specified locality and realism assumptions must obey. Its violation is the operational test for quantum nonlocality.
Local hidden-variable theory: A model in which outcomes are determined by shared variables and each measurement depends only on nearby causes. This is the class of explanations that Bell inequalities constrain.
Aspect experiment: A series of 1980s photon experiments by Alain Aspect and colleagues that tested Bell inequalities with rapidly changed settings. It provided a landmark test of Bell nonlocality using changing analyzer settings.
Quantum non-signaling: The constraint that choices of measurement at one location cannot control outcome statistics at a distant location. Bell nonlocality does not permit controllable faster-than-light communication.
Quantum cryptography: Cryptographic methods that use quantum states or measurements to secure communication. Bell correlations can certify secrecy against broad classes of adversarial strategies.
Bell's theorem: A theorem showing that quantum predictions cannot be reproduced by theories satisfying Bell's assumptions. It establishes why observed correlations challenge local hidden-variable explanations.
Locality (physics): The principle that physical influences do not propagate faster than light between spatially separated events. Bell's locality condition formalizes the causal separation at issue.
Loophole-free Bell test: A Bell experiment designed to close major detection and locality loopholes simultaneously. The 2015 experiments strengthened the empirical case by addressing key escape routes.
Quantum entanglement swapping: A procedure that entangles two systems that have never directly interacted through a joint measurement on their partners. It produces nonlocal correlations between distant systems without direct interaction.
Device-independent quantum cryptography: Cryptography whose security can be established from observed Bell violations without trusting device internals. Nonlocal correlations provide security certification with minimal hardware assumptions.

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