John Stewart Bell

John Stewart Bell was a physicist whose 1964 theorem showed that measurable correlations could distinguish quantum mechanics from local hidden-variable theories.

Bell's theorem: A theorem showing that no theory satisfying specified locality and realism assumptions reproduces all quantum predictions. Bell’s 1964 result turned his central insight into experimentally testable constraints.
Bell inequality: A mathematical bound on correlations predicted by theories satisfying specified assumptions such as locality and measurement independence. Bell derived inequalities that local hidden-variable models must obey.
Freedman–Clauser experiment: A 1972 photon-correlation experiment by Stuart Freedman and John Clauser that tested a Bell inequality. It was the first experiment to demonstrate a violation of a Bell inequality.
Alain Aspect: A French physicist whose experiments tested Bell inequalities using entangled photons. Aspect’s experiments strengthened empirical tests of Bell’s theoretical proposal.
Quantum information science: The study of information processing and communication using quantum systems. Bell-test research helped turn entanglement from a foundational puzzle into a usable resource.
Quantum entanglement: A physical relationship in which a joint quantum state cannot be described as independent states of its parts. Entangled systems produce the correlations Bell used to challenge local explanations.
CHSH inequality: A Bell inequality involving two measurement choices for each of two separated observers. This practical form of Bell’s result is widely used in experiments.
Aspect experiment: A series of 1981–1982 photon experiments by Alain Aspect and collaborators testing Bell inequalities. The experiments varied measurement settings during photon flights to address a locality concern.
John Clauser: An American physicist who performed early experiments testing Bell inequalities and later studied quantum foundations. Clauser helped bring Bell’s theorem from theory into experimental physics.
Quantum cryptography: The use of quantum properties to secure communication against eavesdropping. Bell correlations can support security protocols whose guarantees rely on observed experimental data.

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