Quantum Nonlocality

The problem of quantum nonlocality emerged for the first time in the famous Einstein-Podolsky-Rosen argument against the orthodox (Copenhagen) interpretation of quantum mechanics. The EPR argument employs an entangled state of two particles in which the position and momentum of one particle are correlated respectively with the position and momentum of the other particle regardless of their spatial separation. Under the assumption that quantum mechanics is complete it follows that a measurement of a selected quantity on one particle instantaneously (superluminally) changes the state of the other particle, and this constitutes a standard example of nonlocal quantum correlations. The notion of quantum nonlocality plays an important role in Bell’s theorem, where the joint assumption of locality and realism of properties is shown to have a consequence violating quantum-mechanical predictions. It is common to distinguish two types of quantum nonlocality: outcome dependence and parameter dependence. Only the former is believed to be necessary to account for the predictions of standard quantum mechanics. Modern discussions of quantum nonlocality often rely on the general framework of modal logic, and in particular the logic of counterfactual conditionals.

The EPR argument in its standard version is laid out in the classic article Einstein et al 1935 . Bell’s theorem and its philosophical consequences are discussed in the seminal papers on the foundation of quantum mechanics collected in Bell 1987. The distinction between the two main types of quantum nonlocality is made in Jarrett 1984.