Summary |
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. |