Complementarity, Quantum Entanglement, and Locality

Abstract

The current state of the debate about quantum mechanics is dominated by the arguments concerning the Einstein-Podolsky-Rosen (EPR) type experiments and “quantum entanglement,” the existence of a particular type of correlation between certain spatially separated quantum-mechanical events. This particular way of thinking about the famously strange character of quantum physics emerged in the wake of Einstein, Podolsky, and Rosen’s article, “Can Quantum-Mechanical Description of Physical Reality be Considered Complete?” (EPR 1935) and Bohr’s reply under the same title (Bohr 1935). While quantum entanglement and correlations, sometimes referred to as “the EPR correlations,” were de facto introduced by EPR or, in any event, implied by the EPR experiment (EPR did not consider the question of correlations as such), the shift of focus towards this problematic in the debates concerning quantum mechanics is largely due to subsequent developments. Most notable among them are Schrödinger’s “cat-paradox” paper (1935), which introduced the term entanglement [Verschränkung], David Bohm’s reformulation of the thought experiment proposed by EPR in their paper in terms of spin, John Bell’s and related theorems and special cases of quantum entanglement (which expressly deal with correlations), Alain Aspect’s experiments, and more recently quantum information theory and related experimental work, such as that of Anton Zeilinger and his group, as well as recent investigations of decoherence and related questions. Einstein’s arguments subsequent to EPR’s article refined the EPR argument by more sharply focusing it on the possible nonlocality of quantum mechanics (rather than on its incompleteness as a physical theory) and proposed an alternative between seeing quantum mechanics either as a complete but nonlocal or a local but incomplete theory.³⁰