Quantum nonlocality

In theoretical physics, quantum nonlocality most commonly refers to the phenomenon by which measurements made at a microscopic level contradict a collection of notions known as local realism that are regarded as intuitively true in classical mechanics. However, some quantum mechanical predictions of multi-system measurement statistics on entangled quantum states cannot be simulated by any local hidden variable theory. An explicit example is demonstrated by Bell's theorem, which has been verified by experiment.

Experiments have generally favoured quantum mechanics as a description of nature, over local hidden variable theories. Any physical theory that supersedes or replaces quantum theory must make similar experimental predictions and must therefore also be nonlocal in this sense; quantum nonlocality is a property of the universe that is independent of our description of nature.

Whilst quantum nonlocality improves the efficiency of various computational tasks, it does not allow for faster-than-light communication, and hence is compatible with special relativity. However, it prompts many of the foundational discussions concerning quantum theory.

In 1935, Einstein, Podolsky and Rosen published a thought experiment with which they hoped to expose the incompleteness of the Copenhagen interpretation of quantum mechanics in relation to the violation of local causality at the microscopic scale that it described. Afterwards, Einstein presented a variant of these ideas in a letter to Erwin Schrödinger, which is the version that is presented here. The state and notation used here are more modern, and akin to Bohm's take on EPR. The quantum state of the two particles prior to measurement can be written as

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