Nonlocality vs Quantum EntanglementIt is a fact that the outcomes of measurements performed on spatially separated (i.e. non-communicating) quantum systems sometimes exhibit correlations, which cannot be explained classically, in terms of information shared beforehand.
Such correlations, called non-local, are revealed by the violation of a suitable Bell inequality.
Another peculiarly nonclassical feature of quantum theory is the existence of quantum entanglement, i.e. the property possessed by composite quantum systems whose joint state cannot be written in product form (or, more generally, as a mixture of states in product form).
Even if nonlocality and entanglement are indeed intimately related, it is nowadays widely accepted that they are in fact two well distinct concepts:
- There exist entangled quantum states which behave locally in many aspects
- But quantum states that appear to be maximally non-local are generally not the maximally entangled ones.
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