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Previous work on quantum correlations has focused attention on the influence of the correlation on observables. In this paper we introduce an observable-independent index of correlation based on the information content, or entropy, of that correlation. We analyze a pair of spins and verify that they are maximally correlated when the index of correlation is maximized. The index of correlation suggests that two optical-field modes are most strongly correlated when prepared in a two-mode squeezed state.
Barnett et al. (Fri,) studied this question.