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Information theory allows us to make quantitative statements about the strength and nature of the correlations between systems. Application of this theory to the quantized electromagnetic field reveals a special role for the two-mode squeezed states. The nonclassical properties of these states arise from the intermode correlations, and we apply information-theoretic methods to determine the strength of the correlation between specific pairs of observables. This analysis leads to the important general result that for any correlated pure state a given pair of single-system observables contains at most only half the information about the correlations. We discuss the implications of this result for the distinction between classical and quantum systems.
Barnett et al. (Mon,) studied this question.