This paper is concerned with the limits imposed by quantum mechanics on correlations between statistical sources. Typical examples are the correlations between the transmitter and receiver of a quantum communication channel, and between the measurement results of two detectors that make simultaneous measurements.rnIn the following section mutual information is identified as a natural figure of merit for the degree of correlation between two sources. While mutual information bounds are therefore emphasised in the paper, it should be noted that a number of the methods used to obtain these bounds are in fact applicable to all measures of correlation. In particular, some corresponding results for coincidence rates are given in sections 3-6.rnIn section 3, two basic types of quantum correlation are defined, corresponding to communication and joint-measurement contexts. The non-classical nature of such correlations with respect to mutual information is briefly discussed, via Bell-type inequalities in the joint-measurement context, and a semi-classical "quantum chessboard" example in the communication context.
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