Coherence and indistinguishability.
It has long been known that the interference produced by two light beams is related both to their mutual coherence and also to the intrinsic indistinguishability of the photon paths. With the help of a decomposition of the density operator it is shown that the degree of indistinguishability equals the degree of coherence. This provides the fundamental link between the wave and the particle descriptions.
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Mandel