Abstract
Any electromagnetic signal is representable, in the quantum mechanical description, by a suitable combination of photon states. We consider the question of which of the infinite number of possible combinations should correspond to a classical signal. The characteristic classical criterion we adopt is the indistinguishability of the radiation in two separate channels, whether it has been produced by independent sources or by a single source whose output is divided between the channels. For a quantum source a distinction is in general possible. We prove that the unique quantum state for which a distinction is not possible is the pure state characterized by Glauber as maximally coherent. The connection of this indistinguishability property with characteristic differences between classical and quantum measurements is emphasized.
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