Abstract
A unitary superoperator that represents a quantum phase is introduced by means of a density-matrix formalism of quantum mechanics. In this formalism, a quantum phase can be treated without using any of the additional techniques that are indispensable for defining a unitary phase operator when conventional phase operator methods are used. Furthermore, a phase-space distribution function for number and phase variables is introduced by means of the eigenoperator of the phase superoperator, and the basic properties of the distribution function are investigated.

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