Minimum error discrimination is a general approach for quantum-state discrimination. This work establishes a connection between quantum imaginarity and minimum error discrimination, investigating the conditions under which auxiliary quantum states can enhance the maximum average success probability (MASP) with real positive-operator-valued measures (POVMs) or even achieve the MASP attainable with general POVMs. We prove that an auxiliary quantum state is nonreal if and only if it can enhance the MASP attainable with real POVMs in the composite system for at least one discrimination task. In particular, for pure states, only auxiliary maximally imaginary states can achieve the unrestricted MASP for all discrimination tasks. For a fixed ensemble, the assisted advantage is a convex, strongly monotonic imaginarity quantifier, but it need not be faithful. We characterize the ensembles for which it is faithful through a one-parameter family of discrimination problems and derive a spectral criterion for binary ensembles. These results identify when assisted discrimination efficiency defines a faithful imaginarity measure and clarify the operational role of imaginarity under real measurement constraints.

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• Quantum communication, protocols & technology