Reachability Constraints and Monotones for Quantum-State Texture

Abstract

How a quantum resource can be quantified depends on which operations are free. Yet two proposed quantifiers of quantum state texture can increase under free operations: the Wigner-Yanase-Dyson skew information relative to the unique textureless state and the squared Hilbert-Schmidt distance from that state. We show reachability from every state orthogonal to the textureless state, whether pure or mixed: each can be freely transformed into any other state. Therefore, every texture monotone must assign the same maximal value to all such states, a requirement that neither candidate satisfies. We then construct valid texture monotones by adding the population outside the textureless state to the skew information and by maximizing the squared Hilbert-Schmidt distance over free operations. We prove faithfulness, convexity, and invariance under textureless ancillas, characterize additivity properties and strong monotonicity, satisfied only in the Wigner-Yanase case, and derive tight bounds for both monotones at fixed overlap with the textureless state and characterize all saturating states. Together, these results show how allowed state transformations impose operational constraints on the quantification of quantum state texture.