The problem as to when two noncommuting observables are considered to have the same value arises commonly, but shows a nontrivial difficulty. Here, an answer is given by establishing the notion of perfect correlations between noncommuting observables and applied to obtain a criterion for precise measurements of a given observable in a given state. © 2004 Elsevier B.V. All rights reserved.
Ozawa, M. (2005). Perfect correlations between noncommuting observables. Physics Letters, Section A: General, Atomic and Solid State Physics, 335(1), 11–19. https://doi.org/10.1016/j.physleta.2004.12.003