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Abstract: Two quantum events, represented by positive operators effects, arecoexistent if they can occur as possible outcomes in a single measurementscheme. Equivalently, the corresponding effects are coexistent if and only ifthey are contained in the ranges of a single joint observable. Here we giveseveral equivalent characterizations of coexistent pairs of qubit effects. Wealso establish the equivalence between our results and those obtainedindependently by other authors. Our approach makes explicit use of theMinkowski space geometry inherent in the four-dimensional real vector space ofselfadjoint operators in a two-dimensional complex Hilbert space.

Author: Paul Busch, Heinz-J├╝rgen Schmidt


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