Postselective quantum measurements and the Holevo boundary violation effect

The paper explores post-selective measurements that can be applied in cases where the USD measurement is not applicable. We construct a postselective observable, for which the Holevo value after the measurement is, if successful, is strictly higher than before. We present a class of ensembles for which this jump takes place, including two arbitrary non-commuting states. The role of relative max-entropy is also investigated and a new interpretation is proposed in the context of maximum confidence in distinguishing between ensemble states.