2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/217100I demonstrate that, rather unexpectedly, there exist noisy quantum channels for which the optimal classical information transmission rate is achieved only by signaling alphabets consisting of nonorthogonal quantum states.5 pages, REVTeX, mild extension of results, much improved presentation, to appear in Physical Review LettersQuantum Physicstext