2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/91903We give a direct proof of the additivity of the minimum output entropy of a particular quantum channel which breaks the multiplicativity conjecture. This yields additivity of the classical capacity of this channel, a result obtained by a different method by Matsumoto and Yura [quant-ph/0306009]. Our proof relies heavily upon certain concavity properties of the output entropy which are of independent interest.15 pages, 2 figuresQuantum Physicstext