= Holevo quantity under a quantum channel
{c}
{title2=$\chi(\{p_x,\Lambda(\rho_x)\})\leq\chi(\{p_x,\rho_x\})$}
Attach an orthogonal classical label register to an ensemble. Its <quantum mutual information> with the quantum system equals the ensemble <Holevo quantity>. A local <quantum channel> changes the ensemble states but preserves their labels and probabilities. <Data processing for quantum mutual information> then proves that the output <Holevo quantity> cannot exceed the input value.
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