= Holevo capacity of a qubit depolarizing channel
{c}
For the retention parameter $p$, the <Holevo capacity> is
$$
\chi^*(\Lambda_p)=1-h_2\left(\frac{1+p}2\right).
$$
Every output has <Von Neumann entropy> at least the value on a pure input, $h_2((1+p)/2)$. Two equiprobable orthogonal pure inputs attain this minimum entropy individually and have maximally mixed average output, attaining the bound. <Additivity of Holevo capacity> makes the unassisted <classical capacity of a quantum channel> equal to this value. https://arxiv.org/abs/quant-ph/0204172[King's depolarizing-channel capacity paper] proves additivity with an arbitrary other channel.
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