Degradable channels, less noisy channels, and quantum statistical morphisms: an equivalence relation

Two partial orderings among communication channels, namely “being degradable into” and “being less noisy than,” are reconsidered in the light of recent results about statistical comparisons of quantum channels. Though our analysis covers at once both classical and quantum channels, we also provide a separate treatment of classical noisy channels and show how in this case an alternative self-contained proof can be constructed, with its own particular merits with respect to the general result.