  One of the fundamental concepts of quantum information theory is that of
entanglement purification; that is, the transformation of a partially entangled
state into a smaller-dimensional, more completely entangled state. Of
particular interest are protocols for entanglement purification (EPPs) that
alternate purely local operations with one- or two-way classical communication.
In the present work, we consider a more general, but simpler, class of
transformations, called separable superoperators. Since every EPP is a
separable superoperator, bounds on separable superoperators apply as well to
EPPs; we use this fact to give a new upper bound on the rate of EPPs on
Bell-diagonal states, and thus on the capacity of Bell-diagonal channels.
