Noninvertibility as a requirement for creating a semigroup under convex combinations of channels

Abstract: We study the conditions under which a semigroup is obtained upon convex combinations of channels. In particular, we study the set of Pauli and generalized Pauli channels. We find that mixing only semigroups can never produce a semigroup. Counter-intuitively, we find that for a convex combination to yield a semigroup, most of the input channels have to be noninvertible.