Capacities of highly Markovian divisible quantum channels

Abstract: We analyze information transmission capacities of quantum channels acting on $d$-dimensional quantum systems that are highly Markovian divisible, i.e., channels of the form \begin{equation*} \Phi = \underbrace{\Psi\circ \Psi \circ \ldots \circ \Psi}{l \,\operatorname{times}} \end{equation*} with $l \geq \gamma d^2 \log d$ for some constant $\gamma=\gamma(\Psi)$ that depends on the spectral gap of the dividing channel $\Psi$. We prove that capacities of such channels are approximately strongly additive and can be efficiently approximated in terms of the structure of their peripheral spaces. Furthermore, the quantum and private classical capacities of such channels approximately coincide and approximately satisfy the strong converse property. We show that these approximate results become exact for the corresponding zero-error capacities when $l \geq d^2$. To prove these results, we show that for any channel $\Psi$, the classical, private classical, and quantum capacities of $\Psi\infty$, which is its so-called asymptotic part, satisfy the strong converse property and are strongly additive. In the zero-error case, we introduce the notion of the stabilized non-commutative confusability graph of a quantum channel and characterize its structure for any given channel.