The superadditivity effects of quantum capacity decrease with the dimension for qudit depolarizing channels

TL;DR

This paper investigates how superadditivity effects in quantum capacity diminish as the system dimension increases for qudit depolarizing channels, showing that in high dimensions, coherent information approximates the channel capacity.

Contribution

It proves that superadditivity effects decrease with dimension and that the capacity converges to coherent information as dimension grows large.

Findings

Superadditivity effects decrease with increasing system dimension.

Quantum capacity converges to coherent information as dimension approaches infinity.

High-dimensional qudits' capacity is essentially given by coherent information.

Abstract

Quantum channel capacity is a fundamental quantity in order to understand how good can quantum information be transmitted or corrected when subjected to noise. However, it is generally not known how to compute such quantities, since the quantum channel coherent information is not additive for all channels, implying that it must be maximized over an unbounded number of channel uses. This leads to the phenomenon known as superadditivity, which refers to the fact that the regularized coherent information of $n$ channel uses exceeds one-shot coherent information. In this article, we study how the gain in quantum capacity of qudit depolarizing channels relates to the dimension of the systems considered. We make use of an argument based on the no-cloning bound in order to proof that the possible superadditive effects decrease as a function of the dimension for such family of channels. In addition, we prove that the capacity of the qudit depolarizing channel coincides with the coherent information when $d\rightarrow\infty$. We also discuss the private classical capacity and obain similar results. We conclude that when high dimensional qudits experiencing depolarizing noise are considered, the coherent information of the channel is not only an achievable rate but essentially the maximum possible rate for any quantum block code.