Conditions for equality between entanglement-assisted and unassisted classical capacities of a quantum channel

Abstract

Several relations between the Holevo capacity and the entanglement-assisted classical capacity of a quantum channel are proved, necessary and sufficient conditions for their coincidence are obtained. In particular, it is shown that these capacities coincide if (correspondingly, only if) the channel (correspondingly, the $\chi$--essential part of the channel) belongs to the class of classical-quantum channels (the $\chi$-essential part is a restriction of a channel obtained by discarding all states useless for transmission of classical information). The obtained conditions and their corollaries are generalized to channels with linear constraints. By using these conditions it is shown that the question of coincidence of the Holevo capacity and the entanglement-assisted classical capacity depends on the constraint (even for classical-quantum channels). Properties of the difference between the quantum mutual information and the $\chi$-function (constrained Holevo capacity) of a quantum channel are explored.

26 pages, published version including the result of the first part of arXiv:1202.3449 and the section devoted to the difference between the quantum mutual information and the $\chi$-function (constrained Holevo capacity) of a quantum channel

References in corpus (3)

• Remarks on the classical capacity of quantum channel
• Information capacity of quantum observable
• A criterion for coincidence of the entanglement-assisted classical capacity and the Holevo capacity of a quantum channel

Cited by in corpus (4)

• On the entanglement-assisted classical capacity of infinite-dimensional quantum channels
• A criterion for coincidence of the entanglement-assisted classical capacity and the Holevo capacity of a quantum channel
• Gaussian classical-quantum channels: gain of entanglement-assistance
• On equalities in two entropic inequalities