Superadditivity of private information for any number of uses of the channel

TL;DR

This paper demonstrates that for certain quantum channels, private and coherent information can remain superadditive over unlimited uses, highlighting the need for infinite optimization to accurately assess their capacity for secure communication.

Contribution

The authors construct channels with strictly superadditive private and coherent information over unbounded uses, revealing new insights into quantum capacity and private information behavior.

Findings

Private and coherent information can be strictly superadditive for unbounded channel uses.

Quantum capacity is bounded by private capacity, but non-regularized quantities can be interleaved.

Infinite optimization over channel uses is necessary to evaluate secure communication potential.

Abstract

The quantum capacity of a quantum channel is always smaller than the capacity of the channel for private communication. However, both quantities are given by the infinite regularization of respectively the coherent and the private information. Here, we construct a family of channels for which the private and coherent information can remain strictly superadditive for unbounded number of uses. We prove this by showing that the coherent information is strictly larger than the private information of a smaller number of uses of the channel. This implies that even though the quantum capacity is upper bounded by the private capacity, the non-regularized quantities can be interleaved. From an operational point of view, the private capacity can be used for gauging the practical value of quantum channels for secure communication and, consequently, for key distribution. We thus show that in order to evaluate the interest a channel for this task it is necessary to optimize the private information over an unlimited number of uses of the channel.