Quantum Relay Channels

TL;DR

This paper establishes bounds on the capacity of quantum relay channels using various coding strategies, generalizing classical results and providing new insights into quantum communication with relay assistance.

Contribution

It introduces three bounds for quantum relay channels based on partial decode-forward, measure-forward, and assist-forward strategies, extending classical results to the quantum domain.

Findings

Derived capacity bounds for quantum relay channels.

Generalized classical bounds to quantum communication scenarios.

Identified conditions for capacity determination in Hadamard relay channels.

Abstract

Communication over a fully quantum relay channel is considered. We establish three bounds based on different coding strategies, i.e., partial decode-forward, measure-forward, and assist-forward. Using the partial-decode forward strategy, the relay decodes part of the information, while the other part is decoded without the relay's help. The result by Savov et al. (2012) for a classical-quantum relay channel is obtained as a special case. Based on our partial-decode forward bound, the capacity is determined for Hadamard relay channels. In the measure-forward coding scheme, the relay performs a sequence of measurements and then sends a compressed representation of the measurement outcome to the destination receiver. The measure-forward strategy can be viewed as a generalization of the classical compress-forward bound. At last, we consider quantum relay channels with orthogonal receiver components. The assist-forward bound is based on a new approach, whereby the transmitter sends the message to the relay and simultaneously generates entanglement assistance between the relay and the destination receiver. Subsequently, the relay can transmit the message to the destination receiver with rate-limited entanglement assistance.