Decode-Forward Coding Optimises Quantum Relay Channel Communication Strategies.

The reliable transmission of information over long distances presents a fundamental challenge, and quantum communication offers a potential solution through the use of quantum repeaters. Yigal Ilin and Uzi Pereg, from the Technion – Israel Institute of Technology, along with their colleagues, investigate how these repeaters can best relay quantum information. Their work centres on a three-terminal model that explores the balance between direct transmission and repeater-assisted strategies, proposing a novel ‘decode-forward’ coding scheme to improve reliability. By analysing both scenarios with and without pre-shared entanglement, the researchers demonstrate a versatile framework that recovers existing results and offers new insights into the efficient design of future quantum networks and secure long-distance communication systems. This research represents a crucial step towards building practical quantum repeaters and realising the promise of secure, efficient, and reliable quantum communication.

Relaying quantum information is central to advancements in quantum communication and distributed quantum computing, enabling long-distance transmission and modular architectures. However, quantum information technology faces significant challenges due to the inherent fragility of quantum states, which are highly susceptible to environmental noise. Overcoming these challenges is crucial, as even small disturbances can rapidly destroy quantum information, necessitating methods to protect and preserve quantum states during both transmission and processing.

Reliable long-range quantum communication is severely limited by photon loss and environmental noise, whether transmitted through fiber-optic cables or free space. Quantum repeaters offer a solution, leveraging entanglement swapping and teleportation-based protocols to maintain coherence and extend the distance between communicating parties. Unlike classical repeaters which amplify signals, quantum repeaters rely on quantum error correction or entanglement-based techniques to overcome signal loss and decoherence.

Security concerns surrounding current quantum key distribution implementations stem from the need for trusted intermediate stations, but quantum repeaters promise to address this issue by operating entirely on the quantum level, eliminating the need for trust. Cooperation in quantum communication networks is a major focus of research, driven by advances in both experimental techniques and theoretical developments. Entanglement serves as a valuable resource for network communication, and in point-to-point communication, entanglement assistance between the transmitter and receiver can significantly boost throughput, even with noisy or unreliable entanglement resources.

In multihop networks, multiple rounds of communication take place, making synchronization crucial. Causality constraints in network settings often require the use of block Markov coding, where the transmitter sends a sequence of blocks, each encoding information related to both current and previous transmissions. The quantum communication literature broadly categorizes approaches into repeater-aided and repeaterless transmission.

The Pirandola–Laurenza–Ottaviani–Banchi bound sets the fundamental rate-distance limit for repeaterless communication. Repeater-aided transmission enables the distribution of quantum correlations across long distances by dividing the channel into shorter segments and placing intermediate relay terminals. Through entanglement swapping, quantum repeaters generate entanglement and maintain coherence without violating the no-cloning theorem.

Experimental demonstrations of both repeaterless and repeater-aided communication have been achieved. This research focuses on the transmission of quantum information over quantum relay channels, aiming to characterize the trade-offs between repeater-assisted and repeaterless communication scenarios. Specifically, the three-terminal quantum relay channel is studied, serving as a fundamental building block in cooperative communication and enabling entanglement establishment between network nodes.

The decode-forward strategy is considered, where the relay fully decodes the quantum message received from the sender, re-encodes it, and forwards it to the receiver. The analysis covers both the entanglement-assisted and unassisted cases, and the model permits entanglement between the sender and the relay, leading to improved achievable transmission rates. These findings represent a crucial advancement towards the development of secure, efficient, and reliable quantum networks, ultimately contributing to the practical realisation of quantum repeaters and enabling long-range quantum key distribution.