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Quantum technologies are systems that can compute data, sense their surrounding environment or perform other functions leveraging quantum mechanical effects. Connecting these technologies over long distances has so far proved challenging, as quantum information can easily become scrambled or destroyed following decoherence resulting from the systems' interactions with their surrounding environment.

Over the past few years, quantum physicists and engineers have been trying to devise effective techniques to reliably establish quantum networks, infrastructures that allow quantum information to travel between different devices.

The realization of these networks requires so-called quantum relays, intermediate stations that can forward and redistribute entangled states, extending the distances across which devices can communicate.

Researchers at the University of Science and Technology of China recently developed a new intermediate relay architecture that could be more scalable than other relays introduced in earlier studies. This architecture, outlined in a paper in Nature Â鶹ÒùÔºics, allowed them to realize secure quantum communications between devices that are up to 300 km apart.

"Our work was inspired by recent advances in measurement-device-independent quantum key distribution (MDI-QKD), particularly the mode-pairing scheme, which demonstrated the potential to break the linear rate–loss limit and significantly enhance the signal-to-noise ratio in quantum communication," Xiongfeng Ma, co-senior author of the paper, told Â鶹ÒùÔº.

"This raised a central question for us: could we extend this concept by introducing additional quantum nodes to further improve performance?"

While in principle quantum repeaters should be easy to scale up, most quantum communication systems proposed so far rely on quantum memories, data storage devices that have not yet reached the results that would enable their real-world deployment. As part of their study, Ma and his colleagues thus explored an alternative approach, which entails the use of a single-photon source as a quantum relay.

"Our main objective was to design a modular and scalable quantum network architecture that leverages single-photon sources together with interference-based measurements," said Yang Lu.

"This approach allows long-distance quantum communication without relying on quantum memories. The result is a five-node network with three untrusted intermediate nodes, demonstrating improved security, flexibility, and potential for real-world deployment."

The architecture envisioned by the researchers would only enable quantum communications over long distances if supported by a high-quality single-photon source. This is essentially a device that can reliably release one photon (i.e., light particle) at a time, ensuring the precise transfer of information across a network.

To realize their quantum network architecture, Ma and their colleagues used a state-of-the-art single-photon source that they had introduced in one of their earlier papers.

"Our architecture consists of five nodes," explained Ma. "These include two end-users (Alice and Bob) encoding quantum information onto coherent light pulses, a central single-photon source emitting photons that are split into two paths, and two measurement nodes where the single photons interfere with Alice's and Bob's pulses.

"The interference outcomes are detected by single-photon detectors, enabling quantum information transfer through post-selection."

The new quantum communications architecture introduced by this team of researchers has notable advantages over other solutions introduced in the past. Firstly, they found that their architecture enhances the signal-to-noise ratio, as photons travel shorter distances across the network, which improves its reliability.

"Further advantages of our architecture are its modularity and scalability, as it enables the seamless integration of additional nodes to expand the network," said Teng-Yun Chen.

"Moreover, intermediate relays need not be secure, greatly reducing deployment cost and complexity. In initial tests, we attained an interference visibility above 85% between single photons and coherent states, ensuring robust performance of the system."

With their single-photon source, Ma and their colleagues demonstrated high-quality interference between single photons and coherent states in a quantum communication system.

Notably, this is the first time that such an interference was successfully used to relay quantum information. This team's work could thus open new possibilities for the realization of long-distance quantum communication.

"With our five-node quantum relay network with three untrusted intermediate relays we achieved secure key distribution over 300 km of fiber," said Ma.

"We also successfully integrated a state-of-the-art quantum dot single-photon source, operating at 304.52 MHz with near-unity indistinguishability, which is essential for ensuring robust and scalable performance."

This recent study could contribute to a shift between quantum networks with single-node connections to multi-node networks. The devices used to realize the team's architecture are also compatible with existing fiber networks and could thus be easier to deploy in real-world settings.

In the future, the network architecture devised by the team could be improved further and used to demonstrate secure quantum communications across even longer distances. Currently, the researchers are trying to extend the communication distances attainable using their architecture beyond 1,000 km, by further improving interference visibility and reducing error rates.

"As part of our next studies, we plan to realize larger and more complex network topologies, such as multi-layer star configurations, by integrating additional single-photon sources and measurement nodes," added Chen.

"We will also address critical technical challenges, including spectral overlap, temporal synchronization, and phase noise compensation, through the development of advanced stabilization and synchronization schemes. These improvements will reduce clock-induced errors and enhance the robustness of long-distance quantum communication."

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