AI-rewritten: This is a summary of an article from Phys.org, rewritten by AI (Qwen, running locally) to make it easier to read. The facts come from the original article – read it for the full story.

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Researchers at Shandong University have developed a new method to verify entanglement in quantum networks without needing to trust the detectors used for analysis. Entanglement is a phenomenon where distant particles share a single state, meaning their properties are linked regardless of distance. In this study, led by He Lu, the team successfully tested this approach on a network connecting four users. Their work was published in Physical Review Letters and addresses a major security concern: previous methods could be tricked if measurement devices were tampered with, leading to false positives about entanglement.

The team’s innovation involves using two specific properties of the same photon to perform verification simultaneously. They utilized time-bin encoding, which tracks when photons arrive, and polarization, which refers to the orientation of light waves. By combining these traits, they could confirm that six distinct connections between four users were genuinely entangled. Each link spanned 20 kilometers of optical fiber. Crucially, this process required no additional test photons and did not rely on trusted measurement equipment, solving a limitation of earlier techniques that needed extra resources to function correctly.

He Lu noted that while building the network was only part of the challenge, quantifying the actual amount of entanglement is equally vital for security. Their method establishes a lower bound on the shared entanglement using data from just one type of measurement. This efficiency makes the technology more practical for large-scale networks where complexity grows rapidly. The researchers plan to expand their work to include high-dimensional states and multipartite entanglement, which would allow multiple users to share a single common quantum state. They aim to integrate these scalable verification methods with broadband photon sources to support advanced future communication systems.