Many quantum researchers are working toward building technologies that allow for the existence of a global quantum internet, in which any two users on Earth would be able to conduct large-scale quantum computing and communicate securely with the help of quantum entanglement. Although this requires many more technological advancements, a team of researchers at Shanghai Jiao Tong University in China have managed to merge two independent networks, bringing the world a bit closer to realizing a quantum internet.
Challenges in merging quantum networks
A true global quantum internet will require interconnectivity between many networks, and this has proven to be a much more difficult task for quantum networks than it is for classical networks. While researchers have demonstrated the ability to connect quantum computers within the same network, multi-user fusion remains a major challenge. Fully connected networks using dense wavelength division multiplexing (DWDM) have been achieved, but have scalability and complexity issues.
However, the research team involved in the new study, published in Nature Photonics, has merged two independent networks with 18 different users. All 18 users can communicate securely using entanglement-based protocols using this method. This represents the most complex multi-user quantum network to date.
Multi-user entanglement swapping
To achieve this two system fusion, the researchers used a method called “multi-user entanglement swapping.” This begins with two separate 10-node networks. Initially, the nodes within each network are already entangled with each other, but not with the other network. Then, one node from each network is used to fuse together the two networks, effectively removing them from the set and leaving the other 18 nodes connected to each other.
Bell state measurements are used to entangle—or link together—the two networks, although the measurement itself, in quantum mechanics, forces the two measured photons out by “collapsing” their wave functions. This is why only 18 of the 20 nodes are usable after entanglement swapping.
“Ultimately, the two 10-user fully connected networks are merged into an 18-user network in the quantum correlation layer after the multi-user entanglement swapping. All the end users can communicate with each other using the entanglement-based quantum key distribution protocol,” the study authors write.
The team also developed an active temporal and wavelength multiplexing (ATWM) scheme to conduct their experiment, instead of DWDM, as in previous studies. They verified entanglement quality with two-photon interference and fidelity measurements. Their measurements showed that they achieved high-quality entanglement with fidelities above 84% and interference visibilities above 75% (up to 90.7%), compared to 50% in classical systems.
More hurdles to overcome
Although fusing independent networks is a big achievement in quantum communications research, scaling to even larger or longer-distance networks will require further advances. One of the big hurdles is quantum repeaters, which allow signals to travel farther without losing crucial photons and are needed for larger scale network fusing.
The study authors write, “The most critical challenge in realizing practical long-distance quantum repeater networks is the establishment of robust entanglement between remote quantum memory nodes. Although great progress has been made in quantum memory in recent years, it is still challenging to realize large-scale and long-distance practical quantum communication networks using quantum repeaters with the capabilities of current technology.”
Still, the team is optimistic about their progress, saying, “Our approach opens attractive opportunities for the establishment of quantum entanglement between remote nodes in different networks, which facilitates versatile quantum information interconnects and has great application in constructing large-scale intercity quantum communication networks.”
Written for you by our author Krystal Kasal, edited by Gaby Clark, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive.
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More information:
Yiwen Huang et al, Quantum fusion of independent networks based on multi-user entanglement swapping, Nature Photonics (2025). DOI: 10.1038/s41566-025-01792-0
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Two independent quantum networks successfully fused into one (2025, November 5)
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