The Signal
Three Users Share a QKD Network Through a Passive Circulator Hub
Researchers demonstrated pairwise quantum key distribution among three users through a passive optical circulator hub. The fiber links spanned 20 to 25 kilometers, and all three pairs produced positive secret-key lengths using BB84.
Quantum.world Team · Written with AI · fact-checked ·
“All links achieved low QBERs and yielded positive secret key lengths.”
Youn-Chang Jeong et al., “Quantum key distribution over a three-user fiber network with passive optical circulator,” Scientific Reports
A passive hub connects Alice, Bob and Charlie
A three-user quantum key distribution network can exchange keys between any pair of users without an active optical switch or a trusted intermediary, according to research published in Scientific Reports on October 23, 2026. Youn-Chang Jeong and colleagues built the network around a central hub of three-port optical circulators. Its users, named Alice, Bob and Charlie, connect to the hub over standard telecommunications fiber.
Each user has an identical bidirectional transceiver. The design shares a single asymmetric interferometer between the node’s encoding and decoding paths, reducing duplicated optical components. Rather than routing traffic with an actively controlled switch, the passive circulator arrangement supports key exchange across each of the three user pairs.
Fiber spans reach 25 kilometers between users
Individual fiber links from a user to the central hub measured 10 to 15 kilometers, giving effective distances of 20 to 25 kilometers between users. That distinction matters: the longest reported user-to-user route is not a single 25-kilometer fiber from one user to another, but a path through the central node.
The team used BB84 with two decoy states—a weak pulse and a vacuum state—to generate keys. For each pair, it processed a sifted block of one million bits under a composable security parameter of approximately 10−8. The paper reports that all three links produced positive secret-key lengths.
Cascade produced longer keys than Winnow
The researchers compared estimated key lengths with results from two experimental classical post-processing methods, Winnow and Cascade. Both methods reconcile differences in the users’ sifted keys before privacy amplification reduces information that might have leaked. Cascade consistently yielded longer final keys, which the authors attribute to its higher reconciliation efficiency.
The comparison gives the result more substance than a demonstration of optical connectivity alone: the network produced final keys after post-processing on every pairwise link. But the paper’s reported result is bounded by a three-user system and effective spans of up to 25 kilometers. It does not establish how the architecture performs with a larger number of users or longer routes.
Resource sharing comes with an interferometer constraint
Sharing an interferometer cuts duplicated optical resources at each node, but the architecture is still a specific arrangement of transceivers and fiber links rather than a general-purpose network switch. The reported results show pairwise key generation without active switching at the hub; they do not demonstrate that adding users or increasing distance leaves performance unchanged.
That makes the study a test of a particular design trade-off: passive routing and shared node optics in a compact three-user network, assessed through both theoretical estimates and experimentally distilled keys. Its clearest measured outcome is that Alice, Bob and Charlie each established positive secret-key lengths with both of their counterparts, using fiber paths no longer than 25 kilometers between users.