Quantum key distribution promises communication that cannot be secretly read, but building a network out of it has always forced a choice. Most designs route keys through a hub that users must trust. A more ambitious architecture, the fully connected network, links each user directly with every other user, yet its complexity grows so fast that only tiny proof-of-concept versions have ever been built. A team led by the University of Science and Technology of China (USTC) now reports a fully connected network that scales to 200 users across 200 kilometers of fiber, using a single type of laser chip to sidestep the bottleneck that held earlier designs back. The work appeared in Nature Communications on August 3, 2026.
The security problem that motivated the project is familiar to anyone who has used a VPN: the person running the infrastructure could, in principle, look at what passes through. In quantum key distribution, or QKD, two parties exchange single photons and use the laws of physics, rather than computational difficulty, to guarantee that a third party cannot intercept the key without being detected. But when a network operator routes those photons through its own equipment, the operator’s trustworthiness becomes part of the security model.
Measurement-device-independent QKD, or MDI-QKD, removes that assumption. Both users send photons to a central station, which performs a Bell state measurement and announces the result. Because the central station only ever measures the photons and publicly reports the outcome, it cannot learn the key even if it is fully malicious. The catch has been scale: in a network with N users, a fully connected MDI configuration nominally requires precise frequency locking among a number of laser pairs that grows with the square of N. Previous demonstrations stopped at three nodes.
The USTC team’s solution is the soliton microcomb, a chip-scale device that generates hundreds of evenly spaced optical frequencies, or comb teeth, from a single laser. Each user needs only one seed laser, and every seed laser operates at the same wavelength, locked locally to a rubidium atomic transition. That eliminates the need to synchronize many different wavelengths across the network. Silicon photonic transmitter chips then carve the comb teeth into single-photon pulses and encode them in polarization states. Photons travel up to 100 kilometers on each leg of the link, meet at the untrusted central station, and interfere there in a Hong-Ou-Mandel measurement, the two-photon interference effect that lies at the heart of MDI-QKD.
The results show the approach works at scale. The team verified interference across 80 channel pairs spanning the comb, with an average visibility of 48.5 percent, close to the 50 percent theoretical maximum for this scheme. Across 200 kilometers of standard fiber, each user-to-user connection sustained roughly 62 bits per second on average, with fresh keys extracted every 16.7 minutes. The authors note the architecture could support thousands of users by combining the roughly 200 frequency channels with time-division multiplexing. Compared with earlier fully connected quantum networks, the user count is an order of magnitude larger and the key rate three orders of magnitude higher.
A 62 bit-per-second key rate is not a data channel; it is a key refresh mechanism, and the demonstration ran in a laboratory over spooled fiber with active polarization compensation. The number of channels is currently limited by the passband of the arrayed waveguide gratings used to demultiplex the comb, and the network is designed for metropolitan and intercity distances, not continental ones. The team also emphasizes that this is an enabling demonstration of the architecture, not a deployed service.
What makes the result more than a record is its direction. The same architecture, the authors argue, could be adapted into quantum repeater networks by using comb teeth as pumps to generate entangled photon pairs, with entanglement swapping performed by untrusted relays. If that path holds, the fully connected topology could move from theory papers into the practical infrastructure question of who, exactly, needs to be trusted when cities exchange quantum keys.
Source: Wang, F.-X., Zheng, S.-T., Huang, L., Zhang, G.-W. et al. “Microcomb-driven large-scale fully connected quantum network.” Nature Communications (2026). DOI: 10.1038/s41467-026-75658-6. Preprint: arXiv:2512.17318.