Beijing–Shanghai QKD backbone
The longest deployed terrestrial QKD network. Four-city fibre trunk carrying decoy-state BB84 keys between Beijing, Jinan, Hefei, and Shanghai through 32 trusted-relay nodes, declared operational in late 2017 and integrated with the Micius satellite to span 4,600 km between ground stations.
What it is
The backbone connects four cities with dedicated 1550 nm dark fibre. Each link runs decoy-state BB84 between QuantumCTek transmit and receive modules; at every trusted relay the keys are decrypted to classical bits, re-used for one-time-pad onward encryption, and handed to the next QKD module. The trust model rests on the physical security of the 32 relay nodes — not on quantum-mechanical guarantees between the endpoints. The trunk was integrated with the Micius low-Earth-orbit satellite in 2018, extending the reach of the same trusted-node architecture to a 4,600 km ground-station span through space-to-ground QKD downlinks. Chen et al. 2021
Published papers
Every link on the backbone and its predecessor metro networks runs decoy-state BB84 over dark fibre with trusted relay at every node. The trust model is uniform throughout — the table records what each paper demonstrated and what it established for the deployments that followed.
| Year | Paper | Result | What it implies |
|---|---|---|---|
| 2009 | Chen et al., Opt. Express 17.6540 Chen et al. 2009 | 4-node Hefei field trial over 20 km of dark fibre; decoy-state BB84 with WDM co-existing traffic. | First Chinese multi-node QKD field demonstration — validated that decoy-BB84 survives a live metropolitan fibre plant and sets the hardware template for the backbone nodes. |
| 2010 | Chen et al., Opt. Express 18.27217 Chen et al. 2010 | 8-node Hefei metropolitan all-pass network plus a 67 km inter-city link to Wuhu; any-to-any QKD routing. | Demonstrated the any-to-any trusted-relay topology and optical-switch routing that the four-city trunk would later scale up. |
| 2021 | Chen et al., npj QI 7.134 Chen et al. 2021 | 46-node Hefei metropolitan QAN on commercial fibre; 31-month continuous operation. | Showed that a trusted-relay QKD mesh sustains production-grade uptime and is maintainable with standard telecom tooling — the metropolitan layer that the backbone trunk terminates into. |
| 2021 | Chen et al., Nature 589.214 Chen et al. 2021 | 2,032 km fibre trunk (Beijing–Jinan–Hefei–Shanghai, 32 trusted nodes, 135 links) integrated with Micius satellite to span 4,600 km; >150 users. | Formal peer-reviewed account of the full backbone. Establishes the scale, node count, per-link key rates (kbit/s class), and the trusted-node security model that bounds the end-to-end guarantee. |
| 2025 | Chen et al., npj QI 11.137 Chen et al. 2025 | Carrier-grade expansion to >10,000 km; 145 backbone nodes, 144 links, 20 metropolitan sub-networks, 17 provinces, 80 cities. | Documents the operational phase beyond the 2017 trunk — a national QKD infrastructure built on the same trusted-relay architecture, now with standardised carrier-grade maintenance and monitoring. |
Verified claims
- ~2,032 km total length, four cities, 32 trusted nodes, 135 links — reported by USTC and confirmed in the integrated 4,600 km Nature paper. Chen et al. 2021
- Decoy-state BB84 with polarisation coding, QuantumCTek hardware — the deployment uses prepare-and-measure QKD, not entanglement distribution. Chen et al. 2021
- Trusted-relay architecture, not end-to-end secure. Each relay must be physically secured; compromise of a single node compromises the keys passing through it.
- Operational user base. Reported users include the Industrial and Commercial Bank of China, the Bank of Communications, and government departments along the route — though throughput and uptime figures are not publicly audited.
- 4,600 km integrated span via Micius. The space-to-ground extension uses the Micius satellite as another trusted node, not an entanglement source for the trunk. Chen et al. 2021
Things to note
- Not entanglement-based. The 2017 Micius satellite-to-ground entanglement-distribution result (Yin et al., Science 356.1140) is a separate experiment and is distinct from the trunk. The trunk itself is decoy-state BB84 — single-photon-level coherent pulses, not Bell pairs.
- Not a "quantum internet". The backbone is a service for moving symmetric keys between endpoints; it does not transport quantum states or support distributed quantum-compute primitives.
- Trusted-node security model. The end-to-end security depends on every one of the 32 trusted nodes; the information-theoretic guarantee of point-to-point QKD applies per link, not across the relayed path.
- Throughput sits in the kbit/s range. Reported per-link key rates are in the kbit/s range over the longest hops, with ITS-style key buffering between sessions. The service is sized for symmetric-key replenishment rather than streaming.