Beijing–Shanghai QKD backbone

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}

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.

Related entries

• Applications — QKD as one of several quantum-internet applications.
• Quantum repeaters — the entanglement-based architecture that would replace trusted nodes.
• Links — the 0.2 dB/km fibre regime that bounds reach per hop.
• Standards — ITU-T and ETSI QKD-network standards the deployment aligns with.
• EPB Quantum Network — the entanglement-based contrast.
• BT–Toshiba London QKD — another commercial prepare-and-measure deployment, at metro scale.