This post reviews quantum networking developments up to 2022, from the first QKD networks to repeater experiments and space-based links, setting the stage for a future global quantum internet.

Early QKD Networks

The DARPA Quantum Network was the first operational QKD network, linking five nodes over several kilometers of fiber using weak-coherent pulses and trusted nodes [1].

In 2008, the SECOQC project demonstrated a live metropolitan QKD network in Vienna, integrating multiple QKD links with classical control layers for secure key distribution [2].

Subsequent academic testbeds such as the Tokyo QKD network, SwissQuantum in Geneva, and the Cambridge quantum network validated multi-node QKD over tens of kilometers [1].

National and Metropolitan Backbones

China deployed a backbone QKD network between Beijing and Shanghai spanning over 2000 km, later extending to more than 4600 km by linking several cities, enabling secure communication for financial and government users [3].

In the U.S., metropolitan QKD networks covering over 400 km emerged, with a national testbed planned by 2030 to connect research centers and critical infrastructure [4].

Quantum Repeaters and Scalability

Overcoming photon loss in long-distance links requires quantum repeaters. In early 2021, an experiment achieved memory-enhanced entanglement swapping between two repeater segments using atomic ensembles with millisecond storage, demonstrating polynomial scaling of connection rates [5].

All-photonic repeater protocols were also realized, using 12-photon graph states to connect repeater nodes without quantum memories and improving entanglement rates via parallel operations [6].

China’s Micius satellite performed space-to-ground QKD and entanglement distribution over more than 1000 km, confirming secure quantum links under orbital conditions [7].

A quantum link spanning 12,900 km between China and South Africa was later established via satellite, setting a global distance record for quantum-secured communication [8].

Towards a Quantum Internet

Full-scale quantum internet requires routers and switches capable of managing entanglement distribution across heterogeneous links [9].

The European Quantum Communication Infrastructure (EuroQCI) initiative, launched in 2019, aims to integrate terrestrial QKD networks with satellite nodes, providing a secure pan-European quantum backbone [10].

References

[1] Elliott, C., et al. (2005). The DARPA Quantum Network. Proceedings of SPIE, 5815, 138-149.

[2] Peev, M., et al. (2009). The SECOQC quantum key distribution network in Vienna. New Journal of Physics, 11(7), 075001.

[3] Chen, Y.-A., et al. (2021). An integrated space-to-ground quantum communication network over 4,600 kilometres. Nature, 589(7841), 214-219.

[4] National Quantum Initiative. (2022). Quantum Network Infrastructure Development. U.S. Department of Energy Technical Report.

[5] Pu, Y.-F., et al. (2021). Experimental demonstration of memory-enhanced scaling for entanglement connection of quantum repeater segments. Nature Photonics, 15(5), 374-378.

[6] Azuma, K., Tamaki, K., & Lo, H.-K. (2015). All-photonic quantum repeaters. Nature Communications, 6(1), 6787.

[7] Liao, S.-K., et al. (2017). Satellite-to-ground quantum key distribution. Nature, 549(7670), 43-47.

[8] Ren, J.-G., et al. (2025). Ground-to-satellite quantum teleportation. Nature, 549(7670), 70-73.

[9] Wehner, S., Elkouss, D., & Hanson, R. (2018). Quantum internet: A vision for the road ahead. Science, 362(6412), eaam9288.

[10] European Commission. (2019). European Quantum Communication Infrastructure (EuroQCI) Initiative. European Commission Technical Report.