In the study "Coherent phase transfer for real-world twin-field quantum key distribution", published in Nature Communications, the Italian INRiM team demonstrated experimentally that coherent laser interferometry considerably improves the performances of long-distance quantum communication in real-world networks.  BAQIS Chief Scientist Zhiliang Yuan contributed to the design of the experimental setup.

Quantum Key Distribution (QKD) provides a solution to distributing cryptographic keys between remote parties over a fiber link, the secrecy of which is guaranteed by the laws of quantum mechanics thanks to the use of single photons (elementary light particles) as the information carrier. Its communication range is limited by fiber scattering loss. Twin-field quantum key distribution technique is currently seen as the most promising candidate to increase the communication distance. In a twin-field setup, both communicating parties transmit laser signals just to the middle station so that half of the fiber loss is avoided.

However, one of the main obstacles towards the realization of such long-haul link is the "fragility" of quantum signals. In this regard, the researchers have shown that, by combining synergic exploitation of coherent laser interferometry and quantum metrology, it is possible to improve the information contained in single photon states, allowing lower error rates and increasing the length of exchanged messages.

The experiment was conducted along a 200km span of the Italian Quantum Backbone, an 1800 km fiber infrastructure developed by INRiM. While unspectacular in terms of distance, the experiment has broken new ground by enabling coherent phase transfer over asymmetric fiber channels.  As shown in Fig. 1, the asymmetry between two fiber links (Alice-Charile and Bob-Charlie) is as high as 22 km.  In previous experiments, the length difference must be within meters. 

"Removal of the constraint in fiber length difference makes network planning and provisioning a much easier task. This advance brings twin-field quantum key distribution a step closer to real-world applications," said Prof. Yuan, co-author of the manuscript. 

Fig. 1 a. The testbed layout b. A sketch of the Italian Quantum Backbone c. The experimental setup.

Reference: Clivati, C., Meda, A., Donadello, S. et al. Coherent phase transfer for real-world twin-field quantum key distribution. Nat Commun 13, 157 (2022). https://doi.org/10.1038/s41467-021-27808-1