&Bullet; physics 14, p75
Researchers achieve secure “real” quantum communication over 428 km of glass fiber, the longest terrestrial distance outside of a laboratory environment.
The ability to securely transfer data is critical to today’s digital society. One promising cryptographic approach that uses the fundamental laws of quantum physics is known as twin-field quantum key distribution (twin-field QKD). A research team led by Jian-Wei Pan and Teng-Yun Chen from the University of Science and Technology of China and Xiang-Bin Wang from the Jinan Institute of Quantum Technology, China, has now demonstrated this technique over the longest distance on the Running in a real environment  . The team says the demonstration confirms the feasibility of the technique outside of the laboratory.
In twin-field QKD, two spatially separated parties (“Alice” and “Bob”) secretly code qubits into individual photons, which they send to each other via a third party, “Charlie”. Charlie performs a single photon interference protocol. This interference allows Charlie to infer whether Alice and Bob’s secret qubits are the same or different (00 or 11 versus 01 or 10), but Charlie cannot determine the absolute values, which makes the scheme hack-proof.
In their demonstration, the team conducted a field test using double-field QKD, in which the photons were sent over 428 km of commercial fiber between two users separated by a 300 km line of sight. (Alice was in Jinan, China, and Bob was in Qingdao, China. Charlie was about halfway between the two in Linyi.)
Chen says twin-field QKD was achieved along longer fibers; the previous record was over 500 km. But in this case the fiber was wound up in a lab so the physical separation of Alice and Bob was minimal. It’s time to stage a long-distance demonstration in the real world, says Chen.
Katherine Wright is assistant editor of physics.
- H. Liu et al., “Field test of the double-field quantum key distribution by sending-or-not-sending over 428 km”, Phys. Rev. Lett.126, 250502 (2021).