Three-user quantum-key experiment withstands 59.6 dB system loss

Researchers at Nanjing University experimentally generated a shared secure key for three users under total system loss of approximately 59.6 dB, compared with approximately 21.5 dB in the group's earlier measurement-device-independent conferencing experiment. The laboratory result was published in Physical Review Letters. The protocol pairs single-photon detections recorded at different times instead of requiring simultaneous multiphoton events, improving how key rates scale as users and transmission losses increase. A shared measurement station can announce detections without being trusted. At the highest tested loss, however, the secure-key rate was approximately 4.470 × 10⁻⁹ bits per pulse, and practical intercity deployment remains future work.