QKD quantum secure communication over 1000 km

Chinese researchers have reported the exchange of a secret key for encrypting and decrypting messages over a distance of 1,120 kilometers.

Previous work has demonstrated Quantum Key Distribution (QKD) along up to 404 kilometers of coiled optical fiber in a laboratory, or from a satellite to a ground station up to 1,200 kilometers away. 

However, real-world application between two users has been limited to around 100 kilometers. Photon losses, which increase rapidly with distance, has limited the real-world application around 100 km. In order to extend this distance, repeaters are required but they must be trusted otherwise they introduce security risks.

The team circumvented the need for repeaters by using a satellite to establish a secure link between two ground stations on Earth, using entangled photons. Two telescopes, designed to receive such quantum signals, were built 1,120 kilometers apart in Delingha and Nanshan in China. Entangled photons produced by the Micius satellite are transmitted to the ground as the satellite passes over the stations. Although satellite-based entanglement distribution has been reported before, the authors have now increased their transmission efficiency and reduced error rates enough to use entanglement to transmit quantum keys.

They show that the system produces a secure channel that is resistant to attacks at a finite secret-key rate of 0.12 bits per second, without the need for trusted relays. 

The paper has been published in Nature.