China has sent a quantum-based wireless message that it says cannot be intercepted without detection. It used satellites to overcome distance restrictions that would apply on the ground.

The signal included an encryption key made up entangled photons: in effect light particles which are dependent on one another. Trying to read any of the signal before it is received at its intended location would change the properties of a photon, in turn wrecking the entire sequence.

The technology is already used for terrestrial communications via fibre optic cables but with a big practical limit. Trying to send it over a cable longer than around 125 miles causes the signal to degrade too much. However, using relay stations (which need to decrypt and reencrypt the signal because of the quantum nature) opens up the risk that the station has been compromised and could be intercepted in an unencrypted state.

The Chinese experiment involved sending laser signals via space satellites, which both reduces the problem of signals degrading, and which eliminates the problem of the relay point being physically compromised. The set up meant the signals could be sent from one point in China to two others which were 400 and 745 miles away respectively.

The system is described as having challenges to overcome but China is hoping to use it for carrying confidential financial and defense-related data.