BEIJING, June 16 (Xinhua) -- A team of researchers has demonstrated secure quantum communication between two ground stations, with a record distance of 1,120 km and no intermediate security relays. This is seen as a new step forward toward the practical application of quantum communication.

Quantum key distribution (QKD) is taken as a secure way to allow the exchange of encrypted messages between remote users.

In previous studies, scientists have demonstrated QKD in a laboratory over optical fiber, and between a satellite and a ground station. But QKD between two users on the ground needs trusted relays, or quantum repeaters, to avoid signal loss and extend the distance, which poses security risks.

In the satellite-ground QKD test, for instance, a quantum key was generated, distributed by China's quantum satellite from Quantum Experiments at Space Scale. The satellite, nicknamed Micius, had all the information of the quantum key. If Micius is hacked, there is a risk of information leakage.

In the latest study published online by the journal Nature, the satellite Micius sent pairs of entangled photons to two ground stations 1,120 km apart; in Delingha in Qinghai Province, and Nanshan near Urumqi in Xinjiang Uygur Autonomous Region.

A secured quantum link between the two ground stations was established with no trusted relays, and the satellite Micius had no information about the secure key.

Pan Jianwei, renowned quantum physicist and corresponding author of the research, said that the study increases the secure distance of QKD without trusted relays on the ground tenfold, and ensured secure communication, even when the satellite is compromised. This is a significant improvement in the practical application of quantum secure communication.

Entanglement is a peculiar phenomenon that happens in quantum mechanics. If two particles are entangled, their properties are in a single quantum state, so that once their properties are measured, they remain correlated, no matter how far apart they are.