Beijing, Jun 16: For the first time in history, China has successfully sent pairs of entangled photons from a satellite in orbit to three ground stations in the country, each separated by more than 1,200 km. In a major breakthrough that opens up prospects for practical quantum communications, the photon pairs were demonstrated to still be entangled after travelling long distances.
Quantum entanglement is a phenomenon in quantum physics, which is so confounding that Albert Einstein described it as "spooky action at a distance" in 1948. Scientists found that when two entangled particles are separated, one particle can somehow affect the action of the far-off twin instantly. Quantum physicists have a fundamental interest in distributing entangled particles over increasingly long distances and studying the behaviour of entanglement under extreme conditions.
This satellite-based technology opens up bright prospects for both practical quantum communications and fundamental quantum optics experiments at distances previously inaccessible on the ground, Pan Jianwei, an academic of the Chinese Academy of Sciences said.
The achievement was based on the world's first quantum satellite, Quantum Experiments at Space Scale (QUESS), also dubbed Micius, launched by China on August 16, 2016. This experiment was made through two satellite-to-ground links with a total length varying from 1,600 to 2,400 km.
Previously, entangled particle distribution had only been achieved at a distance up to 100 km due to photon loss in optical fibres or free space. By making use of satellite-based and space-based technologies, the team was able to send entangled photons to the three different ground stations: Delingha Observatory in Qinghai, Nanshan Observatory in Xinjiang and Gaomeigu Observatory in Yunan.
Compared with the previous methods of entanglement distribution by direct transmission -- using the best performance and most common commercial telecommunication fibres respectively -- the effective link efficiency of the satellite-based approach is between 12 and 17 orders of magnitude higher, Pan said.