Chinese quantum satellite sends pairs of entangled photons to ground

Quantum entanglement makes faster-than-light communication a possibility

For the first time, 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 be still entangled after travelling long distances.

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 academician 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, and was published as a cover article in the latest issue of the academic journal Science, state-run Xinhua news agency reported today.

This experiment was made through two satellite-to-ground downlinks with a total length varying from 1,600 to 2,400-km.

The obtained link-efficiency is many times higher than that of the direct bi-directional transmission of the two photons through telecommunication fibres, said Pan, who is also the lead scientist of QUESS.

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.

Previously, entanglement distribution had only been achieved at a distance up to 100-km due to photon loss in optical fibres or terrestrial free space.

One way to improve the distribution lies in the protocol of quantum repeaters, whose practical usefulness, however, is hindered by the challenges of quantum storage and readout efficiency, Pan said.

Another approach is making use of satellite-based and space-based technologies, as a satellite can conveniently cover two distant locations on Earth. The main advantage of this approach is that most of the photons' transmission path is almost in a vacuum, with almost zero absorption and de- coherence, Pan said.

After feasibility studies, Chinese scientists developed and launched QUESS for the mission of entanglement distribution. Cooperating with QUESS are three ground stations: Delingha Observatory in Qinghai, Nanshan Observatory in Xinjiang and Gaomeigu Observatory in Yunan.

For instance, one photon of an entangled pair was beamed to Delingha and the other to Gaomeigu. The distance between the two ground stations is 1,203-km. The distance between the orbiting satellite and the ground stations varies from 500 to 2,000 kilometres, said Pan.

Due to the fact that the entangled photons cannot be amplified as classical signals, new methods must be developed to reduce the link attenuation in the satellite-to-ground entanglement distribution. To optimise the link-efficiency, Chinese scientists combined a narrow beam divergence with a high-bandwidth and a high-precision acquiring, pointing, and tracking (APT) technique.

By developing an ultra-bright space-borne two-photon entanglement source and the high-precision APT technology, the team established entanglement between two single photons separated by 1,203-km, the Xinhua report said.

Compared with the previous methods of entanglement distribution by direct transmission of the same two-photon source -- using the best performance and most common commercial telecommunication fibres respectively -- the effective link-efficiency of the satellite-based approach is 12 and 17 orders of magnitude higher, Pan said.

He said the distributed entangled photons are readily useful for entanglement-based quantum key distribution, which, so far, is the only way to establish secure keys between two distant locations on Earth without relying on a trustful relay, the report said.