China’s quantum leap in space opens up prospects of practical quantum communications

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China has demonstrated it is possible to send entangled photons over great distances thereby opening up the possibility of practical quantum communications.

According to Chinese Academy of Sciences, researchers were able to send pairs of entangled photons from Quantum Experiments at Space Scale (QUESS), also dubbed Micius, quantum satellite in orbit to three ground stations in China separated by more than 1,200-km. The achievement opens up bright prospects for both practical quantum communications and fundamental quantum optics experiments at distances previously inaccessible on the ground.

scientists involved with the study revealed that the experiment was carried out using two satellite-to-ground downlinks with a total length varying from 1,600 to 2,400-km. Scientists working with QUESS revealed that they managed to obtain link-efficiency that is many times higher than that of the direct bi-directional transmission of the two photons through telecommunication fibres.

Quantum entanglement was described by Albert Einstein 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.

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, scientists with the latest study 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.

The QUESS was launched 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 kilometers, 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. 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.

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 trustful relay.

The study is published in journal Science.

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