A team of Chinese scientists has realized the satellite-based distribution of entangled photon pairs over 1200 km. The photon pairs were demonstrated to be still entangled after travelling long distances and Bell's inequality was shown to be violated under strict Einstein locality conditions. 

This experiment was made through two satellite-to-ground downlinks with a summed length varying from 1600-2400 km. The obtained link efficiency is orders of magnitude higher than that of the direct bidirectional transmission of two photons through telecommunication fibers. 

Quantum communication scientists have a fundamental interest in distributing entangled particles over increasingly long distances and studying the behavior of entanglement under extreme conditions. So far, entanglement distribution has only been achieved at a distance up to ~100 km due to photon loss in optical fibers or terrestrial free space. 

One way to improve distribution lies in the protocol of quantum repeaters, whose practical usefulness, however, is hindered by the challenges of simultaneously realizing and integrating all key capabilities. 

Another approach makes use of satellite- 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 vacuum, with almost zero absorption and de-coherence. 

To prove the feasibility of satellite- and space-based distribution research, ground-based studies were done that demonstrated bidirectional distribution of entangled photon pairs through a two-link terrestrial free-space channel, over distances of 600 m, 13 km, and 102 km, with an ~80-dB effective channel loss. Quantum communications on moving platforms in a high-loss situation and under turbulent conditions were also tested. 

After these feasibility studies, a quantum science experiment satellite - Micius - was developed and launched from Jiuquan, China on August 16, 2016 with a mission of entanglement distribution. Cooperating with Micius are three ground stations (Delingha in Qinghai; Nanshan in Urumqi, Xinjiang; and Gaomeigu Observatory in Lijiang, Yunnan). The distance between Delingha and Lijiang (Nanshan) is 1203 km. The distance between the orbiting satellite and these ground stations varies from 500-2000 km. 

Due to the fact that the entangled photons cannot be amplified as classical signals, new methods must be developed to reduce link attenuation in satellite-to-ground entanglement distribution. To optimize link efficiency, the scientists combined narrow-beam divergence with a high-bandwidth and high-precision acquiring, pointing, and tracking (APT) technique. By developing an ultra-bright space-borne two-photon entanglement source and high-precision APT technology, the team established entanglement between two single photons separated by 1203 km, with an average two-photon count rate of 1.1 Hz and state fidelity of 0.869 ± 0.085. Using the distributed entangled photons, the scientists performed the Bell test at space-like separation and without locality and freedom-of-choice loopholes. 

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

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. Another immediate application is to exploit distributed entanglement to perform a variant of quantum teleportation protocol for remote preparation and control of quantum states. 

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.

The principle allows for “a secure communication channel that is unhackable,” said Jonathan Dowling, a physicist at Louisiana State University. “When the Chinese roll out this type of communications nationwide, which is their plan,” said Dowling, “then no matter how many NSA computers you string together, you are never going to be able to tap into their system.”