Chinese researchers have successfully demonstrated satellite-based distribution of entangled photon pairs to two locations separated by 747.5 miles (1,203 km) on Earth, through two satellite-to-ground downlinks with a summed length varying from 994 to 1,491 miles (1,600-2,400 km).

The distribution of quantum entanglement, especially across vast distances, holds important implications for quantum teleportation and communication networks.

Yet, efforts to entangle quantum particles, essentially ‘linking’ them together over long distances, have been limited to about 60 miles (100 km) or fewer, mostly because the entanglement is lost as they are transmitted along optical fibers, or through open space on land.

One way to overcome this issue is to break the line of transmission into smaller segments and repeatedly swap, purify and store quantum information along the optical fiber.

Another approach to achieving global-scale quantum networks is making use of lasers and satellite-based technologies.

Using the Chinese quantum space satellite Micius, launched last year and equipped with a specialized quantum optical payload, the researchers demonstrate the latter feat.

Micius was used to communicate with three ground stations across China (Delingha in Qinghai; Nanshan in Urumqi, Xinjiang; and Gaomeigu Observatory in Lijiang, Yunnan), each up to 747.5 miles apart.

The separation between the satellite and these ground stations varied from 310 to 1,243 miles (500-2,000 km).

A laser beam on Micius was subjected to a beam splitter, which gave the beam two distinct polarized states.

One of the spilt beams was used for transmission of entangled photons, while the other was used for photon receipt.

In this way, entangled photons were received at the separate ground stations.

“By developing an ultrabright spaceborne two-photon entanglement source and high-precision acquiring, pointing, and tracking technology, we established entanglement between two single photons separated by 747.5 miles, with an average two-photon count rate of 1.1 Hz and state fidelity of 0.869,” the scientists said.

“Using the distributed entangled photons, we performed the Bell test at spacelike separation and without the locality and the freedom-of-choice loopholes.”

The results were published in the June 16, 2017 issue of the journal Science.

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Juan Yin et al. 2017. Satellite-based entanglement distribution over 1,200 kilometers. Science 356 (6343): 1140-1144; doi: 10.1126/science.aan3211