Future quantum communications networks will depend on the successful exchange of entangled states between distant locations. That’s not easy to do. Take photon-based schemes: Optical fibers are too lossy for reliable long-distance transport of single photons, and atmospheric obstacles hinder free-space transmission. An intriguing alternative is relaying signals via satellites, since the photons’ transit would take place largely in the near-vacuum of the upper atmosphere. Now Jian-Wei Pan at the University of Science and Technology of China and his colleagues have used a recently launched satellite to distribute polarization-entangled photons to ground stations more than 1200 km apart. The previous distance record, achieved over a free-space channel, was just over 100 km.
The researchers made use of the $100 million Micius satellite, which last August was inserted into a 500-km-altitude orbit. At about 1:30am each night, the satellite passes over ground stations in Delingha in northwest China and Lijiang in southwest China. A pump laser and crystal generate 5.9 million entangled photon pairs per second; one of each pair is sent toward Delingha, the other to Lijiang. The researchers report that the satellite and receivers at the two stations achieved link efficiencies that are 12 orders of magnitude as high as that of the highest-performance optical fibers. Pan and his team analyzed 1167 corresponding photon pairs that were received over 17 minutes and found that measurements of their polarization violated the Bell inequality, which demonstrates that entanglement was preserved.
Now that the researchers have demonstrated entanglement distribution, they plan to try using Micius for quantum teleportation and key distribution. Pan’s team will have competition—other groups are planning similar experiments with satellites and with an instrument on the International Space Station. (J. Yin et al., Science 356, 1140, 2017.)
