(Nature) Nature’s “News & Views” summarizes the efforts of Yuan Yin and his team demonstrating that cryptographic solutions can be deployed over distances exceeding 1,000 kilometres via satellite, without compromising the security promised by the underlying quantum technology.
A key way to understand their result is to observe how it was achieved, starting from previous work by some of the current authors and their colleagues in 2017. In that paper, the researchers demonstrated the distribution of entangled states generated on board the satellite Micius and sent through two communication links to optical ground stations in China separated by 1,200 km. Although that work was a milestone for the field, the transmission efficiency achieved was too low for QKD to be carried out in practical conditions.
Yin et al. remedied this problem by implementing major technological enhancements. These included installing highly efficient telescopes at the ground stations and optimizing equipment components at all stages of the optical path. The authors’ meticulous optimization also involved cutting-edge signal acquisition, pointing and tracking systems and synchronization techniques for both the satellite and the ground stations. Their efforts led to a fourfold increase in transmission efficiency compared with the previous experiment and, consequently, produced low enough error rates for a secret key to be extracted. The authors also verified the stability and reliability of their findings over multiple satellite orbits.
several shortcomings will need to be overcome for these findings to become relevant for truly practical high-security applications. For instance, the experiment produced keys at extremely low rates. Also, the experiment was carried out only at night, and using a wavelength that is incompatible with the optical-fibre networks used for telecommunication that would interface with space-based networks in infrastructures for global quantum communication. Moreover, QKD can be achieved only between ground stations that are visible simultaneously from the satellite.

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