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Considered a promising technology, quantum communication (QC) is significantly different from conventional communication. Specifically, traditional telecommunications are not technically bound to a region. Signals travel around the world in milliseconds, without users recognizing if the server is on one continent or another. Simply put, signal distribution and amplification are everywhere.

With QC, there is most often a source of entangled photon pairs. These photons are sent directly to two receivers. A sequence of such single photons serves as a key for subsequent encryption of the actual message, transferred on classical channels. This concept is called quantum key distribution (QKD). The source of the photons can be right beside one of the receivers, or far away. The distribution of such signals through optical fibers is limited to a few hundred kilometers at best as the photons get absorbed in the fiber. Amplifiers are not allowed.

This article reviews a recently proposed constellation where the source of entangled photons is on a satellite, sending entangled photons to two ground stations. This is a first step for long haul connections, limited to the free line of sight. For a truly global network, a chain of satellites and a technique of storing the quantum information—the so-called quantum memories—is necessary.

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