(TechnologyReview) Sumeet Khatri and colleagues at Louisiana State University in Baton Rouge have studied the various ways a quantum internet could be built and say the most cost-effective approach is to create a constellation of quantum-enabled satellites capable of continuously broadcasting entangled photons to the ground.
At the heart of any quantum network is the strange property of entanglement. The problem is that entanglement is fragile and hard to preserve. Any small interaction between one of the photons and its environment breaks the link. Indeed, this is exactly what happens when physicists transmit entangled photons directly through the atmosphere or through optical fibers.
One option is to create the entangled pairs of photons in space and broadcast them to two different base stations on the ground. These base stations then become entangled, allowing them to swap messages with perfect secrecy.
A Chinese satellite called Micius showed for the first time that entanglement can indeed be shared in this way. It turns out that photons can travel much further in this scenario because only the last 20 kilometers or so of the journey is through the atmosphere.
Khatri and co say that a constellation of similar satellites is a much better way to create a global quantum internet. The key is that to communicate securely, two ground stations must be able to see the same satellite at the same time so that both can receive entangled photons from it.
Khatri and co suggest that the best compromise is a constellation of at least 400 satellites flying at an altitude of around 3,000 kilometers. By contrast, GPS operates with 24 satellites. Ghe maximum distance between base stations will be limited to about 7,500 kilometers. This means that such a system could support secure messaging between London and Mumbai, which are 7,200 km apart, but not between London and Houston, 7,800 km apart—or indeed between any cities that are farther apart.

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