Why the quantum internet should be built in space - MIT Technology Review
And that raises an interesting question: How should scientists and engineers go about the task of building a quantum internet that spans the globe?
Today we get an answer thanks to the work of Sumeet Khatri and colleagues at Louisiana State University in Baton Rouge. This team has 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. In other words, the quantum internet should be space-based.
So another 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.
In 2017, 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, provided the satellite is high in the sky and not too close to the horizon.
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.
At what altitude should the satellites fly to provide coverage as broad as possible? And how many will be needed? “Since satellites are currently an expensive resource, we would like to have as few satellites as possible in the network while still maintaining complete and continuous coverage,” say Khatri and co.
To find out, the team modeled such a constellation. It turns out there are a number of important trade-offs to take into account. For example, fewer satellites can provide global coverage when they orbit at a high altitude. But higher altitudes lead to greater photon losses.
Also, satellites at lower altitudes can span only shorter distances between base stations, because both must be able to see the same satellite at the same time.
Given these limitations, 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.