A space based quantum internet - and why it will be needed

It hadn't sunk into my consciousness 'til now, the matter of why a quantum internet was being keenly pursued as a research topic; but this article explains it's because of quantum computing's potential to break all present, mathematically based, encryption.  It then goes onto explain the technical issues with creating a quantum internet, and paints a pretty convincing picture that it is going to be best based in space, in orbiting satelittes:

First some background. At the heart of any quantum network is the strange property of entanglement. This is the phenomenon in which two quantum particles share the same existence, even if they are separated by vast distances. It ensures that a measurement on one of these particles immediately influences the other, a marvel that Einstein called “spooky action at a distance.”

Physicists usually distribute entanglement using pairs of photons created at the same point and instant in time. When the photons are sent to different locations, the entanglement linking them can be exploited to send secure messages.

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. The photons interact with other atoms in the atmosphere or the glass, and the entanglement is destroyed. It turns out the maximum distance over which entanglement can be shared in this way is just a few hundred kilometers.

How then to build a quantum internet that shares entanglement across the globe? One option is to use “quantum repeaters”—devices that measure the quantum properties of photons as they arrive and then transfer these properties to new photons that are sent on their way. This preserves entanglement, allowing it to hop from one repeater to the next. However, this technology is highly experimental and several years from commercial exploitation.

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....

I won't extract the next few paragraphs, detailed the new paper's calculations as to how many satellites at what orbit would be needed, and cut to the conclusions:

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.

Even then, the 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. That’s a significant drawback.

Nevertheless, a space-based quantum internet significantly outperforms ground-based systems of quantum repeaters, say Khatri and co. Repeaters would have to be spaced at intervals of less than 200 kilometers, so covering long distances would require large numbers of them. This introduces its own set of limitations for a quantum internet. “We thus find that satellites offer a significant advantage over ground-based entanglement distribution,” say Khatri and co.

That's pretty fascinating, and it sounds like China is really interested in it.  Maybe Huawei will build it!