Why did China build a quantum satellite?
I’ve written before about the next generation of quantum computers and the new encryption which we’ll need to hide our secrets from them. So I’ve read a few stories about China’s new quantum-encryption satellite:
China's quantum satellite achieves 'spooky action' at record distance
Quantum entanglement-physics at its strangest-has moved out of this world and into space. In a study that shows China's…
This is a scientific achievement, but it’s not practically useful at all.
Note that quantum encryption, as used by the satellite, is a completely different field from the algorithms which your computer can run today in post-quantum encryption. The ends of a quantum encrypted conversation are using entangled particles to guarantee that they have an uninterrupted line of communication. If someone attempts to spy on the contents of the conversation or do a man-in-the-middle attack, the sender and receiver will not have matching entangled states and know that they’re being watched.
This makes quantum encryption foolproof for detecting eavesdropping — but as the linked article points out, previous ground tests have been limited to a little over a kilometer in distance. In a practical system we cannot have dedicated fiber optic cable between every two terminals, and we need routers and other network devices to relay messages. We also can’t expect both ends of the conversation to have special hardware that monitors particle’s quantum state. This type of connection might make sense for the Pentagon-White House or other short highly-classified line, but this covers only a few cases. I strongly disagree with the quote at the end of the article about a future internet being built with these systems.
The satellite-ground communication is an interesting test of what’s physically possible in quantum encryption, but do satellite connections often get man-in-the-middle attacks? Wouldn’t you see this sneaky satellite/plane? Their satellite is sending a radio beam from fixed locations inside China/Tibet to a constant orbit, not through an untrusted network or a situation where signals will come in unexpectedly. I assume that Mission Control already uses shared encryption keys, so the satellites know if a radioed command like hey, do a barrel roll comes from Mission Control, a ham radio operator, or a hostile nation. Maybe it adds an extra layer of difficulty for a foreign actor to control the satellites, making stealing the satellite communication key not enough to break communication.
tl;dr quantum encryption exists and works, but I’m not sure if anyone has a practical use for it, even this satellite
by the author.