China launches the first quantum communications satellite – and what is that, exactly?

Congratulations are in order for China: by launching the world’s first quantum communications satellite, the country has achieved an interesting — if somewhat difficult to explain — milestone in space and cryptography.

Quantum Experiments at Space Scale (QUESS), nicknamed Micius after the philosopher, lifted off from Jiuquan Satellite Launch Center at 1:40 AM local time (late yesterday in the U.S.) and is currently maneuvering itself into a sun-synchronous orbit at 500 km.

So what’s in the package that’s so exciting?

QUESS is an experiment in the deployment of quantum cryptography — specifically, a prototype that will test whether it’s possible to perform this delicate science from space. I’ll attempt to explain, but bear in mind that hardly anyone on the planet truly understands quantum physics, and some of them are probably bluffing. So this is just the basics — and feel free to correct me if I’m wrong, professor.

Inside QUESS is a crystal that can be stimulated into producing two photons that are “entangled” at a subatomic, quantum level. Entangled photons have certain aspects — polarization, for example — that are the same for both regardless of distance; if one changes, the other changes. The how and the why are beyond our pay grade here, so just take entanglement as a given.

The trouble is that photons are rather finicky things, and tend to be bounced, absorbed, and otherwise interfered with when traveling through fibers, air, and so on. QUESS will test whether sending them through space is easier, and whether one of a pair of entangled photons can be successfully sent to the surface while the other remains aboard the satellite.

If this is possible, the entangled photons can be manipulated in order to send information; the satellite could, for example, send binary code by inverting its photon’s polarization, one way for 1, the other way for 0. The ground station would see its photon switching back and forth and record the resulting data.

The critical thing about this is that there is no transmission involved, or at least not one we understand and can intercept. Whatever links the two photons is intangible and undetectable — you can’t entangle a third one to listen in, and if even if you managed to interfere with the process, it would be immediately noticed by both sides of the process, which would see unexpected changes to the photons’ states.

As you can imagine, an undetectable and perfectly secure channel for digital communications is of enormous potential value for an endless list of reasons. China is early to the game with QUESS, but they’re not the only ones playing. Other quantum satellites, though none quite so advanced, are in the ether right now, and more are sure to come. The experiments from the whole set will definitely be interesting — if anyone can find a way to explain what’s going on in them.

Featured Image: Chinese Academy of Sciences

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