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?

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 — in fact, the satellite will test that at 1,200 km, which will set a new record. The how and the why are beyond our pay grade here, so just take entanglement as a given. And let’s not even get into the faster-than-light communication argument here.
The trouble with this tech 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 proves possible, the satellite will attempt quantum key distribution via these entangled photons. When measured, a photon will show its observers a random polarization state — but critically, entanglement means the other photon will always show the same random state. These correlated polarizations can be the basis of a cryptographic key known only to the observers. (Note: the explanation that was here before was incorrect and has been changed.)
The best thing about this is that apart from the original distribution of the photons, 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 the observers of the original entangled photons, which would cease to be perfectly correlated.
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.
