China Launches The First Quantum Communications Satellite

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 the 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.

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Quantum key distribution is only used to produce and distribute a key, not to transmit any message data. This key can then be used with any chosen encryption algorithm to encrypt (and decrypt) a message, which can then be transmitted over a standard communication channel. The algorithm most commonly associated with QKD is the one-time pad, as it is provably secure when used with a secret, random key.^[1] In real-world situations, it is often also used with encryption using symmetric key algorithms like the Advanced Encryption Standard algorithm.

From Wikipedia

Jon, you're right, it can be copied; IMHO Xinhua is reaching with the statement:

"It is hence impossible to wiretap, intercept or crack the information transmitted through it."

though it could easily be corrected to

"It is hence impossible to wiretap, intercept or crack the information transmitted through it, without it being detected"

Which is certainly impressive, but not quite the same. For instance, if you were transmitting a private SSL cert key, its great to know that it was intercepted, but the damage is already done.

Quantum entanglement is like a camera's tamper alert but for particles; it doesn't stop someone from destroying it, it just lets you know that they did.