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So I recently read a news article about how China is using Quantum entanglement to send information. Now I thought this was very strange because I know for a fact that you cannot use quantum entanglement to send information through the entangled pair of objects. (See No-communication theorem).

How are they using entanglement to send the information then?

News Article I read

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  • $\begingroup$ Not instantaneous communication but read this physics.stackexchange.com/q/170454 and this sciencealert.com/… $\endgroup$ – user108787 Aug 16 '16 at 23:00
  • $\begingroup$ Related: physics.stackexchange.com/q/274646/2451 $\endgroup$ – Qmechanic Aug 16 '16 at 23:08
  • $\begingroup$ "But a quantum communication satellite uses a crystal that produces a pair of entangled photons whose properties remain entwined even as one is transmitted over a large distance. Messages could be sent by manipulating these properties." I take this to mean sending information through entanglement. $\endgroup$ – user2705260 Aug 17 '16 at 0:08
  • $\begingroup$ Two experiments will be carried out, one is a BB84 like key distribution, another is the teleportation. If the long distance teleprotation can be achieved, then it can surely be used to transfer information. $\endgroup$ – XXDD Aug 17 '16 at 8:33
  • $\begingroup$ Sorry, I was wrong about the key distribution part. It's directly based on the measurement of the entangled pairs. $\endgroup$ – XXDD Aug 17 '16 at 9:13
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The linked Nature Article is a bit more precise here:

At the heart of their satellite is a crystal that produces pairs of entangled photons, whose properties remain entwined however far apart they are separated. The craft’s first task will be to fire the partners in these pairs to ground -stations in Beijing and Vienna, and use them to generate a secret key.
(...)
The team will also attempt to ‘teleport’ quantum states, using an entangled pair of photons alongside information transmitted by more conventional means to reconstruct the quantum state of a photon in a new location.

The first part, generating a secret key, does not involve any transfer of information. The key will be totally random. However since the photons are entangled, both parties will be able to generate the same, unique key and they will be able to detect, if some third-party has messed with their key-generation. This key can then be used to encrypte information, transferred through a classical connection.

The second part also needs a classical connection to transfer information in order to recreate the 'teleported' quantum state. Again the two entangled photons pretty much just create a 'common base'.

It looks like the New York Times article simplified a bit too much or even mixed two experiments/concepts. The Nature article seems totally reasonable to me.

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  • $\begingroup$ Calling it a quantum satellite is certainly mixing up concepts. $\endgroup$ – user108787 Aug 16 '16 at 23:05
  • $\begingroup$ I've asked a related question and linked this answer there. $\endgroup$ – uhoh Aug 24 '16 at 8:28

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