Thursday, 15 August 2019

How is China's 'Quantum Satellite' going to send information through entangled photons? - no-communication theorem



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



Answer



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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