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This is for an application in cryptography. There is a concept called "time based cryptography", where a message can be decrypted only after a certain time, Say "12/12/2060, 12:30 GMT". There are some ideas based on "proof-of-work" but they have many problems. Is it possible to use quantum mechanics to do this in a nicer way?

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Quite a fascinating question. I fail to see why the downvotes. @Jus12: Maybe if you give some more background explanation (and links) in your question to show that the question is well motivated and not random, readers might take it more seriously. –  Siva May 17 '13 at 8:06
    
It took some thinking and researching but I understand the motivation behind his question also. It's a good question. This question crypto.stackexchange.com/questions/606/… gives more background on Time-based crypto. The only thing I can think of in QM that is 'predictably' time-dependent is radioactive decay... –  John May 17 '13 at 17:38
    
Maybe consider asking this question on the cryptography stack exchange... –  John May 17 '13 at 19:19
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The only related idea I'm aware of is position based quantum key distribution, which means that you can only distribute keys to a partner located in a specific place. But they only work as long as eavesdroppers have limited resources. You can find more information in "Position-Based Quantum Cryptography: Impossibility and Constructions" (http://arxiv.org/abs/1009.2490)

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Presumably the time coordinate could be analogous to a position coordinate. –  Siva May 17 '13 at 8:04
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Well, this should be feasible.

You generate entanglate EPR pairs of qubit. For each pair you keep the qubit A, you send the other qubit B on a spaceship one light-year away. You have to prepare that in advance because it will take more than one year to reach this place.

Today you mesure your qubits and XOR your message with the result to encode it. The spaceship was programmed to mesure its qubit the same day, and send back the measurement to earth. Everyone will be able to decode the message, but only in a year.

Of course, you have to know the spaceship was not compromised before reaching its destination.

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Of course, you may send a classical one time pad, that would work the same way. –  MatthieuW May 17 '13 at 16:00
    
This scheme can beat one-time pad in a certain sense. Using a quantum locking protocol, one can achieve the same security as a one-time pad, but the key can be much, much smaller. I still don't envision anybody sending anything a lightyear away though... –  Dan Stahlke May 18 '13 at 15:54
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