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Theory of Computation Theoretical Computer Science - Automata Theory, Computability Theory, and Computational Complexity Theory


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February 14th, 2011, 10:15 AM   #1
 
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Quantum Entanglement - Reusing particles

For clarification do two quantum computers, communicating with entangled particles, collapse the state of a particle on one end to communicate a message to the other, such as a down spin state? If so, are these particles now useless for further communication due to the collapse?

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James
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April 21st, 2011, 12:52 PM   #2
 
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Re: Quantum Entanglement - Reusing particles

You cannot use an entangled state alone to communicate: if you could then this would allow faster than light communication.

The typical scenario one images is this: two particles are put into an entangled state, one is sent to Alice, one is sent to Bob. Now, quantum theory tells us that there will be non local effects: if Alice measures her particle, then it will effect what Bob gets. However, Alice only gets her results probabilistically, and Bob cannot tell whether or not Alice has even made a measurement. Thus, no communication will be possible (even though the particles 'agree' on what the results should be, apparently faster than light).

Once a measurement has been performed, the particle collapse so that they are no longer entangled, thus they cannot be reused.

- This is not as 'useless' as it sounds: this technique can be used in combination with classical communications to achieve transfer of more information than is possible in just the classical state: for instance 'super-dense coding' and 'quantum teleportation'
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