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February 14th, 2011, 10:15 AM  #1 
Joined: Feb 2011 Posts: 1  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? Thanks, James 
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April 21st, 2011, 12:52 PM  #2 
Joined: Apr 2011 Posts: 4  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 'superdense coding' and 'quantum teleportation' 

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