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Feb23-12, 04:48 PM
P: 887
Quote Quote by edguy99 View Post
Entanglement generally means if you measure one of the particles you will automatically know what the state of the other particle is. It does not imply "manipulating one particle we can at the same time change the other".
This is the wrong way to look at it. Entanglement doesn't say that the individual states of the two particles is connected. It says that there is a single state for the whole system which specifies both particles. If you measure a single particle, you learn something about the state of the system, and this can help you predict the results of a measurement on the other particle.

Suppose you have two spin 1/2 particles, and the system is in an eigenstate of 0 total spin. The state of particle 1 and the state of particle 2 cannot be treated as if they were separate, independent things. Rather, there is only a state of the system, which might be something like
1/sqrt(2) (|UD> + |DU>) or 1/sqrt(2) (|UD> - |DU>)
This state cannot be factored into two independent states.