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Asked by underscoredash to Dave, David, Jack on 27 Jun 2013.
Question: Can two entagled partices be "un-tagled"? If yes, how and if no, why?
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Jack Miller answered on 27 Jun 2013:
Hi underscoredash,
It’s very easy to ‘untangle’ an entangled pair of particles — all you have to do is measure one of them! At that point, you know exactly the state of the other particle, and they’re no longer in this spooky quantum state of uncertainty. The whole point about entanglement is that you don’t know what state particle A is in, but you DO know that if A is in one state, B has to be in another state. If we measure A, we know instantly what state B is in — and they’re no longer an entangled pair, just two particles we know something about.
Hope that helps!
— Jack
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David Freeborn answered on 27 Jun 2013:
Hi underscoredash,
Yes, and quite easily!
The most obvious way to untangle the particles is to fire a photon at one to observe it. Say the the two particles could have spin up or down and are in an entangled state. As soon as we use the photon to “observe” one particle’s spin, it will “choose” one of the two spins, and the other particle will be forced to “choose” the other spin. Now both the particles have a well-defined spin, and they are no longer entangled!
In fact, it’s quite difficult to *keep* two particles in an entangled state! We need to keep the two entangled particles from interacting with anything else, or the entanglement could break. If you want the tangled particles to travel far apart, this becomes quite a challenge.
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