Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: DoctorBeaver on 22/07/2007 20:31:52
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Can quantum entanglement be applied to photons? If so, I have a further question.
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Yes
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OK, thanks.
Right, I'm not quite sure what happens with entanglement, but I think I'm right in saying that if something happens to 1 particle of an entangled pair, the other somehow reflects that. So, if the polarisation of 1 was altered, would the other also change polarisation? Or what if the wavelengh of 1 photon of an entangled pair was altered or it was passed through a medium that slowed it down? Would the wavelength of the other 1 change or would it slow down also?
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OK, thanks.
Right, I'm not quite sure what happens with entanglement, but I think I'm right in saying that if something happens to 1 particle of an entangled pair, the other somehow reflects that. So, if the polarisation of 1 was altered, would the other also change polarisation? Or what if the wavelengh of 1 photon of an entangled pair was altered or it was passed through a medium that slowed it down? Would the wavelength of the other 1 change or would it slow down also?
AFAIK, entanglement with wavelenght hasn't found it (I can be wrong, however).
Edit: I intended: "has not been discovered yet".
Sorry for my english.
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I'm not an expert, especially on recent experiment, so someone else might have a better explanation. Naively, I don't see a problem with entangling states in frequency (energy), but there might be practical problems.
Entanglement works based on the idea that quantum measurements define the system, so if entanglement is set up so that state |A1> of particle A is always seen with state |B1> of particle B, and similarly with states |A2> and |B2>, you would be able to force photon B to take on state |B1> just by measuring photon A to be in state |A1>.
Right, I'm not quite sure what happens with entanglement, but I think I'm right in saying that if something happens to 1 particle of an entangled pair, the other somehow reflects that.
You have to be careful in what things can happen to particle A to change particle B. Entanglement lies in the correlations between measurements. If photon A is changed such that all its states |A1>,|A2> are changed in the same way, I'm pretty sure that particle B won't change since you haven't changed the correlations, just the values of state A.
So, if the polarisation of 1 was altered, would the other also change polarisation?
If the states I mentioned above correspond to polarization states, yes.
Or what if the wavelengh of 1 photon of an entangled pair was altered or it was passed through a medium that slowed it down? Would the wavelength of the other 1 change or would it slow down also?
I don't think just uniformly slowing down A would effect B, since entanglement has to do with relations between measurements, but...
If state |Afast> was a fast frequency, and always correlated with measuring |Bfast>, and state |Aslow> was a slower frequency, and always correlated with |Bslow>, and you set up a system that would force particle A to pick state |Aslow> (that is, to only allow slow frequencies through), then particle B would have to "slow down" its frequency too, since they're correlated.
But as you can see, if you just set up a system that said "slown down particle A, no matter what state it's in," then you wouldn't change the state of particle B.
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lightarrow & jpetruccelli - thank you for your replies. There was something I was thinking about with regard ghosts but I think you've shown that it wouldn't happen.
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There was something I was thinking about with regard ghosts but I think you've shown that it wouldn't happen.
GHOST PARTICLE THEORY :)
http://www.newscientist.com/article.ns?id=dn6495
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If state |Afast> was a fast frequency, and always correlated with measuring |Bfast>, and state |Aslow> was a slower frequency, and always correlated with |Bslow>, and you set up a system that would force particle A to pick state |Aslow> (that is, to only allow slow frequencies through), then particle B would have to "slow down" its frequency too, since they're correlated.
Would I be correct in saying that in this situation, one requirement is that the wavelength of particle B cannot be known prior to measuring that of Particle A? That is, the best you can say about particle B before the slowing of Particle A, is that it could be in either state but we don't know which?
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If state |Afast> was a fast frequency, and always correlated with measuring |Bfast>, and state |Aslow> was a slower frequency, and always correlated with |Bslow>, and you set up a system that would force particle A to pick state |Aslow> (that is, to only allow slow frequencies through), then particle B would have to "slow down" its frequency too, since they're correlated.
Would I be correct in saying that in this situation, one requirement is that the wavelength of particle B cannot be known prior to measuring that of Particle A? That is, the best you can say about particle B before the slowing of Particle A, is that it could be in either state but we don't know which?
Exactly. Since they're fully correlated, if you knew the wavelength of particle B, you would know the wavelength of particle A. So if one is unknown at the start, they both have to be, and once one is known, they both are.
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If you simultaneously measure the position of particle A and the velocity of particle B, would you then know the velocity and position of particle A? Or would you still somehow fall foul of the uncertainty principle?
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The short answer is no, you can't beat the uncertainty principle and know the position and momentum through entanglement.
The long answer is that I'm not even sure if you could entangle particles in both position and momentum at the same time: I believe that if you try to correlate a given position of A with a given position of B, the uncertainty principle says that your momentums should be uncorrelated. You might be able to "loosely correlate" position measurements (such as "If particle A is in my house somewhere, I'm pretty sure particle B is in my house somewhere"), in which case, you can probably loosely correlate momentums too, but since every thing's loosely correlated, knowing the position of A and momentum of B would only tell you a range of numbers for the momentum of A and the position of B (which couldn't beat the uncertainty principle).
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OK. I had a feeling it shouldn't be possible.
Thank you.