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Entanglement - For Dummies.


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#1 Vexer

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Posted 5 April 2008 - 04:27 AM

I'm sorry if this has been gone-over recently.


I have read a few web-pages that have undermined my simple understanding of this.

Here’s what I thought I knew about quantum entanglement:

If two (or more) ‘particles’ (‘photons’ in the experiments) are created at the same time/place, they are Entangled. Alain Aspect, etc.

Is this wrong?


I have read about experiments that try to Entangle macro objects…. But… (I don’t understand them).


I’m wondering why Everything isn’t Entangled via the alleged Big Bang?


(Again, if you reply, keep in mind that I (obviously) know Nothing, so err on the side of crayon drawings). But I want to know.
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#2 swansont

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Posted 5 April 2008 - 12:37 PM

If two (or more) ‘particles’ (‘photons’ in the experiments) are created at the same time/place, they are Entangled. Alain Aspect, etc.

Is this wrong?


You have to have some related or conserved quantity, such as spin or polarization.

I have read about experiments that try to Entangle macro objects…. But… (I don’t understand them).


I’m wondering why Everything isn’t Entangled via the alleged Big Bang?


(Again, if you reply, keep in mind that I (obviously) know Nothing, so err on the side of crayon drawings). But I want to know.


Because interactions tend to remove the entanglement.
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#3 5614

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Posted 5 April 2008 - 03:16 PM

If you have two crayons that are entangled and you accidentally drop one of them then it ruins the entanglement or correlation between the two of them. :-p

If two photons are entangled, but one of them interacts with something (for example hits a material, where it is absorbed and then re-emitted, this is what happens when light hits a surface, i.e. how we see) then the two photons will no longer be entangled.
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#4 Vexer

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Posted 14 April 2008 - 05:17 AM

Wow. So, nothing is Entangled for long, at all?

I wonder why among all the talk of Entanglement, I've never heard the "interaction kills Entanglement" thing?


This really puts a damper on my Cosmic Unity via Entanglement thing....



Thanks, all. (I'll probably come back to this one, once I've looked more).
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#5 emcelhannon

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Posted 20 August 2009 - 12:32 PM

So is it possible to entangle 3 or more particles?
How many quantities or even combinations can particles be entangled? If they are few, what are they?
Is evey particle entangled in some way with the last particle it enteracted with?
Does it automatically loose all entanglement with its previous partner upon impact?
Is polarization the wave counterpart to the particle spin?
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#6 swansont

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Posted 20 August 2009 - 03:37 PM

Multiple particles have been entangled. IIRC both photons and ions.

You need some property that is conserved and not uniquely determined in the interaction. Most interactions don't qualify, e.g. scattering conserves momentum, but doesn't allow for multiple final states (given identical initial states).
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#7 emcelhannon

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Posted 20 August 2009 - 07:44 PM

You're the man, Swansont,and I think I'm keeping up. Is polarization related to spin, or are they separate properties?
Pardon me if this question belongs in a different thread.
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#8 swansont

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Posted 20 August 2009 - 09:53 PM

Polarization in these discussions refers to photons and the orientation of the electric field. Spin is intrinsic angular momentum, and in these discussions is usually describing a massive particle. However, you can have spin polarization — orienting all of the spins of an ensemble in a certain direction.
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#9 J.C.MacSwell

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Posted 21 August 2009 - 12:17 AM

Polarization in these discussions refers to photons and the orientation of the electric field. Spin is intrinsic angular momentum, and in these discussions is usually describing a massive particle. However, you can have spin polarization — orienting all of the spins of an ensemble in a certain direction.


That would require interaction with something other than the ensemble, correct? (I'm guessing)

Also, what would a typical ensemble consist of? Is this where you meant massive particles?

Edited by J.C.MacSwell, 21 August 2009 - 12:26 AM.

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#10 emcelhannon

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Posted 21 August 2009 - 01:45 AM

Can the photon's electric field rotate or tumble, (as in N up S up N) or is it's orientation fixed between interactions? Being massless, it's hard to imagine angular momentum. Is it proven that the photon is massless?
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#11 swansont

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Posted 21 August 2009 - 11:29 AM

That would require interaction with something other than the ensemble, correct? (I'm guessing)

Also, what would a typical ensemble consist of? Is this where you meant massive particles?


You can spin-polarize atoms optically pumping them, putting them all in the same spin state. The light will need to be be polarized in order to do this.

But typically if someone is talking at a general level about polarization, they are talking about light.

Merged post follows:

Consecutive posts merged

Can the photon's electric field rotate or tumble, (as in N up S up N) or is it's orientation fixed between interactions? Being massless, it's hard to imagine angular momentum. Is it proven that the photon is massless?


If you have elliptical polarization, the electric field changes orientation, sweeping out a helix over time (and distance). The experimental upper limit on the photon mass is very small and consistent with zero.
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#12 emcelhannon

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Posted 21 August 2009 - 04:13 PM

The experimental upper limit on the photon mass is very small and consistent with zero.

Thank you,
What about volume?
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#13 swansont

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Posted 21 August 2009 - 06:38 PM

That issue is a bit slipperier; the "size" of a photon has been discussed a little bit here (and probably elsewhere)
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