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Old 24th September 2009, 03:05 AM   #1
martu
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Are entangled photons ‘touching’ in time?

At the creation of a pair of entangled particles we can say that the time of creation for each photon is equal, that is for a pair of entangled photons p1 and p2

te1 = te2 = 0

where ten is the creation time of photon n.

Here we’re going to use a slightly modified spacetime diagram, in this spacetime diagram the x axis and y axis represent the spatial dimensions and the time axis, ct, is the z axis perpendicular to the (x, y) plane. The z dimension is suppressed to help us picture what is going on.

Picture the two entangled photons as created at the point (0, 0, 0) and they go in opposite directions along the x axis. So for our entangled photons p1 and p2 we can say at the time of emission

(x1, y1) = (x2, y2) = (0, 0)

And also

(dx1/dy1) = (dx2/dy2)

Here is the extra step – our time axis is not a plane but a line. What does that mean? Well it means for each event en we define it’s spatial and temporal co-ordinates not as (x, y, t) but (x, y) and (t) that is we plot the path of an object through time as a line in the (x, y) plane and a different line ‘up’ the time axis. If your spacetime diagram is on a page our path through time can be represented as a line coming out of the page. All objects move ‘up’ the time axis at the same rate, c.

A consequence of this is that for our entangled pair of photons they travel ‘up’ the time line together, at least until one of them experiences an event. Or to put it another way at any time tn the two photons will be on the t axis at the same point. Any objects that are created at the same time and at the same spatial dimension are ‘touching’ in time and can, and indeed will, influence the other when a conserved property of one changes, the follows directly from the conservation principle.

Another way to look at it is that the photons share a clock from the event of their creation until one of them experiences another event. Indeed for every event you can consider that a unique point is created on the t axis which ‘moves’ up the t axis. This point is our clock and any objects that share a clock will influence each other, objects will no longer share a clock when they experience another event.

Does this make sense to anyone? We have a violation of locality but only for a very special subset of events where time and space are exactly equal. This is very rare in the universe hence we do not see locality violated often. We also cannot violate causality – the only direction you can go in time is ‘up’ the time axis so even though we have faster than light influence we can’t go back in time at all.
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Old 24th September 2009, 03:49 AM   #2
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The problem with discussions of this nature is that they impinge so little on "normal reality" that common sense scarcely applies- so it's honestly near impossible for the average person to say what is "sensible in this context" and what is not.

Your argument sounds superficially credible to me, but we need someone who can say if it makes sense mathematically.
(Not that such is definitive, but it does seem likely to be a shade more definitive than a merely verbal argument.)
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Old 24th September 2009, 04:05 AM   #3
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It seems to make sense to me. The photons are "touching in time" and separated by space, i.e. they are traveling separately through spacetime.

However quantum entanglement is not really a "conservation principle".
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Old 24th September 2009, 05:48 AM   #4
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Originally Posted by Reality Check View Post
It seems to make sense to me. The photons are "touching in time" and separated by space, i.e. they are traveling separately through spacetime.

However quantum entanglement is not really a "conservation principle".
Can you elaborate on the last sentence please?
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Old 24th September 2009, 05:49 AM   #5
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Originally Posted by Soapy Sam View Post
The problem with discussions of this nature is that they impinge so little on "normal reality" that common sense scarcely applies- so it's honestly near impossible for the average person to say what is "sensible in this context" and what is not.

Your argument sounds superficially credible to me, but we need someone who can say if it makes sense mathematically.
(Not that such is definitive, but it does seem likely to be a shade more definitive than a merely verbal argument.)
Agreed.
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Old 24th September 2009, 06:02 AM   #6
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Originally Posted by martu View Post
Can you elaborate on the last sentence please?
Quantum entanglement is that measurement of an entangled property of one photon determines the property of the other photon (as in your OP).

A conservation principle is that a system has a conserved quantity at all times.
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Old 24th September 2009, 06:11 AM   #7
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Originally Posted by Reality Check View Post
Quantum entanglement is that measurement of an entangled property of one photon determines the property of the other photon (as in your OP).

A conservation principle is that a system has a conserved quantity at all times.
Yep and what I was trying to say is that the two entangled photons should be considered as one system until they experience another event because they are 'touching’ in time. This explains how measuring one affects the other no matter how far apart spatially they are.
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Old 24th September 2009, 09:01 AM   #8
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I don't know if this makes any sense, but is it plausible to think of them as the same photon for the applicable time?
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Old 24th September 2009, 09:20 AM   #9
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Originally Posted by gnome View Post
I don't know if this makes any sense, but is it plausible to think of them as the same photon for the applicable time?
I'm pretty sure the maths is OK but whether this answers the questions raised by QM and locality is one for the experts.

To answer your question I don't think so as they are in different locations in space. Same location in time however.
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Old 24th September 2009, 12:51 PM   #10
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Originally Posted by martu View Post
Yep and what I was trying to say is that the two entangled photons should be considered as one system until they experience another event because they are 'touching’ in time. This explains how measuring one affects the other no matter how far apart spatially they are.
The 'touching’ in time concept unfortunately does not explain this. It also applies to any 2 non-entangled photons and measuring one of these does not affect the other. Think about 2 photons emitted from 2 different atoms at the same time (te1 = te2 = 0).
It is the entanglement that is the reason for the correlation between the 2 photon measurements.
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Old 24th September 2009, 12:53 PM   #11
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Originally Posted by martu View Post
I'm pretty sure the maths is OK but whether this answers the questions raised by QM and locality is one for the experts.

To answer your question I don't think so as they are in different locations in space. Same location in time however.
I'm thinking of the same photon, simultaneously located in two different places (effectively).

I have no idea if this image is at all useful.
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Old 24th September 2009, 02:32 PM   #12
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Originally Posted by Reality Check View Post
The 'touching’ in time concept unfortunately does not explain this. It also applies to any 2 non-entangled photons and measuring one of these does not affect the other. Think about 2 photons emitted from 2 different atoms at the same time (te1 = te2 = 0).
It is the entanglement that is the reason for the correlation between the 2 photon measurements.
Those two photons are not created at the same point (x, y).
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Old 24th September 2009, 06:18 PM   #13
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Originally Posted by martu View Post
Those two photons are not created at the same point (x, y).
Neither do the entangled photons have to be (as in your example).
Quantum entanglement of particles does not require them to be in the same location at some point in time, e.g. Experiment demonstrates quantum entanglement between atoms a metre apart.

Last edited by Reality Check; 24th September 2009 at 06:31 PM.
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Old 25th September 2009, 02:05 AM   #14
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Originally Posted by Reality Check View Post
Neither do the entangled photons have to be (as in your example).
Quantum entanglement of particles does not require them to be in the same location at some point in time, e.g. Experiment demonstrates quantum entanglement between atoms a metre apart.
That does require that the two photons emitted are at the same point at the same time namely in the beam splitter.

Let’s see if I have understood that article. We have an event at the same point in time and space where the photons are emitted by the ions creating an entangled state between one ion and one photon. Then when the photons ‘mingle’ at the beam splitter there is a chance that the two photons will be entangled. Conclusion - every time there is an entanglement ‘created’ between two objects they are at the same point and time.

Is this in error?

Are there any experiments where two entangled particles do not ‘touch’ each other spatially at all? Yes I get that these two atoms aren’t touching but they are in an entangled state with photons that do touch.
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Old 25th September 2009, 02:10 AM   #15
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Originally Posted by gnome View Post
I'm thinking of the same photon, simultaneously located in two different places (effectively).

I have no idea if this image is at all useful.
That would mean that this one photon had two paths through spacetime which I do not think is permissable but I may very well be wrong, I'll see if I can dig anything up about this. One of the physicists here may know the answer to this immediately.
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Old 25th September 2009, 02:19 AM   #16
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Originally Posted by gnome View Post
I'm thinking of the same photon, simultaneously located in two different places (effectively).

I have no idea if this image is at all useful.
Actually thinking about this a different way if it was the same photon what would it's spin be?
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Old 25th September 2009, 05:49 AM   #17
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Originally Posted by martu View Post
That does require that the two photons emitted are at the same point at the same time namely in the beam splitter.

Let’s see if I have understood that article. We have an event at the same point in time and space where the photons are emitted by the ions creating an entangled state between one ion and one photon. Then when the photons ‘mingle’ at the beam splitter there is a chance that the two photons will be entangled. Conclusion - every time there is an entanglement ‘created’ between two objects they are at the same point and time.

Is this in error?
Yes that is in error.
The experiment produces 2 entangled atoms not photons. The atoms are never in contact. They are in 2 different magnetic traps over a metre apart.
The photons in the experiment are used to demonstrate that the atoms are entangled.
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Old 25th September 2009, 07:22 AM   #18
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Originally Posted by Reality Check View Post
Yes that is in error.
The experiment produces 2 entangled atoms not photons. The atoms are never in contact. They are in 2 different magnetic traps over a metre apart.
The photons in the experiment are used to demonstrate that the atoms are entangled.
No it produces entangled ions and photons it says so clearly in the text:

Quote:
…collapses the wavefunction into a state in which the two ions, as well as the photons, are entangled with each other.
I take it there is something I am missing here?
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Old 25th September 2009, 07:30 AM   #19
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There's no reason why two entangled particles ever had to occupy the same spot. There's also no reason why two non-entangled particles couldn't be produced in the same place at the same time (to as much accuracy is entangled particle pairs sometimes are).

So it's hard to see how this idea would work, even if it were necessary to explain something.
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Old 25th September 2009, 08:27 AM   #20
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Originally Posted by sol invictus View Post
There's no reason why two entangled particles ever had to occupy the same spot.
Is this opinion based on theory or experiment?

Originally Posted by sol invictus View Post
There's also no reason why two non-entangled particles couldn't be produced in the same place at the same time (to as much accuracy is entangled particle pairs sometimes are).
How could we know they weren't entangled? How could you produce a pair of non entangled particles at the same point wont you violate conserved properties for the system?

Originally Posted by sol invictus View Post
So it's hard to see how this idea would work, even if it were necessary to explain something.
I think the idea that two or more particles can be at the same unique spot in time no matter how far apart they are could explain entanglement don't you? Completely theoretically mind you, I am well aware it seems to be wrong.
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Old 25th September 2009, 08:42 AM   #21
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Originally Posted by martu View Post
Is this opinion based on theory or experiment?
Both. You can, for example, entangle two atoms using EM fields. The atoms were never in the same spot. That works theoretically and experimentally.

Quote:
How could we know they weren't entangled?
By measuring them.

Quote:
How could you produce a pair of non entangled particles at the same point wont you violate conserved properties for the system?
It depends what they are and how you produce them. It also depends on what you mean by "same point".

Quote:
I think the idea that two or more particles can be at the same unique spot in time no matter how far apart they are could explain entanglement don't you? Completely theoretically mind you, I am well aware it seems to be wrong.
No, to be honest I don't see how such an idea could work.
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Old 25th September 2009, 09:11 AM   #22
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Originally Posted by sol invictus View Post
Both. You can, for example, entangle two atoms using EM fields. The atoms were never in the same spot. That works theoretically and experimentally.
Can you show me a link to this experiment please?

As ever thanks again to you and RealityCheck for the use of your physics brains.
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Old 25th September 2009, 06:32 PM   #23
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Originally Posted by martu View Post
Actually thinking about this a different way if it was the same photon what would it's spin be?
An interesting point. My cursory study of this shows that entangled particles tend to have opposite spin. That might squash the usefulness of trying to think of them as the same particle co-located.
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Old 25th September 2009, 11:55 PM   #24
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Originally Posted by martu View Post
No it produces entangled ions and photons it says so clearly in the text:

I take it there is something I am missing here?
There are two entanglements that I can see.
The first is between each ion and the photon that it emited.
The second is between the ions. This is established when the "experimenters select only those excitation events that result in photons recorded by both detectors within 15 ns after the excitation".

As far as I know, the photons are not entangled with each other.
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Old 26th September 2009, 03:45 AM   #25
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Originally Posted by Reality Check View Post
There are two entanglements that I can see.
The first is between each ion and the photon that it emited.
The second is between the ions. This is established when the "experimenters select only those excitation events that result in photons recorded by both detectors within 15 ns after the excitation".

As far as I know, the photons are not entangled with each other.
Ok. When are the ions entangled before or after the photons mingle? What does the author mean by mingle anyway?
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Old 26th September 2009, 05:01 AM   #26
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Originally Posted by gnome View Post
An interesting point. My cursory study of this shows that entangled particles tend to have opposite spin. That might squash the usefulness of trying to think of them as the same particle co-located.
Exactly they have to be different.
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Old 26th September 2009, 11:55 AM   #27
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Originally Posted by Reality Check View Post
Neither do the entangled photons have to be (as in your example).
Quantum entanglement of particles does not require them to be in the same location at some point in time, e.g. Experiment demonstrates quantum entanglement between atoms a metre apart.
Can you do this experiment without the two photons mingling?
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Old 26th September 2009, 10:33 PM   #28
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(Derail)

Every time QM weirdness -- with its 11 dimensions, all hidden somewhere incredibly close to us yet untouchable, time moving backward, etc. -- is discussed, I am reminded of the old limmerick:

The creatures of other dimensions
Cause earthlings great consternation.
They can sneak up to you
And give you a screw
Before you divined their intention.
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Old 27th September 2009, 02:46 AM   #29
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Originally Posted by martu View Post
Can you do this experiment without the two photons mingling?
The 2 photons are never "mingled" as in quantum entangled.
What they mean by "mingle" is pass through the half-mirror within a 15 ns window.

Last edited by Reality Check; 27th September 2009 at 02:52 AM.
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Old 27th September 2009, 02:54 AM   #30
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Originally Posted by martu View Post
Ok. When are the ions entangled before or after the photons mingle? What does the author mean by mingle anyway?
The ions quantum states are entangled by the selection of the photon that each emit. This selection is done after the photons are detected, i.e. after they "mingle".
What the author means by "mingle" is pass through the half-silvered mirror within a 15 ns window.
If they wanted to say entangled then they would have said entangled.

ETA:
The middle column on page 17 states that the photons are entangled by the requirement that both detectors detect photons.

Last edited by Reality Check; 27th September 2009 at 03:16 AM.
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Old 27th September 2009, 03:17 AM   #31
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Originally Posted by Reality Check View Post
The 2 photons are never "mingled" as in quantum entangled.
What they mean by "mingle" is pass through the half-mirror within a 15 ns window.
Why is mingling required?
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Old 27th September 2009, 05:23 AM   #32
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Originally Posted by Reality Check View Post
The ions quantum states are entangled by the selection of the photon that each emit. This selection is done after the photons are detected, i.e. after they "mingle".
What the author means by "mingle" is pass through the half-silvered mirror within a 15 ns window.
If they wanted to say entangled then they would have said entangled.

ETA:
The middle column on page 17 states that the photons are entangled by the requirement that both detectors detect photons.
Erm forgive my dumbness but doesn't your ETA contradict your original post?
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Old 27th September 2009, 05:33 AM   #33
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Originally Posted by Skeptic View Post
(Derail)

Every time QM weirdness -- with its 11 dimensions, all hidden somewhere incredibly close to us yet untouchable, time moving backward, etc. -- is discussed, I am reminded of the old limmerick:

The creatures of other dimensions
Cause earthlings great consternation.
They can sneak up to you
And give you a screw
Before you divined their intention.
If time is the 4th dimension
there is one thing I aught to mention
though this photon is here
and that photon is there
if entagled they're at one location
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Old 27th September 2009, 05:58 AM   #34
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Originally Posted by martu View Post
Erm forgive my dumbness but doesn't your ETA contradict your original post?
Yes if you mean "As far as I know, the photons are not entangled with each other.". Now I know better.
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Old 27th September 2009, 06:00 AM   #35
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Originally Posted by martu View Post
Why is mingling required?
To entangle the quantum state of the ions as stated in the middle column of page 17.

The experiment shows that quantum state of ions separaterd by a metre or more can be entangled The demostraion of the entanglement is described on the next page - page 18.
There is also another interesting aspect: The entanglement of the ions is establshed by causing the entanglement of photons at an arbitary distance from the ions.

Last edited by Reality Check; 27th September 2009 at 06:07 AM.
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Old 27th September 2009, 07:54 AM   #36
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Originally Posted by Reality Check View Post
To entangle the quantum state of the ions as stated in the middle column of page 17.

The experiment shows that quantum state of ions separaterd by a metre or more can be entangled The demostraion of the entanglement is described on the next page - page 18.
There is also another interesting aspect: The entanglement of the ions is establshed by causing the entanglement of photons at an arbitary distance from the ions.
Right thanks.

I contend that when the photons mingle we have the two photons and the two ions at the same point in time hence the ions become entangled. Before the photons mingle the ions are not entangled.
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Old 27th September 2009, 11:42 AM   #37
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Originally Posted by martu View Post
Right thanks.

I contend that when the photons mingle we have the two photons and the two ions at the same point in time hence the ions become entangled. Before the photons mingle the ions are not entangled.
That is right. Before the photons are mingled the ions are not entangled. If the photons are then detected within the 15 ns window the ions should be entangled. This is then demonstrated to be the case with the detection of the hyperfine state of the ions.
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Old 27th September 2009, 08:21 PM   #38
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Originally Posted by martu View Post
Can you show me a link to this experiment please?
Here's an example, with links to the papers: http://blogs.discovermagazine.com/80...ntangled-ions/

By the way, all the atoms in a Bose-Einstein condensate are entangled with each other, even though the condensate can be relatively large, and it can be achieved simply by cooling the atoms sufficiently.
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Old 27th September 2009, 10:54 PM   #39
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Originally Posted by sol invictus View Post
Here's an example, with links to the papers: http://blogs.discovermagazine.com/80...ntangled-ions/

By the way, all the atoms in a Bose-Einstein condensate are entangled with each other, even though the condensate can be relatively large, and it can be achieved simply by cooling the atoms sufficiently.
I read your link and it seems to infer that this entanglement involves a mechanical system. One pair of ions is set vibrating, the other pair responds. So we get back to the Mars thing again: If I set two ions vibrating on Earth, two entangled particles do likewise on Mars. Why is this not an instantaneous switch?
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Old 28th September 2009, 01:52 AM   #40
martu
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Originally Posted by sol invictus View Post
Here's an example, with links to the papers: http://blogs.discovermagazine.com/80...ntangled-ions/.
From your link:

Quote:
The first step to achieving these synchronized vibrations relied on standard techniques to entangle the spins of the beryllium ions in each pair.
What are these standard techniques? Do they involve bringing the entangled pairs together at one point in space and time?

Originally Posted by sol invictus View Post
By the way, all the atoms in a Bose-Einstein condensate are entangled with each other, even though the condensate can be relatively large, and it can be achieved simply by cooling the atoms sufficiently.
From this paper Entanglement concentration in Bose-Einstein condensates

Quote:
Since entanglement cannot be created by local operations on separate systems, entangled pairs of systems need to be created at a source and then distributed to distant parties.
Bose-Einstein condensates fit my picture as you have to ‘distribute’ the entanglement which I would say is bringing two particles together at the same point in time then doing this over and over again resulting in all the atoms sharing a point in time.
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