Quantum Entanglement

Superposition?

I don't think (and I'm quite sure of that) that entanglement has anything to do with superposition (I assume you misinterpret what entanglement is about - but I may always be wrong).

Just because we do not know the spin an entangled particle has after it has been created does not mean it is necessary for it to be in superposition. Even if it was to have a defined spin right after it has been created it does not change the peculiar results it gives - measuring one particle results in automatical and instant measurement of the paired entangled particle 'far away'.

This is the "spooky action at a distance" - that's what entanglement is all about. Supposedly separate and separated particles can interact with eachother no matter how long the distance between them is.

This is also why conservative physicist find these results very, very peculiar - it forces us to assume that either information can transfer superluminously (instantly, infact) or that locality assumption is incorrect. Both can't be true or there would be no "spooky action at a distance" whatsoever.

Bear in my this is only my opinion and point of view on the QE - nothing else.

KaerbEmEvig,

it is possible that term "supposition" is not entirely adequate for describing the official notion about a spin of a photon but one thing about the quantum entanglement is certain.

If two entanglemented photons , A and B, would have defined spin on the certain axis, each of them, let us say A up and B down, before the measurement, then there is no magic of the quantum entanglement to wonder about. In this case we would have two photons which are mirror image/state of each other and of course that if we observe one of those photons with spin up on the certain axis then we would of course know that other has spin down on that axis because they were and they are in that correlation since they have been created, therfore nothing changed and there is nothing magical or nonlocal about that, that is a simple noncausal correlation.

The notion which makes the quantum entanglement magical is that either of those entaglemented photons MAY assume either of the possible spins, that is, A can be up or down and B can be up or down, that is, the spin of each of those photons is undefined since they have been created. It is not that we just do not know what is the spin for each of them, it is that each of them does not have defined spin UNTIL one of them is observed/measured, which by the way breaks entanglement.

There is a magic of nonlocality, and that is the reason why I am questioning the notion that A or B are in the state of undefined spin BEFORE an observation/measurement.

Anton

This post has been edited by Mate on Jul 10 2007, 01:24 PM

I think you misunderstood what the magic is about. The magic of entanglement is not the fact that characteristics of entangled particle pair are opposite - it's not that. The peculiarity of QE is the fact one particle influence the other.

Measuring one particle results in instantly measuring the other - that is: when you measure A, B's interference pattern (fringes) magically disappear, without any local measurement of the B particle.

That's what the magic of QE is about - not the fact the y bear opposite spins.

If they were simply separate particles with opposite characteristics after the creation, then we would not be witnessing the fact that one particle influence the other no matter how far the distance is.

Once again - QE has nothing to do with superposition. QE is all about one entangled particle influencing its counterpart instantly - even at the distance of hundreds of light years.

Now I have noticed that I am using "supposition" instead "superposition". Nonetheless it was understandable what I meant. I hope.

KaerbEmEvig,

what you are describing is the delayed choice quantum eraser experiment. Indeed that is a mystery.

Although what is troubling me about that experiment is that as soon as any of the entanglemented photons is observed in some manner then the entanglement between them is suppose to be broken, that is, one entanglemented photon can influence the other just once. But in DCQE one entanglemented photon hits the detector, in which moment that act of an observation in accordance with the quantum entanglement should determine the state of the one photon in regard to other, and vice versa.

However, in this experiment it is more chaotic than table tennis.

That other photon continues to be deflected by prism toward the beam splitter, through which that photon either passes or it is reflected by the beam splitter, and then that photon arrives at one of four possible detectors, and depending which detector it will hit that would determine is there an interference of not on the screen where the first photon hit the screen, long long time ago.

The question is. Again, as I understand quantum entanglement between two photons is broken after one of them is being observed once. In QCDE photon A hitting the detector, photon B reflects from prism, passing or reflecting by beam splitter, and finally hitting one from four possible detectors. When the entanglement has been broken between those two photons?


To go back on the subject matter.

http://en.wikipedia.org/wiki/Quantum_Entanglement

"Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated. This leads to correlations between observable physical properties of the systems. For example, it is possible to prepare two particles in a single quantum state such that when one is observed to be spin-up, the other one will always be observed to be spin-down and vice versa, this despite the fact that it is impossible to predict, according to quantum mechanics, which set of measurements will be observed. As a result, measurements performed on one system seem to be instantaneously influencing other systems entangled with it."

It seems to me that what I am saying about spin up or down is what the quantum entanglement is saying.

And when it is said,

"..this despite the fact that it is impossible to predict, according to quantum mechanics, which set of measurements will be observed",

what is meant by that? Impossible to predict the spin because any of the entanglemented photons may assume randomly either spin up or spin down or impossible to predict which of entanglemented photons has spin up ( or down )?

Anton

This post has been edited by Mate on Jul 10 2007, 03:50 PM

It's the second answer (you can deduce it from the academic artles I have presented - I have many more on the subject, if you wish). It's impossible as in: the probability equals to 50% for either option and they it is assigned 'randomly' - thus there's no trace you could base your predictions on - impossible to predict.

The impossibility of prediction which photon has spin down does not imply that it's in superposition. It simply states that only means to determine which spin it posseses is to measure it.

KaerbEmEvig,

but if it is the second then where is the nonlocality there? That is what motivated me to start this thread in the first place.

If it is the second then that is akin of having two chairs in some dark room, each of the chair placed near the opposite wall of that room. What we know is that one chair is red and another is green, but we do not know in which chair we are sitting until someone would put a light on the other chair near opposite wall of the room.

And we see that that chair is the red one. Now we know that we are siting in the green one even we still do not see the color of the chair we are sitting in.

Where is nonlocality in this metaphorical situation?

Anton

Hi Anton, KaerbEmEvig ,

This might help .. http://www.mtnmath.com/whatrh/node78.html

Best wishes, -C2.

Edit .. and the next page.

This post has been edited by Confused2 on Jul 10 2007, 05:38 PM

Nonlocality? The fact that particle here has an effect on the particle 'there' instantly (this is not superluminous information transfer because those particles can be/are described by one wavefunction from what I know - they are a singularity in two places at a time - nolocality).

You constantly commit the same mistake - you leave out the fact that the up particle, when measured, influences the down particle, so that the latter is now in the state after-the-measurement - without actually being measured (interference fringes disappear). Vide this: grad.physics.sunysb.edu/~amarch/

Where is the nonlocality in the analogy? Nowhere - the analogy is incorrect and does not resemble entangled system.

This post has been edited by KaerbEmEvig on Jul 10 2007, 05:47 PM

KaerbEmEvig,


I already addressed your remark about an interference fringes disappearing in few posts ago. That is different matter than relationship between the spins of a entanglemented photons. Disappearing fringes of an interference is matter of wave/particle duality. Here there is no post observation state of another photon after observing one photon, if that observed photon had determined spin of the certain axis BEFORE it has been measured.

If that is the case then there is no non locality when we measure/observe a photon A with spin up because photon A had a spin up on that axis since it has been created. And measuring it's spin does not influence a spin of a photon B because photon B had a spin down on that axis since it has been created.

Do you understand what I am saying here?

Now, I do not claim that that is really the "relationship between spins of the entanglemented photons, but if you hold opinion that they have defined spin before the measurement then that is the implication of that notion. Clearer now?

Anton

This post has been edited by Mate on Jul 10 2007, 06:26 PM

I get the impression I haven't helped much. I'll try again - at least you can both blame me if I'm wrong.

Looking at http://grad.physics.sunysb.edu/~amarch/

Their 'x' polarisation is my Horizontal polarization and 'y' is my Vertical polarisation

Which-Way Marker diagram
The p photon cannot be vertically AND horizontally polarized at the point of detection so 'the system' must choose which one to go for... hence the path is also chosen:- QWP1 for pH and QWP2 for pV. No interference.

Quantum Erasure diagram
The source pH or pV polarisation is made indistinguishable by a polarizer so detection no longer forces s (the other photon) into a defined state. Now s isn't forced into a defined state it can take either path and we get interference.

Forcing 'the other' photon into a defined state is the entanglement, doing it somewhere else is the non-locality and doing it in the 'wrong order' is the erasure.

Looks very elegant to me. If you cannot be kind then please be gentle.

Best wishes - C2.

This post has been edited by Confused2 on Jul 10 2007, 08:29 PM

No argument here, I'm just trying to look at things from different angles to see if something might have gone unnoticed.

Your description gives me an idea. We should look at the consequences of changing the wave packets to see how it all fits together. The polarizers used in the delayed choice dse don't block the energy, they reshape it.

Blah, no time to think on this, japanese to study.

C2: You've been plenty of help, thanks.

This post has been edited by Wulf on Jul 10 2007, 08:43 PM

But the truth is - just because they have defined spins does not mean they exhibit the nature of measured particles. They will still create interference fringes but after measurement they will create a point. Measuring particle A results in particle B behaving as if it has also been measured. That's nonlocality (or superluminous information transfer).

This is exactly what I was saying all the time. I just didn't have time to back it up with the article like you did. Thanks for your effort. Your explanation clearly explains what entanglement is and when nonlocality occurs.

This post has been edited by KaerbEmEvig on Jul 10 2007, 09:13 PM

Just want to throw an idea I have out there.

Alright lets consider the Quantum erasure DSE, wether it is delayed or not dosent matter.

| /_ In the unobserved run of the experiment 3 states are reaching the target Spin up down and circular. So every detection event has the full spectrum of possible spins. We observe an interference pattern.

|_ In the wich way run detectors in place only 2 states are reaching the targets, for some reason the interference pattern dissapears.

|_ + / Finally in the erasure step we add a 45deg polarizer before the event counter and the pattern suddenly re-appears.

Notice anything different between these three runs of the experiment?



The / component being removed destroys the pattern. Re-introducing it brings it back.

Now in the Delayed choice DSE the distribution of detection events is based on both photons, adding a 45deg filter to the detector to destroy the wich way information also reduces the number of detection events by 2/3.

Let us consider a little more of my stunning ascii art:

|||||||||
|||||||||

| | |
| | |

This kind of looks like an interference pattern dosen't it?



So where is the spooky action at a distance? This looks more like a failure to consider the properties of the sample set you are basing the measurements off of.

This seems to do away with the whole spooky action at a distance and time travel effects. That is, unless I'm missing something.

This also seems to relate to the nature of the inerference pattern as well, but I'm too tired to think about it right now. Any thoughts or comments?

C2,

thanks for the papers you are posting but there was not enough time to read them thoroughly because yesterday I was playing a polo with my friends , and I can tell you that it is difficult to read about a physics and ride a horse simultaneously.

A few comments nonetheless...



"The strangeness of quantum mechanics makes it difficult to describe these experiments coherently. On the one hand the photon does not have a definite polarization until and unless it is detected."



Now, this is a guess. How we can be reasonably sure that photon does not have a definitive polarization until and unless it is detected? On which experimental finding that assertion lies?

Snip......



"Consider a single quantum event that creates a pair of photons. Conservation laws require a correlation in properties like polarization for the elements of such pairs. The probability that both will pass though a pair of polarizers is $\cos(\theta)$ where $\theta$ is the angle between the polarizers. Note this says nothing about the polarization angle of the photons. That does not exist until it is observed!"



Is my proposal of the experiment of using one fitter to have perfectly polarized beam of photons exiting that filter at precise angle of polarization as the fitter is set up, and other filter through which ( or not ) would particular angles of polarization pass that vertical filter, a possible solution? Now we can experiment with photons for which we know exactly what is their angle of polarization in regard to that other filter.

Yes? No? We do not know? Go back and play polo?



Anton

Hello, Wulf

Look, the photon in the experiment will exhibit two nature - but only one at a time - it will either exhibit the corpuscular nature or the wave nature. The wave nature is present when we do not place which'way detectors between the slits and the targeted area - we can clearly see fringes.

When we place a which-way detector, so that we are able to obtain the information through which slit the photon went, the photon will exhibit its corpuscular nature - correct?

Now comes the nonlocality and quantum entanglement.

Quantum entanglement means that if we observe/measure one of the entangled particles to have a specific spin, the other will have an opposite spin.

We obtain the which-way information through measuring the spin of the photon (it doesn't matter whether the photon has a defined spin right after it has been created or not - it's not superposition that defines interference fringes, it's the lack of which-way information - lack of the measurement of the spin).

This means that obtaining (or rather being able to obtain it - doesn't matter whether we look at the collected data or not) the 'which-way information, the information about what the spin is, forces the photon to 'change' the exhibited nature from wave-like to particle-like.

Where's the entanglement? When we obtain which-way information for photon A by measuring its spin we simultaneously obtain the information on the B photon's spin - thus we also obtain the which-way information for the latter photon. This means that by influencing photon A (to change the exhibited nature from wave-like to particle-like) we have at the same time influenced photon B - without actually measuring. One photon influenced the other.

Where's the nonlocality? When one entangled photon influences the other the distance doesn't matter - it all happens instantly (this means we would either have to assume that information is transfered superluminously or that entangled photons are actually on 'being' in two places - nonlocality). Scientists assume it's nonlocality becase there's alot more goin' against the former assumption.