Implications of the Delayed Choice Quantum Eraser

[Kansas_Cowboy]

My mind was blown upon discovering this experiment. Subsequent attempts at putting my brain back together have all failed miserably. All posts trying to demystify the experiment have either appeared flawed or were too complex for my primitive liberal arts brain to understand. Now I fear such brain contusions may simply be a side effect of studying quantum physics in general, but I'm hoping you guys can help me out anyway.

Basically, my understanding of the experiment leads me to the uncanny conclusion that the behavior of these photons is dependent on our access to information concerning which specific slot they initially pass through. When this information is discernible, the correlating photons act like particles. Yet when this information is unavailable, the correlating photons produce interference patterns as if they were waves. Not only that, but entangled pairs display the same behavior even though one reaches its detector before the other one even reaches the beam splitter, the results of which determine whether or not such information will be available. And SOMEHOW, I find this latter prospect to be much more reasonable and understandable than the first.

Is this understanding correct? Should I even bother looking for the remaining bits of brain I have yet to recover post-explosion?

 

[Kansas_Cowboy]

One guy was like, "The truth is actually quite obvious and less mysterious that often presented. It is of the "eating your cake and having it" kind. You cannot detect a photon just after it has passed the slits and still expect the same photon to reach the screen. At the very least your efforts will disburb/deflecting the photons. Therefore, by attempting to detect photons at the slits, you have introduced a disturbance which perturbs your interference pattern!"

However, this seems like a misunderstanding of the experiment. All of the idler photons are eventually detected. It just so happens that if by chance they reach D3 or D4, then we may indirectly infer which slot they went through. If they instead pass through the initial beam splitter and reach D1 or D2, then the information is lost. If mere detection produces a disturbance interfering with the interference pattern, then one would not expect to see an interference pattern anywhere in this experiment.

 

[bhobba]

Without going into the details its not nearly as mind blowing as all that - its simply that decoherence, while usually irreversible, can be undone in simple circumstances.

But if you want to understand it there is no substitute of slogging through the details including the math. Sadly there is no lay way of getting to the bottom of this experiment - you must delve into the details of QM.

Its examined in chapter 20 here:
http://quantum.phys.cmu.edu/CQT/index.html

But to understand chapter 20 you need the time and patience to go through the earlier chapters. Even though it contains math this book has been designed with a minimum of it to be accessible to as wide an audience as possible but it will still require your close attention.

Thanks
Bill

 

[Kansas_Cowboy]

Without going into the details its not nearly as mind blowing as all that - its simply that decoherence, while usually irreversible, can be undone in simple circumstances.

But if you want to understand it there is no substitute of slogging through the details including the math. Sadly there is no lay way of getting to the bottom of this experiment - you must delve into the details of QM.

Its examined in chapter 20 here:
http://quantum.phys.cmu.edu/CQT/index.html

But to understand chapter 20 you need the time and patience to go through the earlier chapters. Even though it contains math this book has been designed with a minimum of it to be accessible to as wide an audience as possible but it will still require your close attention.

Thanks
Bill

I assume by decoherence, you're referring to the collapse of the wave function? Is that correct?

Also, I could be wrong, but that chapter seemed to deal largely with refuting retrocausality. I'm particularly interested in why the results differ between D1/D2 and D3/D4. Could you provide any insight on that issue? Is there anything wrong with the following simple statement?: In this experiment, photons exhibit particle-like behavior when information is available regarding the specific slit through which they first passed and wave-like behavior when this information is unavailable?

Perhaps I'm asking too much. Perhaps this aspect of quantum physics must necessarily remain esoteric, a golden idol of knowledge hidden deep within an ancient temple of complex mathematics. But it would be great if any of you Indiana Jones's could break into the temple, grab the idol, get out while avoiding all the complex quantum math booby-traps, and fill me in on the escapade. The math must represent something after all.

 

[bhobba]

I assume by decoherence, you're referring to the collapse of the wave function? Is that correct?

Yes and no - its a subtle issue. But at a lay level you can say collapse is undone.

Could you provide any insight on that issue?

I thought it addressed that issue.

Is there anything wrong with the following simple statement?: In this experiment, photons exhibit particle-like behavior when information is available regarding the specific slit through which they first passed and wave-like behavior when this information is unavailable?

This wave particle thing is basically a myth:
http://arxiv.org/abs/quant-ph/0609163

Perhaps I'm asking too much.

Things are as they are. If you want to understand how a transistor radio works you have to understand how transistors work. If you want to understand the quantum eraser you must understand QM. That will take time and effort. There is some things in QM that can be explained in lay terms - however the eraser experiment isn't one of them. Even when you know QM it requires effort to get to grips with it.

Thanks
Bill

 

[Derek Potter]

Perhaps I'm asking too much. Perhaps this aspect of quantum physics must necessarily remain esoteric, a golden idol of knowledge hidden deep within an ancient temple of complex mathematics. But it would be great if any of you Indiana Jones's could break into the temple, grab the idol, get out while avoiding all the complex quantum math booby-traps, and fill me in on the escapade. The math must represent something after all.

No, it's extremely simple. The system remains in a superposition of all possible outcomes until after both photons have been detected. In the language of Schrodinger's Cat the system is in a state of [D0 & D1], [D0 & D2], [D0 & D3], [D0 & D4] at the same time. Each outcome has a weighting factor according to x. When this superposition is resolved by observation, one of those four is observed with the correct probability. That's all there is to it.

 

[Kansas_Cowboy]

No, it's extremely simple. The system remains in a superposition of all possible outcomes until after both photons have been detected. In the language of Schrodinger's Cat the system is in a state of [D0 & D1], [D0 & D2], [D0 & D3], [D0 & D4] at the same time. Each outcome has a weighting factor according to x. When this superposition is resolved by observation, one of those four is observed with the correct probability. That's all there is to it.

Thanks, Derek. If I understand correctly, this solves the mystery of retrocausality.

I'm also wondering why the results then differ between D1/D2 and D3/D4 in the way they do. Is this something that scientists simply haven't been able to explain yet, or is there some rationale? Perhaps something to do with the photons' interactions with beam splitters or mirrors?

 

[Derek Potter]

Thanks, Derek. If I understand correctly, this solves the mystery of retrocausality.
I'm also wondering why the results then differ between D1/D2 and D3/D4 in the way they do. Is this something that scientists simply haven't been able to explain yet, or is there some rationale? Perhaps something to do with the photons' interactions with beam splitters or mirrors?

If you look at the ray paths, D1 and D2 can only see one slit, D3 and D4 see both. It's as simple as that.

 

[DrChinese]

You cannot detect a photon just after it has passed the slits and still expect the same photon to reach the screen. At the very least your efforts will disburb/deflecting the photons. Therefore, by attempting to detect photons at the slits, you have introduced a disturbance which perturbs your interference pattern!"

As you pointed out, this is incorrect. As a matter of fact, you can determine which slit a photon goes through and it still reach the screen. One way to do that is to place a V polarizer at one slit and an H polarizer at the other. The interference pattern will disappear. On the other hand, if both polarizers are V, there will be an interference pattern.

 

[DrChinese]

Thanks, Derek. If I understand correctly, this solves the mystery of retrocausality.

Unfortunately, it does not. It is not clear why delayed choice experiments seem to have a retrocausal component.

A different delayed choice experiment shows: you can observe perfect correlations with a pair of entangled photons (ie observe both at any angle and the result will match the entangled state statistics). The punch line is that the photons are entangled AFTER they are detected, and the photons were never in a common light cone.

http://arxiv.org/abs/quant-ph/0201134

See page 5, where the delayed choice version is discussed:

"A seemingly paradoxical situation arises — as suggested by Peres [4] — when Alice’s Bellstate analysis is delayed long after Bob’s measurements. This seems paradoxical, because Alice’s measurement projects photons 0 and 3 into an entangled state after they have been measured. Nevertheless, quantum mechanics predicts the same correlations. Remarkably, Alice is even free to choose the kind of measurement she wants to perform on photons 1 and 2. Instead of a Bell-state measurement she could also measure the polarizations of these photons individually. Thus depending on Alice’s later measurement, Bob’s earlier results either indicate that photons 0 and 3 were entangled or photons 0 and 1 and photons 2 and 3. This means that the physical interpretation of his results depends on Alice’s later decision.

"Such a delayed-choice experiment was performed by including two 10 m optical fiber delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore, this result indicate [sic] that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres [4]: this paradox does not arise if the correctness of quantum mechanics is firmly believed."

 

[Derek Potter]

Unfortunately, it does not. It is not clear why delayed choice experiments seem to have a retrocausal component.

Not clear to whom? It seems clear to me that the appearence is due to imposing definiteness on observations. Keep everything in superposition and the paradoxes go away.

I'll wade through the entanglement swapping paper some time as entanglement swapping is rather new to me and keeping track of a four-photon state is a bit taxing to my poor brain. But I'd be utterly amazed if the same approach - no collapse, not even at observation - fails here at last, having successfully accounted for EPR and DCQE.

 

[bhobba]

Not clear to whom? It seems clear to me that the appearence is due to imposing definiteness on observations. Keep everything in superposition and the paradoxes go away.

In the early days of QM Von Neumann showed that's not possible - the Von Neumann cut must occur - but can be placed anywhere.

There are outs such as MW and Consistent Histories but without one of those outs its unavoidable.

If you don't want it you must clearly state what your out is and show it actually is an out.

Thanks
Bill

 

[Derek Potter]

In the early days of QM Von Neumann showed that's not possible - the Von Neumann cut must occur - but can be placed anywhere.
There are outs such as MW and Consistent Histories but without one of those outs its unavoidable.
If you don't want it you must clearly state what your out is and show it actually is an out.

Not really. The onus is on proponents of "cut" interpretations to say why they want to have a cut at all.
All my picture says is that the cut, if any, can be placed after the idler photon has been detected and this is sufficient to avoid paradox.

 

[bhobba]

Not really. The onus is on proponents of "cut" interpretations to say why they want to have a cut at all.

I disagree. The onus is on the no cut idea to show how it gets an outcome. All decoherence does is have a mixed state. That doesn't have an outcome without some other assumption.

Consider the mixed state 1/2 |a><a| + 1/2 |b><b|. What's its outcome?

Thanks
Bill

 

[Derek Potter]

I disagree. The onus is on the no cut idea to show how it gets an outcome. All decoherence does is have a mixed state. That doesn't have an outcome without some other assumption.
Consider the mixed state 1/2 |a><a| + 1/2 |b><b|. What's its outcome?

Whoah there! I do not want to confuse the already-exploded OP's brain with a review of all conceivable classes of interpretation. There is, as you know, an ongoing thread that I started in order to nail down the outcome problem. Let's discuss it there.
You can have a quantum/classical cut if you like - just don't place it too early in the experiment.

 

[jerromyjon]

I'm also wondering why the results then differ between D1/D2 and D3/D4 in the way they do.

Do you mean the π phase shift? That is exactly what I want to know.

 

[Derek Potter]

If you look at the ray paths, D1 and D2 can only see one slit, D3 and D4 see both. It's as simple as that.

Oops, wrong numbering, it's the other way round. Sorry.

 

[Kansas_Cowboy]

If you look at the ray paths, D3 and D4 can only see one slit, D1 and D2 see both. It's as simple as that. (fixed)

But none of the detectors truly see anything. They simply detect the photons. The experiment is setup so as to not interfere with the photons at the point in which they pass through the slits. Instead, this information is provided implicitly when the idler photons bounce off the initial beam splitter to reach D3 or D4, and unavailable if they happen to pass through. Correct?

 

[Kansas_Cowboy]

Do you mean the π phase shift? That is exactly what I want to know.

So is it simply a mystery? I would be ecstatic if I could get a simple "we don't know" as an answer to my question. This would allow me to stop scratching my head and leave it to greater minds than mine to scratch theirs.

Here's a quote from one of Richard Feynman's lectures in reference to the classic double slit experiment, "We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by "explaining" how it works. We will just
tell you how it works."

Now this was a very intelligent man, but I assume much progress has been made since he died. So I must ask, all of the answers that people are providing here, all of the answers produced in scientific papers thus far...Are they essentially just explaining how it works, explaining merely what occurs in the experiment? Or do they truly solve the mystery?

 

[Derek Potter]

So is it simply a mystery?

No, it's not a mystery. jerrymyjon asked you whether you meant the fact that the D0/D1 pattern is complimentary to the D0/D2 pattern, see figs 3 and 4 of the Kim paper.. He doesn't understand why they are different but that does not mean that nobody knows! I could work out the details and show you but it's tedious and really has nothing to do with the questions you asked at the beginning.

Feynman's reference to mystery did not mean that nobody knew how to explain phenomena with quantum mechanics, it meant that nobody knew why the world is quantum. Suppose the Lizard People are actually quite benign and are making sure that the Matrix simulation that we live in is self-consistent. What constraints does that place on our experience of physics? Must it be Newton's classical world with his Laws of Motion? Or must it be the magical world of Harry Potter and Hogwarts? Or is the quantum world the only possible self-consistent one? QM clearly steps outside the paradigm of classical physics and, being physics, it doesn't find an underlying, more fundamental science to stand on. Biology has chemistry and chemistry has physics but QM is pretty well on its own. To date no-one can provide a neat and tidy underlying picture that explains why it is what it is. But we do know what it is even if we don't know why. And we have no difficulty (in principle) in explaining quantum phenomena within that framework.

 

[Derek Potter]

But none of the detectors truly see anything. They simply detect the photons.

What is the difference between seeing a slit and detecting photons from a slit?

The experiment is setup so as to not interfere with the photons at the point in which they pass through the slits. Instead, this information is provided implicitly when the idler photons bounce off the initial beam splitter to reach D3 or D4, and unavailable if they happen to pass through. Correct?

If photons behaved ballistically then you would be right. But they don't and saying they "happen to pass through" is an interpretive assumption that they do. If you make that assumption then you will run into huge problems like retrocausality and so on. Quantum mechanically the probabilty amplitude (~wave function for our purposes) both bounces off and passes through the beam splitter. As I have said, follow the possibilities through the apparatus, don't assume anything is either/or but keep it all in superposition until the final reckoning.
You can couch the "which-path" information idea in quantum mechanical terms with complete rigour. But saying that "the interference is restored when the information is erased" needs to be put in those terms otherwise it becomes yet another "Gee-wizz isn't quantum mechanics wierd?" vibe which we can do without.

 

[Kansas_Cowboy]

What is the difference between seeing a slit and detecting photons from a slit?

If photons behaved ballistically then you would be right. But they don't and saying they "happen to pass through" is an interpretive assumption that they do. If you make that assumption then you will run into huge problems like retrocausality and so on. Quantum mechanically the probabilty amplitude (~wave function for our purposes) both bounces off and passes through the beam splitter. As I have said, follow the possibilities through the apparatus, don't assume anything is either/or but keep it all in superposition until the final reckoning.
You can couch the "which-path" information idea in quantum mechanical terms with complete rigour. But saying that "the interference is restored when the information is erased" needs to be put in those terms otherwise it becomes yet another "Gee-wizz isn't quantum mechanics wierd?" vibe which we can do without.

So it's the interaction between all of the possibilities that produce interference patterns? D1 and D2 include possibilities from both slits resulting in the interference pattern, while the possibilities relevant to D3 and D4 include only one slit, which then results in the apparent particle like behavior? Is this a fair characterization?

 

[Derek Potter]

So it's the interaction between all of the possibilities that produce interference patterns? D1 and D2 include possibilities from both slits resulting in the interference pattern, while the possibilities relevant to D3 and D4 include only one slit, which then results in the apparent particle like behavior? Is this a fair characterization?

Yes though I'd query a couple of the terms you use, not because I wish to be pedantic but to avoid any possible confusion.

1 I don't think interaction is a good word to use for interference. The waves are simply passing through each other. A true interaction would mean something in one "wave" collided with something in the other producing debris or altering the paths of the waves.

2 Possibilities don't do anything except happen - possibly :) It is of course the probability amplitudes that add together (bearing in mind they are complex numbers) and actual probabilities emerge via the usual Rules of Quantum Mechanics.

Note that it's detections by D0 and D1 at the same time that shows the interference pattern: there is no interference pattern actually at D0, nor at D1, so the idea of ordinary waves is misleading. (And as an entirely different matter, it's probably wrong for massless spin-1 entities like photons, for extremely deep reasons that I have not got a clue about). However, a complex number that represents the final probability doesn't have to live in real space, it's just a number.

3 Finally, I do not agree that the two patterns, one showing interference fringes and the other not, should be characterized as wave-like and particle-like. They are both wave-like patterns, one of a wave which has passed through two slits, the other of a wave that has only passed through one. To me, the term "particle-like" suggests a small compact object bouncing around. If that happens at all, it happens in the detector and is irrelevant. Unfortunately Kim et al use these terms and it is hard to buck the trend.

 

[DrChinese]

Not clear to whom? It seems clear to me that the appearence is due to imposing definiteness on observations. Keep everything in superposition and the paradoxes go away.

I'll wade through the entanglement swapping paper some time as entanglement swapping is rather new to me and keeping track of a four-photon state is a bit taxing to my poor brain. But I'd be utterly amazed if the same approach - no collapse, not even at observation - fails here at last, having successfully accounted for EPR and DCQE.

You might take the time to learn something new. The reason I posted the link is because DCQE experiment discussions quickly devolve to issues of comprehending the details of the setup. That gets pretty complicated, and I see from recent posts that the thing is happening here.

The DCES reference is easier to follow: Bell test violation from pairs of photons which are entangled after they are detected. So the question is how can you entangle something that no longer exists? You can talk about appearances, but a little examination of the experiment will show it lays out the issues you are discussing without the baggage you are tossing around.

Or you can ignore it, that is the more common route anyway.

 

[DrChinese]

So is it simply a mystery? I would be ecstatic if I could get a simple "we don't know" as an answer to my question. This would allow me to stop scratching my head and leave it to greater minds than mine to scratch theirs.

Here's a quote from one of Richard Feynman's lectures in reference to the classic double slit experiment, "We choose to examine a phenomenon which is impossible, absolutely impossible, to explain in any classical way, and which has in it the heart of quantum mechanics. In reality, it contains the only mystery. We cannot make the mystery go away by "explaining" how it works. We will just
tell you how it works."

Now this was a very intelligent man, but I assume much progress has been made since he died. So I must ask, all of the answers that people are providing here, all of the answers produced in scientific papers thus far...Are they essentially just explaining how it works, explaining merely what occurs in the experiment? Or do they truly solve the mystery?

The mystery is as Feynman says: QM describes the results to expect, but no one can tell you much deeper than that. We know some of the things that cannot be correct, but the underlying mechanisms rate a big fat "we don't know".

 

[DrChinese]

One difference between quantum probabilities and classical probabilities is that quantum probabilities can cancel each other out (classical do not). This side of interference between paths (histories) leads to the distinctive interference patterns. Only one of the paths is actually realized (randomly as anyone can best tell), but its likelihood is a function of all of the paths.

And the probabilities are controlled by the entire context of a setup. A quantum context can have past components, future components, local components and nonlocal components. Obviously, this too is different than a classical setup. We see this in the delayed choice type experiments.

 

[Kansas_Cowboy]

1 I don't think interaction is a good word to use for interference. The waves are simply passing through each other. A true interaction would mean something in one "wave" collided with something in the other producing debris or altering the paths of the waves.

2 ...Note that it's detections by D0 and D1 at the same time that shows the interference pattern: there is no interference pattern actually at D0, nor at D1, so the idea of ordinary waves is misleading.

Alright, I'm with you on three. Just a couple things on one and two.

First off, I'm not sure of a better word to use. Perhaps it is not an interaction in the classical sense, but it is the various possibilities together which result in the interference pattern. As DrChinese wrote, "Quantum probabilities can cancel each other out" with this feature of QM resulting in the interference patterns of these types of experiments. If the probabilities had no influence on each other, then the interference patterns couldn't exist. Therefore, as per my understanding at least, they must somehow interact.

Also, could you elaborate on the bit of quote I left for #2? As I understand, D0 doesn't exhibit the regular interference pattern when you take all results into account. When you look at the results of the signal photons associated with D1 and D2 hits, then you get the interference pattern. Whereas the signal photons associated with D3 and D4 lack the interference pattern. Is this correct? Also, before, I was under the assumption that the results of D1 and D2 themselves exhibited interference patterns.

The mystery is as Feynman says: QM describes the results to expect, but no one can tell you much deeper than that. We know some of the things that cannot be correct, but the underlying mechanisms rate a big fat "we don't know".

Interesting. Do you think we will ever know?

Personally, I think it could be impossible. No matter how clever the experiment we devise, it seems that we'll merely be validating our equations. What actually happens in the interim, the period between initiation of some phase of an experiment and the results observed, the actual physical manifestation of our equations and the underlying mechanisms of such manifestations...I fear this knowledge is forever beyond the grasp of the human mind as it has evolved.

 

[jerromyjon]

As I understand, D0 doesn't exhibit the regular interference pattern when you take all results into account.

Yes, it does. Only when the entangled photon is detected by D1 or D2 then you get interference patterns at D0 from the idler photon. When D3 or D4 gets a hit AFTER the photon is detected at D0, then you do not get interference at D0. Crazy, huh?

I think that is why Derek Potter likes to refer to the photons as "seeing the path they are going to take" because it is a way to rationalize what occurs. There is no physical evidence that the photons know where they are going to hit, only the logical implication that is contradicted by normal reality.

 

[Derek Potter]

You might take the time to learn something new. The reason I posted the link is because DCQE experiment discussions quickly devolve to issues of comprehending the details of the setup. That gets pretty complicated, and I see from recent posts that the thing is happening here.

That is what I said. I will take the time to learn about entanglement swapping. It it's what it looks like then the experiment is much simpler to describe than the Kim setup but I'm not going to be pressured into attempting an explanation until I am sure exactly what it is that I'm explaining.

The DCES reference is easier to follow: Bell test violation from pairs of photons which are entangled after they are detected. So the question is how can you entangle something that no longer exists?

Clearly you cannot if entanglement is a physical state that is mysteriously imposed on the photons. Clearly there is no difficulty at all if it is merely a correlation which is brought about by Alice telling Victor which pairs are to be considered to be correlated.

You can talk about appearances, but a little examination of the experiment will show it lays out the issues you are discussing without the baggage you are tossing around.

I have no idea what you mean by tossing baggage around. I travel light.

Or you can ignore it, that is the more common route anyway.

Yes, ignoring a problem is often much easier than resolving it. But we don't want to do that. Do we?

 

[jerromyjon]

I have no idea what you mean by tossing baggage around. I travel light.

Amen to that!

Yes, ignoring a problem is often much easier than resolving it. But we don't want to do that. Do we?

Depends who you ask I imagine. You seem like me, committed to finding an acceptable understanding why things are, rather than being content with simply how they are.

 

[Derek Potter]

Alright, I'm with you on three. Just a couple things on one and two.

First off, I'm not sure of a better word to use. Perhaps it is not an interaction in the classical sense, but it is the various possibilities together which result in the interference pattern. As DrChinese wrote, "Quantum probabilities can cancel each other out" with this feature of QM resulting in the interference patterns of these types of experiments. If the probabilities had no influence on each other, then the interference patterns couldn't exist. Therefore, as per my understanding at least, they must somehow interact.

No, probability is given by the square of the magnitude of something called probability amplitude. Probability amplitude is a complex number. Probability amplitudes are added together like vectors (Pythagorus!) where two contributions arrive together. There is no change in the state that gives rise to the probability amplitude The result can be bigger than either or less than both. Hence interference.

Also, could you elaborate on the bit of quote I left for #2? As I understand, D0 doesn't exhibit the regular interference pattern when you take all results into account. When you look at the results of the signal photons associated with D1 and D2 hits, then you get the interference pattern. Whereas the signal photons associated with D3 and D4 lack the interference pattern. Is this correct?

Yes.

Also, before, I was under the assumption that the results of D1 and D2 themselves exhibited interference patterns.

I thought the same thing at one time too, but I was wrong.

Interesting. Do you think we will ever know?
Personally, I think it could be impossible. No matter how clever the experiment we devise, it seems that we'll merely be validating our equations. What actually happens in the interim, the period between initiation of some phase of an experiment and the results observed, the actual physical manifestation of our equations and the underlying mechanisms of such manifestations...I fear this knowledge is forever beyond the grasp of the human mind as it has evolved.

Well, don't give up. A lot has happened since I grappled with Schrodingers cat at University. My own feeling is that we are almost there. I doubt whether things will happen overnight - one day there are 20 plausible interpretations, each one a bit unsatisfactory, the next day we have one complete and obvious picture. But it's likely, in my opinion, that we will very soon be able to constrain what form viable interpretations may take far tighter than the present free-for-all. I don't want to derail this thread by discussing it though, there's a lot to it. On the subject of which, a certain amount of discussion about "what goes on", i.e. interpretation, is tolerated on this forum, but generally anything that smells of philosophy is anathema and will get a thread terminated in less time than it takes for Schrodinger's cat to decohere.

 

[DrChinese]

Clearly you cannot if entanglement is a physical state that is mysteriously imposed on the photons. Clearly there is no difficulty at all if it is merely a correlation which is brought about by Alice telling Victor which pairs are to be considered to be correlated.

In QM, entanglement is a state. Is it "physical"? Hmmm. I guess it would call it physical but I get that you might interpret differently. No problem there.

Yes, it could be "merely" correlated to something, but remember that these are perfect correlations - a far more difficult measurement state to have occur. For there to be a correlation (in this case), there should definitely be a common cause somewhere. What is the common cause? Presumably, the cause should precede the perfect correlation. But that is not a requirement of QM, only of classical correlation.

Alice's bell state measurement is done after the other 2 photons are measured. Or before, doesn't matter. The other 2 photons can exist at any point in space time, including points nonlocal and/or non-overlapping in time with each other. And you get to decide whether to instruct Alice to even perform the bell state measurement, and when to do it. Alice has no way to even know what the outcome of the other measurement is (or will be, as the case may be). So the perfect correlations of processes outside of each others' light cones have a common cause of... what exactly?

And even if there is FTL physical collapse: what is collapsing, and when and where? And why would there be a perfect correlation between separately collapsed photons? So yes, I think there is plenty of mystery here.

 

[quantumfunction]

My mind was blown upon discovering this experiment. Subsequent attempts at putting my brain back together have all failed miserably. All posts trying to demystify the experiment have either appeared flawed or were too complex for my primitive liberal arts brain to understand. Now I fear such brain contusions may simply be a side effect of studying quantum physics in general, but I'm hoping you guys can help me out anyway.

I think that seems to be a problem in some cases. I think people start with the premise "This can't be true." So there must be an explanation that fits what they believe to be true. So they "demystify" these things but I don't see how that's possible.

What this experiment seems to be saying on it's face is that particles "know" what the position of other particles will be even in the future. So the particle at D0 behaves as a wave if the entangled pair hits D1 or D2 even after the particle has hit D0. If the particle hits D3 or D4 then which path information is known and the particle at D0 behaves like a particle.

So the obvious question is how does the particle at D0 know when which path information can or can't be known? I don't think it's something that needs to be "demystified" because that just means a person believes the experiment can't mean what it means so we have to explain it away or say it's too complicated to understand.

The same thing happens with Entanglement swapping delayed choice experiment.

The vertical axis represents time, moving into the future as you go up. They start with two pairs of entangled photons, which are sent into optical fibers. Two of these (one from each pair) go directly to detectors that record their polarizations roughly 35 ns after they were produced. The other two go into very long fibers, and are sent to a detector that either records the two original polarzations, or makes a joint measurement of the two together. If they measure the individual polarizations, the original pairs remain independent of one another, but if they make a joint measurement of the two, that entangles their states, meaning that the polarizations of the othertwo photons are now entangled with each other, and should be correlated.

Since these photons went into much longer fibers (104m vs. 7m), though, the entangling measurement is made after the two photons whose states are being entangled have had their polarizations measured– about 520 ns after they were produced.

In keeping with the silly jargon of the field, the two photons that are detected immediately (Photons 1 and 4) go to detectors that are imagined to be held by people named “Alice” and “Bob.” The two that are measured together to determine the entanglement (Photons 2 and 3) go to a third imaginary person named “Victor,” and it’s Victor’s measurement that determines everything.

So how did Alice and Bob's particles "know" whether Victor would entangle/not entangle after their particles have already been measured? Why isn't Alice and Bob's measurement independent of Victor's choice to entangle/not entangle?

These are the obvious questions on it's face that can't be easily explained away because in the end it really depends on which interpretation of QM you choose to accept.

Here's a couple of links:

http://scienceblogs.com/principles/2012/05/04/entangled-in-the-past-experime/

http://arstechnica.com/science/2012...cts-results-of-measurements-taken-beforehand/

This goes to things like is the wave function real or not. There's some Scientist even saying the wave function is real but a NON PHYSICAL Reality.

The wave-function is real but nonphysical: A view from counterfactual quantum cryptography

Counterfactual quantum cryptography (CQC) is used here as a tool to assess the status of the quantum state: Is it real/ontic (an objective state of Nature) or epistemic (a state of the observer's knowledge)? In contrast to recent approaches to wave function ontology, that are based on realist models of quantum theory, here we recast the question as a problem of communication between a sender (Bob), who uses interaction-free measurements, and a receiver (Alice), who observes an interference pattern in a Mach-Zehnder set-up. An advantage of our approach is that it allows us to define the concept of "physical", apart from "real". In instances of counterfactual quantum communication, reality is ascribed to the interaction-freely measured wave function (ψ) because Alice deterministically infers Bob's measurement. On the other hand, ψ does not correspond to the physical transmission of a particle because it produced no detection on Bob's apparatus. We therefore conclude that the wave function in this case (and by extension, generally) is real, but not physical. Characteristically for classical phenomena, the reality and physicality of objects are equivalent, whereas for quantum phenomena, the former is strictly weaker. As a concrete application of this idea, the nonphysical reality of the wavefunction is shown to be the basic nonclassical phenomenon that underlies the security of CQC.

http://arxiv.org/abs/1311.7127

There were other Scientist who also reached similar conclusions.

I think it all comes back to MWI and Everett's postulate on isolated systems. There's a global superposition of all possible states so you can't look at any observable state in this system as having an independent classical existence.

I think there's less need to try and "demystify" these things with MWI.

 

[Derek Potter]

In QM, entanglement is a state. Is it "physical"? Hmmm. I guess it would call it physical but I get that you might interpret differently. No problem there.

Entanglement can, of course, be described as a non-separable wavefunction. But a non-separable wavefunction can always be expanded as a superposition of separable states. I actually asked about this on this forum as I wasn't quite sure about the "always", but it turns out it's a theorem. Since superposition is a consequence of linearity, if we assume that it persists, there is no need to posulate a mysterious physical tie between the particles. To be quite clear about this, Bartlemann's socks are perfectly correlated with classical correlation. They were made as a pair and sent on their way. Alice and Bob's photons were made as a pair too, and were sent on their way. The difference is, the socks were sent as a mixed state |red>|blue> OR |blue>|red> but the photons are created (because of spatial symmetry) as a superposition of |H>|V> AND |V>|H>. |H> and |V> span the same state space as |a> and |a+90>, where a is an arbitrary angle, so the EPR correlation is maintained for just as long as the superposition does not collapse or otherwise decohere.

Yes, it could be "merely" correlated to something, but remember that these are perfect correlations - a far more difficult measurement state to have occur. For there to be a correlation (in this case), there should definitely be a common cause somewhere. What is the common cause? Presumably, the cause should precede the perfect correlation. But that is not a requirement of QM, only of classical correlation.
Alice's bell state measurement is done after the other 2 photons are measured. Or before, doesn't matter. The other 2 photons can exist at any point in space time, including points nonlocal and/or non-overlapping in time with each other. And you get to decide whether to instruct Alice to even perform the bell state measurement, and when to do it. Alice has no way to even know what the outcome of the other measurement is (or will be, as the case may be). So the perfect correlations of processes outside of each others' light cones have a common cause of... what exactly?

You are dragging me into discussing detail I am not sure of! Still, I am willing to bet that Victor cannot determine the entanglement until after Alice has told him which pairs to consider entangled and Bob has told him the detection results. It's only if you insist that the photons really were physically affected that the effect precedes the cause. Since the superposition interpretation of entanglement does not need this effect there is no requirement for a cause, common or otherwise.

And even if there is FTL physical collapse: what is collapsing, and when and where?

Collapse? FTL? Such things may go on in Copenhagen but they don't exist in my universe.

And why would there be a perfect correlation between separately collapsed photons?
So yes, I think there is plenty of mystery here.

No, just the need to be consistent. Before the infamous von Neumann cut everything is superposition, neither the photons nor the detector states collapse, the correlation emerges as an improper mixed state. After the cut you can treat it as a proper mixture. I would say "FAPP" but bhobba would probably say "because we can't tell the difference" - there is a subtle difference in the ontology. Your argument hinges on placing the cut at the first detection, effectively denying that the superposition persists and insisting on a collapse.

 

[Derek Potter]

I think there's less need to try and "demystify" these things with MWI.

In all the examples I know about, an MWI description of the system predicts the results without the slightest mystery. But there's no need to go to full-blown MWI, all that you need to do is make sure you place the "infamous" von Neumann cut late enough in the experiment that superposition can work its magic. You can even add a handful of particles, say 10⁹³ of them, bobbing along on the wavefunction if you want to get rid of all those dead cats, I mean all those embarrassing other worlds.