Who is puzzled by the delayed choice?

[Ken G]

Yes, I think we very much agree, our ways of describing it are merely affected by our various perspectives and perhaps different ways of saying it might resonate with different readers, or stimulate different objections to our stance. I'm sure you cringe just as much as I do when you read a lot of what is said about DCQE (like photons that were originally determined to go through only one slit being told by some choice in a later experiment to go back in time and split and go through both slits instead, or some other such highly forced and awkward interpretation), so I think this was a good thread for you to start!

I also think you made a good point in the OP in regard to the interpretations. It seems like people go to great lengths to settle on one or other self-consistent interpretation, but all of a sudden when DCQE comes up, they toss out their favored interpretation and immediately resort to pseudo-classical kinds of language that would never even stand up to their own favored interpretation! Or, we get popularized articles that feel no need to adopt any standard interpretation, because those "weren't meant for DCQE" or some such thing. This abandoning of interpretations will always lead to paradoxical sounding results, the same thing happens with that darned cat!

 

[StandardsGuy]

...what is said about DCQE (like photons that were originally determined to go through only one slit being told by some choice in a later experiment to go back in time and split and go through both slits instead, or some other such highly forced and awkward interpretation),

You just answered your own question of post 28. As I see it, any interpretation is as awkward as any other. You may have made a good point in #28, though. Are you saying that the observation of the interference pattern (the wave) or the scatter pattern (the particle) is just the Heisenberg Uncertainty principle at work?

 

[StandardsGuy]

What do you mean by "photons", if not the wave function itself? The only consistent way (I am aware of) of talking about photons different from waves is in terms of Bohmian interpretation. But I have explained in the first post that this interpretation (number 7) does not involve any change back in time.

I mean, by my understanding of this interpretation, that the wave (of the past) taking two paths, has changed into particles taking one path. You seem to not understand that a delay means that time has passed in which the wave has already traveled some distance. Or do you think the wave is everywhere at once?

I know nothing about the Bohmian interpretation except what you posted. So it seems there is still a wave through each slit which both have to collapse, but only one particle materializes. Can you explain why that happens only when people are looking for particles?

 

[Ken G]

You just answered your own question of post 28. As I see it, any interpretation is as awkward as any other. You may have made a good point in #28, though. Are you saying that the observation of the interference pattern (the wave) or the scatter pattern (the particle) is just the Heisenberg Uncertainty principle at work?

I believe the analogy has merit, yes. But mostly what I'm saying is that the presence of interference, or what kind of interference is present, is not a physically demonstrable fact. The same pattern, or absence of pattern, can be interpreted in many ways, some involving various types of interference between the slits, others not involving any interference between the slits. Let me give you a clear example-- put a linear polarizer in each slit, with a 90 degree relative rotation. You don't get a two-slit pattern on the wall, because you have which-way information. You can say that the reason you don't get a two-slit pattern is that the orthogonal polarizations don't interfere, but is it an objective fact that no interference is occurring? No, it isn't-- each linear polarization can be thought of as a superposition of opposite circular polarizations. But each of those circular polarizations, by themselves, certainly do interfere with the same circular polarization coming from the other slit! Analyzed this way, the linear polarizers do indeed undergo two-slit interference, but it isn't apparent in the final pattern because the different interferences end up being incoherent with each other and don't result in a pattern (sound like the two correlated DCQE signals?).

And, to complete the analogy, we can recover the two-slit pattern by "erasing" the which-way information, by inserting a polarizer at 45 degrees to both of the polarizers in the slits, before the photons reach the detecting wall. Now some of the interference that was always there but wasn't showing up on the wall is allowed to show up on the wall, and the two-slit pattern appears, even though there are polarizers in the slits that, with a different setup, would provide "which way" information. This is true even if the 45 degree polarizers are a light year from the slits, and were put in place long after the light passed through the slits. The morals of the story:
1) the apparatus as a whole determines the outcome, not pieces of the apparatus, and it makes no difference at what time the elements of the apparatus were placed there, and
2) what interference does or does not happen is a function of our analysis, not the apparatuses. Any description that gets the answer right is just as valid a way to talk about what "really happened" as any other.

 

[Demystifier]

I mean, by my understanding of this interpretation, that the wave (of the past) taking two paths, has changed into particles taking one path.

I think that such interpretation of QM does not make sense.

I know nothing about the Bohmian interpretation except what you posted. So it seems there is still a wave through each slit ...

Correct.

... which both have to collapse,

Not correct. In Bohmian interpretation wave function never really collapses. It only splits into separate branches that do not know about each other.

but only one particle materializes.

Not correct. There is only one particle which takes only one path, but the particle does not "materialize". Instead, it exists all the time, irrespective of the wave function. The purpose of the wave is only to guide the MOTION of the particle, not to create the particle.

Can you explain why that happens only when people are looking for particles?

People are irrelevant. What is relevant is the measuring apparatus. The wave function that guides the particle interacts with the wave function that guides the apparatus-particles. This interaction changes the particle-guiding wave function, which affects the particle trajectory AFTER the interaction.

For more details about the Bohmian interpretation, see e.g.
http://xxx.lanl.gov/abs/quant-ph/0611032

 

[harrylin]

I believe now I finally understood the reason why so many physicists are puzzled by the delayed choice. Let me share it with you.

The origin is in the fact that such experiments are closely related to the so-called "WHICH WAY" experiments. The name of such experiments suggests that they really measure the way along which the particle travels. So, naive physicists are inclined to think that the particle chooses only one path (through only one of the slits) in these experiments, and therefore that there cannot be any interference effect. From that, it is easy to get a paradox and conclude that there must be a change-of-the-past involved.

But this, of course, is wrong. The so-called "which way" experiments do NOT really measure the way along which the particle travels. Indeed, these experiments only measure the FINAL particle position at the detector, from which the rest of the particle's "path" is RECONSTRUCTED, rather then really measured. Moreover, this reconstruction is based on a naive classical reasoning about motion of particles [..] - it is completely unjustified. [..] all these interpretations agree that there is a wave function traveling through BOTH slits (even though they disagree on what this wave function actually is), which is ultimately why the interference is possible.

To conclude, [..] They are just misleadingly called so.

Right. A few years ago I was also riddled about such "quantum eraser" experiments (I hope that that's indeed the same type of experiments but with a different label); and as I didn't find an answer I asked my office mate who had to get more credits for his PhD to choose a QM course so that he could explain it to me (he's very smart and I was lazy).
His explanation was similar to your point 4 as well as your statements here above; if I correctly recall it, we can after-the-fact select a part of the interference data, and the pattern that we then observe simply depends on our selection.
I remember having seen one paper in which this was even clear by just looking at the plots, but I forgot which one.

 

[Demystifier]

It seems like people go to great lengths to settle on one or other self-consistent interpretation, but all of a sudden when DCQE comes up, they toss out their favored interpretation and immediately resort to pseudo-classical kinds of language that would never even stand up to their own favored interpretation!

Yes, that's exactly how it looks to me too.

Even Wheeler, who made a lot to popularize and explain the many-world interpretation, apparently abandoned it when proposed the first version of a delayed choice experiment.

 

[RUTA]

Thanks, RUTA!

We just had this paper “Being, Becoming and the Undivided Universe: A Dialogue between Relational Blockworld and the Implicate Order Concerning the Unification of Relativity and Quantum Theory,” Michael Silberstein, W.M. Stuckey & Timothy McDevitt, http://arxiv.org/abs/1108.2261 accepted in Foundations of Physics. Check it tomorrow when the newest version will be posted.

As you will see therein, the RBW interpretation suggests a correction to GR with consequences on cosmological scales. We explored this as regards dark energy and produced a fit of the supernovae data (Union2 data) using an appropriately modified Regge calculus version of the Einstein-deSitter cosmology (flat, decelerating universe, no Lambda, no accelerating expansion, no dark energy). Our fit (sum of squares error = 1.77) is just as good as the Einstein-deSitter model with Lambda (SSE = 1.79, aka LCDM, reigning cosmology model in which universe has accelerating expansion). See: “Modified Regge Calculus as an Explanation of Dark Energy,” W.M. Stuckey, Timothy McDevitt & Michael Silberstein, Classical & Quantum Gravity 29 055015 (2012). http://arxiv.org/abs/1110.3973. This has obvious implications for quantum gravity.

Additionally, there are consequences for particle physics and the unification of forces. These are outlined in the FoP paper as well. In other words, RBW is not a mere interpretation of QM, but has the potential to produce new physics.

 

[StandardsGuy]

...the RBW interpretation suggests a correction to GR with consequences on cosmological scales. We explored this as regards dark energy and produced a fit of the supernovae data (Union2 data) using an appropriately modified Regge calculus version of the Einstein-deSitter cosmology (flat, decelerating universe, no Lambda, no accelerating expansion, no dark energy). Our fit (sum of squares error = 1.77) is just as good as the Einstein-deSitter model with Lambda (SSE = 1.79, aka LCDM, reigning cosmology model in which universe has accelerating expansion). See: “Modified Regge Calculus as an Explanation of Dark Energy,” W.M. Stuckey, Timothy McDevitt & Michael Silberstein, Classical & Quantum Gravity 29 055015 (2012). http://arxiv.org/abs/1110.3973. This has obvious implications for quantum gravity.

Additionally, there are consequences for particle physics and the unification of forces. These are outlined in the FoP paper as well. In other words, RBW is not a mere interpretation of QM, but has the potential to produce new physics.

I just looked through a couple PDFs by Googling. On the surface it looks like in RBW, reality exists, but time does not, (but is an illusion?) as Einstein was quoted to have said. He also said time is the fourth dimension, which contradicts the quote. Putting that aside, if time doesn't exist, what does the measurement of the speed of light mean? In a book, Paul Davies said that there is no passage of time for light, and I think he said light is everywhere at once. With this notion, there is no problem with "delayed choice."

How does RBW account for the acceleration of the expansion of the universe, or does it deny it? Two separate teams of scientists measured it separately. Can we ignore that?

 

[harrylin]

[..] we can recover the two-slit pattern by "erasing" the which-way information, by inserting a polarizer at 45 degrees to both of the polarizers in the slits, before the photons reach the detecting wall. Now some of the interference that was always there but wasn't showing up on the wall is allowed to show up on the wall, and the two-slit pattern appears [..]

Exactly - it was to that kind of observations that I alluded, thanks for saying it so clearly.

 

[Ken G]

Perhaps a good way to sum much of this up is to note that the DCQE experiment teaches us that "which way information" can either be thought of as information in the head of some physicist analyzing some specific data, or equivalently as some physical aspect of a complete apparatus that gives rise to such a dataset, but it is quite problematic to interpret it as a "physical happening" that is actually occurring to a particle at the time that it passes the slits. Since the Bohmian interpretation involves the most direct connection between the experimental outcome and "what is actually happening" to the particle as a function of time, it would be interesting to consider how that interpretation navigates this lesson. The other interpretations seem to navigate it best by viewing a particle as a kind of a priori construct that emerges holistically from the complete apparatus, and/or from the mind of the physicist.

 

[San K]

The morals of the story:
1) the apparatus as a whole determines the outcome, not pieces of the apparatus, and it makes no difference at what time the elements of the apparatus were placed there, and

not sure if I got this...

if a piece of the apparatus is placed in the path after the photon has passed (based on time = distance/C), there is no effect...so time does matter, it seems...

 

[StandardsGuy]

not sure if I got this...

if a piece of the apparatus is placed in the path after the photon has passed (based on time = distance/C), there is no effect...so time does matter, it seems...

You got it wrong, it changes it. Not only that but there is an experiment in which the interference pattern is in time, not space: http://physicsworld.com/cws/article/news/2005/mar/02/new-look-for-classic-experiment. This was in another discussion here.

Who is confused now?

 

[RUTA]

I just looked through a couple PDFs by Googling. On the surface it looks like in RBW, reality exists, but time does not, (but is an illusion?) as Einstein was quoted to have said. He also said time is the fourth dimension, which contradicts the quote. Putting that aside, if time doesn't exist, what does the measurement of the speed of light mean? In a book, Paul Davies said that there is no passage of time for light, and I think he said light is everywhere at once. With this notion, there is no problem with "delayed choice."

How does RBW account for the acceleration of the expansion of the universe, or does it deny it? Two separate teams of scientists measured it separately. Can we ignore that?

Time exists in RBW as a subset of spacetimematter.

In our Class. Quant. Grav. paper, the supernova data is explained without accelerating expansion. No one has "measured" the accelerating expansion. The time evolution of the expansion rate is read off the cosmology model that best fits mu vs z. The model that best fits this data (without changing GR) and is consistent with WMAP data is Einstein-deSitter (flat, matter-dominated) with a cosmological constant (called LambdaCDM or the concordance model). Our flat, matter-dominated, decelerating cosmology model fit the supernova data as well as LCDM and is consistent with WMAP data, but it constitutes a change to GR.

 

[StandardsGuy]

Time exists in RBW as a subset of spacetimematter.

In our Class. Quant. Grav. paper, the supernova data is explained without accelerating expansion. No one has "measured" the accelerating expansion. The time evolution of the expansion rate is read off the cosmology model that best fits mu vs z. The model that best fits this data (without changing GR) and is consistent with WMAP data is Einstein-deSitter (flat, matter-dominated) with a cosmological constant (called LambdaCDM or the concordance model). Our flat, matter-dominated, decelerating cosmology model fit the supernova data as well as LCDM and is consistent with WMAP data, but it constitutes a change to GR.

Thanks for the explanation. Could you elaborate on the change to GR?

 

[RUTA]

Thanks for the explanation. Could you elaborate on the change to GR?

We modified GR via its graphical counterpart called Regge calculus. We let the link lengths be large and modified the relationship between proper distance and luminosity distance. These changes are motivated in the FoP paper linked above.

 

[zonde]

You got it wrong, it changes it.

Your idea leads to Bell telephone. Try to defend that (that Bell telephone is possible).

Not only that but there is an experiment in which the interference pattern is in time, not space: http://physicsworld.com/cws/article/news/2005/mar/02/new-look-for-classic-experiment. This was in another discussion here.

First this experiment is about electrons but electrons move at different speeds.
Basically there are two paths leading to detection event - one is where electron is emitted at earlier time but moves slower and second is where electron is emitted later but moves faster. Two indistinguishable histories leading to the same detection event means we observe interference. Nothing magical.

Who is confused now?

You of course

 

[harrylin]

Your idea leads to Bell telephone. Try to defend that (that Bell telephone is possible).

Indeed, such ideas lead to impossibilities. The kind of thing in which time plays no role, is our selection of information that was produced by the system.

 

[StandardsGuy]

Your idea leads to Bell telephone. Try to defend that (that Bell telephone is possible).tongue:

That wasn't my idea, it was yours. Apparently you don't know what delayed choice means. Look here:http://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment

BTW, I have owned a Bell telephone for years. Do you want a picture of it?

 

[zonde]

That wasn't my idea, it was yours.

Really?

if a piece of the apparatus is placed in the path after the photon has passed (based on time = distance/C), there is no effect...

You got it wrong, it changes it.

If we place a piece of the apparatus in the path of the photon after it has passed and this produces measurable effect on photon we can construct FTL communication device.

Apparently you don't know what delayed choice means. Look here:http://en.wikipedia.org/wiki/Wheeler's_delayed_choice_experiment

And what this has to do with placing a piece of the apparatus in the path of photon after it has passed?

 

[Ken G]

not sure if I got this...

if a piece of the apparatus is placed in the path after the photon has passed (based on time = distance/C), there is no effect...so time does matter, it seems...

Yes, I don't mean you can place pieces of the apparatus after the particle has passed, I mean it doesn't matter at what time various particles pass various pieces of the apparatus. Entangled particles can encounter the apparatus at times that are widely different and still get the same correlations if their encounters were simultaneous (indeed that very issue is frame dependent). I'm referring to the fact that some people are puzzled when an entangled photon encounters some part of the apparatus outside the light cone of some other entangled photon encountering some other part of the apparatus, yet the outcomes can be correlated (as in EPR), and the correlations can be altered by making a choice of apparatus outside the light cone of some other part of the experiment (as in DCQE). The "moral" is that it doesn't matter when the particles encounter the apparatus (not that it doesn't matter if they encounter the apparatus!).

 

[San K]

Yes, I don't mean you can place pieces of the apparatus after the particle has passed, I mean it doesn't matter at what time various particles pass various pieces of the apparatus. Entangled particles can encounter the apparatus at times that are widely different and still get the same correlations if their encounters were simultaneous (indeed that very issue is frame dependent). I'm referring to the fact that some people are puzzled when an entangled photon encounters some part of the apparatus outside the light cone of some other entangled photon encountering some other part of the apparatus, yet the outcomes can be correlated (as in EPR), and the correlations can be altered by making a choice of apparatus outside the light cone of some other part of the experiment (as in DCQE). The "moral" is that it doesn't matter when the particles encounter the apparatus (not that it doesn't matter if they encounter the apparatus!).

ok...yes, agreed.

 

[StandardsGuy]

If we place a piece of the apparatus in the path of the photon after it has passed and this produces measurable effect on photon we can construct FTL communication device.

You can't measure the photon itself, but the outcome of the experiment is changed. No FTL communication is possible IMO. From my Notes: This phenomenon is known as non-locality. Einstein called it ‘spooky action at a distance.’ The Austrian Erwin Schrodinger thought that all this was ridiculous, and came up with his story of the cat in the box, which appeared in print in 1935. Albert Einstein, Boris Podolsky, and Nathan Rosen also came up with a ‘thought experiment’ about the same time. It is known as the ‘EPR paradox.’ In the mid 60’s, John Bell, an Irish physicist, found a way to express the paradox in terms of an experiment that could be carried out on pairs of photons emitted from an atom simultaneously in two different directions. Several researchers took up the challenge. The most conclusive of these experiments was carried out by Alain Aspect & colleagues in the early 1980’s. They demonstrated beyond any reasonable doubt that non-locality really does rule in the quantum world, and that Einstein and the others were wrong.

And what this has to do with placing a piece of the apparatus in the path of photon after it has passed?

OK, this isn't the experiment I thought. From my notes: In the mid 1980s, a group from the University of Maryland, and a separate group from the University of Munich carried out a version of the two holes experiment. They used a beam of laser light that went through a beam-splitting mirror. One of the split beams was phase-shifted and then the two split beams were recombined to form the interference pattern. Detectors known as Pockels cells were placed in each of the split beams to monitor the passage of photons, and detectors at the far end looked to see if it was producing interference or not. The Pockels cells could be switched on or off within 9 billionths of a second. The length of each path of light took about 15 billionths of a second for light to travel that far, so the detectors could be switched on (or off) after the light had passed the detectors. The decision of whether the cells were on or off was made by a computer at random (with no human intervention). Both groups found that the light behaved as particles or waves depending on which choice was going to be made, even though the decision had not been made when the light passed the detectors! See John Gribbin’s, book Schrodinger’s Kittens and the Search for Reality.

Would not the Pockels cells be an apparatus in the path of the photocells? If I am off base here, please explain how.

 

[Demystifier]

If we place a piece of the apparatus in the path of the photon after it has passed and this produces measurable effect on photon we can construct FTL communication device.

How exactly can we construct it (the FTL communication device)?

 

[Ken G]

Would not the Pockels cells be an apparatus in the path of the photocells? If I am off base here, please explain how.

Your notes must have left something out, because you cannot affect the outcome of an experiment by doing something to Pockels cells after the photon has passed. There are important details like what is the bandwidth of the laser, because by the HUP the inverse of the laser bandwidth tells you the precision in the time that you can say when the photon passed various places. If we shine photons one at a time, each photon will have a time-dependent wave function, and it will not be affected by changes in an apparatus that does not overlap with that wave function. If it did, it wouldn't be quantum mechanics, and we would all have heard about it.

 

[Ken G]

How exactly can we construct it (the FTL communication device)?

I can field that one, it's pretty easy. The information being communicated FTL would be what was done to the apparatus after the photon passed, since if that could affect the arriving photon, then the photon would carry that information, and since it is purported to have happened after the photon passed, it would be FTL. Hence there can never be any detectable affect on the nature of the arriving photon created by any changes in the apparatus done after the photon passed the changed apparatus part. I'm sure you realize this, so you must be asking about DCQE. But there the apparatus that is changed "later" is something that the photon that will interact with it has not yet passed. Of course, the location of the entangled paired photon at that time is both irrelevant and frame dependent.

 

[Dadface]

One answer to the question given in the title is "me".By reading through the thread I got the impression that the quantum eraser could be something interesting so I started to read up on it.The result is I am puzzled.
Allow me to ask a question which may be dopey.Is it true that no interference is ever observed at the detectors where the paths are known but interference is always observed at the other detectors where the paths are not known? If that is true then I don't see why I should be puzzled,but that in itself is puzzling.
I think what I'm really asking for is access to a well written and clear and precise description of the experimental set up including a description of the actual observations and how they are made.I have done a lot of searching and a lot of reading on this over the last couple of days but nothing has come quite up to scratch.Probably a lot of it has gone over my head.Any recommendations please?

 

[harrylin]

[..] I think what I'm really asking for is access to a well written and clear and precise description of the experimental set up including a description of the actual observations and how they are made.I have done a lot of searching and a lot of reading on this over the last couple of days but nothing has come quite up to scratch.Probably a lot of it has gone over my head.Any recommendations please?

The following may be helpful:
http://departments.colgate.edu/physics/research/Photon/Photon research/Quantumlan07/lab3eraser09.pdf
(a little disappointment: I have an earlier version of 2005 which contains more explanations but is not available anymore from their website. As a result, it has become less helpful).

Note that in view of the above discussion, in the last paragraph it would perhaps be clearer to put "we regain visible interference".

 

[Dadface]

Hello harrylin.The above article was one I had already found and read.I think it added a bit to my understanding but I am looking for something clearer and more detailed.Thank you very much for posting.

 

[Cthugha]

Is it true that no interference is ever observed at the detectors where the paths are known but interference is always observed at the other detectors where the paths are not known?

No, you never see interference patterns at the single detectors. At least not if we are talking about the delayed choice quantum eraser. The interference patterns are only present in the coincidence counts between both detectors which is one very important point.

[...] I am looking for something clearer and more detailed.

I am not sure whether that really helps, but to really understand DCQE it is necessary to understand the physics of spontaneous parametric down conversion. Some of the authors of one of the DCQE experiments have written a detailed review article on the physics and correlations in SPDC which was published in Physics Reports. The ArXiv version can be found here:http://arxiv.org/abs/1010.1236.

This is a detailed paper, but goes more into the details of SPDC and spatial correlation patterns and therefore only mentions DCQE partially. If you have some spare time this is a good introduction to get a feeling for what is important for understanding SPDC and DCQE experiments. If you, however, are more interested in a short review especially about DCQE just disregard this link.