DCQE - how does/can the pattern change?

[unusualname]

Nope .. I am definitely "getting it". The principal question with delayed choice is whether or not it violates causality. My analysis above, which is not really convoluted but is rather simple once you understand it, uses the principles of quantum mechanics to show unequivocally why the answer is, "no, delayed choice does not violate causality".

Er, yeah right, that was what the argument was about all along was it, I don't think so..

Your "explanation" is zero-value, it adds nothing apart from a vacuous and unnecessary analysis of how "phases" might exist in the experiment. You might as well argue about the statistical thermal properties of the apparatus, I'm sure they could be made to seem relevant by a similar convoluted "analysis", but totally irrelevant.

 

[unusualname]

[EDIT] By the way, you characterized this as an "alternative explanation" .. what other explanations are there that are consistent with QM?

[EDIT2] Also, in what way does the explanation I gave fail to explain what happens in the experiment?

edit1: i wrote "interpretative"

edit2: you can't explain what happens in the experiment, you can calculate it and you can interpret it, that's all.

 

[San K]

Just to clarify, there is a phase-based quantum explanation of the 2007 Aspect delayed-choice experiment as well, and just as with the DCQE, it serves to dispel much of the apparent mysticism associated with the popular analysis.

The 2007 Aspect experiment can be explained using an even simpler phase-based argument than the DCQE experiments. In this case, there is only one-photon. Interaction with the first 45º-polarized photon into a superposition state, which becomes entangled with the two spatial paths in the interferometer .. one polarization state (S or P) travels down each arm. At the detector, we have two choices based on the setting of an electo-optic modulator (EOM)

1) in the open configuration, the (EOM) does not affect the two orthogonally polarized beams, and they are sent through to a Wollaston-prism which collapses the spatial-entanglement, with a 50-50 probability distribution between the two detection channels, irrespective of the phase of the interferometer.

2) in the closed configuration, the (EOM) acts like a second beamsplitter in a Mach-Zender interferometer, converting the spatially-entangled state back into a polarization-entangled state in a *phase-sensitive fashion*. That is, since the quantum state is coherent, there is a well-defined phase relationship between the two polarization components as they travel through the two arms of the interferometer. These two paths have different lengths (controlled by tilting an optic on the detection stage in this experiment), and therefore the two output beams have different phases, and interfere with each other, causing their associated polarization components to have different contributions to the final superposition, and this gives rise to the different intensities in the S & P channels after the Wollaston-prism, depending on the phase difference between the interferometer arms.

There are two important things to notice about this interpretation. First, it is inherently NON-LOCAL .. it relies on the entanglement of the photon along two spatially separated paths through the interferometer. Second, it relies on the COHERENCE of the quantum superposition throughout the apparatus. Both of those are features that are exclusive to quantum mechanics, and cannot be explained by a classical interpretation.

Another important thing to notice about the explanation is that it completely removes any question of whether there is a causality violation in the experiment. Since the photon always travels through both arms of the interferometer, there is no paradox about "which path" information being selected "after" the photon has "chosen a path". With all due respect to Prof. Wheeler, it is my opinion that those are features of a classically-based MIS-interpretation of the experiment.

I will go over your post, in detail, later. You are not saying that sub-sampling & phase are competing theories, right?...because I think they are exclusive and non-competitive...

However don't we need all three, listed below, depending upon the experiment, to explain the observations?:

sub-sampling
phase (coherence/decoherence)
non-location

not sure what you mean by the below:

With all due respect to Prof. Wheeler, it is my opinion that those are features of a classically-based MIS-interpretation of the experiment.

it's not classically based, its non-local...not sure what you mean...

 

[SpectraCat]

i wrote "interpretative"

Well, that's even worse, because it's not even correct. The analysis I gave does not rely on any interpretation, just the usual mathematics of QM, which are actually quite simple for this case.

Er, yeah right, that was what the argument was about all along was it, I don't think so..

Your "explanation" is zero-value, it adds nothing apart from a vacuous and unnecessary analysis of how "phases" might exist in the experiment. You might as well argue about the statistical thermal properties of the apparatus, I'm sure they could be made to seem relevant by a similar convoluted "analysis", but totally irrelevant.

As before, until you have something of substance to offer in response to my posts, instead of just vague dogmatic objections, then all I can do is simply stop perpetuating this and allow the readers to draw their own conclusions. I presented a fairly simple and straightforward analysis that is consistent with the experimental results and with QM, and shows why there is no reason to expect that causality is violated in this experiment, which is the primary question the experiment was designed to address. If you want to show that my explanation is "vacuous" and "zero-value", then you need to explain in detail why it fails to show that causality is not violated. Otherwise you aren't adding anything to this thread.

Also, I'll point out that it's odd that you think that a phase-based analysis is somehow irrelevant to this experiment, when the entire experiment hinges on the observation of interference, i.e. spatial coherence, i.e. an experimental phenomenon that depends explicitly on phase.

 

[San K]

Just to clarify, there is a phase-based quantum explanation of the 2007 Aspect delayed-choice experiment as well, and just as with the DCQE, it serves to dispel much of the apparent mysticism associated with the popular analysis.

The 2007 Aspect experiment can be explained using an even simpler phase-based argument than the DCQE experiments. In this case, there is only one-photon. Interaction with the first 45º-polarized photon into a superposition state, which becomes entangled with the two spatial paths in the interferometer .. one polarization state (S or P) travels down each arm. At the detector, we have two choices based on the setting of an electo-optic modulator (EOM)

1) in the open configuration, the (EOM) does not affect the two orthogonally polarized beams, and they are sent through to a Wollaston-prism which collapses the spatial-entanglement, with a 50-50 probability distribution between the two detection channels, irrespective of the phase of the interferometer.

2) in the closed configuration, the (EOM) acts like a second beamsplitter in a Mach-Zender interferometer, converting the spatially-entangled state back into a polarization-entangled state in a *phase-sensitive fashion*. That is, since the quantum state is coherent, there is a well-defined phase relationship between the two polarization components as they travel through the two arms of the interferometer. These two paths have different lengths (controlled by tilting an optic on the detection stage in this experiment), and therefore the two output beams have different phases, and interfere with each other, causing their associated polarization components to have different contributions to the final superposition, and this gives rise to the different intensities in the S & P channels after the Wollaston-prism, depending on the phase difference between the interferometer arms.

There are two important things to notice about this interpretation. First, it is inherently NON-LOCAL .. it relies on the entanglement of the photon along two spatially separated paths through the interferometer. Second, it relies on the COHERENCE of the quantum superposition throughout the apparatus. Both of those are features that are exclusive to quantum mechanics, and cannot be explained by a classical interpretation.

Another important thing to notice about the explanation is that it completely removes any question of whether there is a causality violation in the experiment. Since the photon always travels through both arms of the interferometer, there is no paradox about "which path" information being selected "after" the photon has "chosen a path". With all due respect to Prof. Wheeler, it is my opinion that those are features of a classically-based MIS-interpretation of the experiment.

SpectraCat, Please post the link to the paper, diagrams etc. I tried searching.

i found this just now...http://www.nature.com/nature/journal/v446/n7138/box/446866a_BX1.html

 

[SpectraCat]

SpectraCat, Please post the link to the paper, diagrams etc. I tried searching.

I just used the arXiv link that unusualname gave earlier: http://arxiv.org/abs/quant-ph/0610241.

 

[unusualname]

Well, that's even worse, because it's not even correct. The analysis I gave does not rely on any interpretation, just the usual mathematics of QM, which are actually quite simple for this case.
As before, until you have something of substance to offer in response to my posts, instead of just vague dogmatic objections, then all I can do is simply stop perpetuating this and allow the readers to draw their own conclusions. I presented a fairly simple and straightforward analysis that is consistent with the experimental results and with QM, and shows why there is no reason to expect that causality is violated in this experiment, which is the primary question the experiment was designed to address. If you want to show that my explanation is "vacuous" and "zero-value", then you need to explain in detail why it fails to show that causality is not violated. Otherwise you aren't adding anything to this thread.

Also, I'll point out that it's odd that you think that a phase-based analysis is somehow irrelevant to this experiment, when the entire experiment hinges on the observation of interference, i.e. spatial coherence, i.e. an experimental phenomenon that depends explicitly on phase.

Can you not understand that a phase analysis works irrespective of the delayed-choice nature of the experiment? (ie it works for an experiment without delayed choice)

A phase analysis does not explain the delayed-choice part of the experiment (or the eraser part in DCQE).

How much more simply can I state this?

 

[SpectraCat]

Can you not understand that a phase analysis works irrespective of the delayed-choice nature of the experiment? (ie it works for an experiment without delayed choice)

A phase analysis does not explain the delayed-choice part of the experiment (or the eraser part in DCQE).

How much more simply can I state this?

Apparently you are using a different definition of the word "explain" than I am accustomed to. In both cases, DCQE and normal delayed choice, the explanations Cthugha and I have put forward follow the photons from source to detector, and give well-supported reasons in terms of standard QM for each of the experimental observations. In addition, in each case these explanations show why some of the often-quoted "mysteries" about these experiments are not really new or different, but rather stem from the principal mystery of QM, namely the non-local character of quantum entanglement. How is that not explaining the experiments?

Do you not agree that one of the principal questions these experiments were designed to address is whether or not the delayed choice violates causality? Do you not see why the explanations we have put forth demonstrate that the experiments can be explained in a consistent manner that does not involve causality violation?

[EDIT] Also, would you care to provide some justification for the claims you made above? You are not an established expert or an authority on QM or physics, so therefore I do not accept the validity of the claims you are making unless you can support them somehow, particularly since Cthugha and I have given detailed reasons why the statements you make above are wrong.

 

[unusualname]

Apparently you are using a different definition of the word "explain" than I am accustomed to. In both cases, DCQE and normal delayed choice, the explanations Cthugha and I have put forward follow the photons from source to detector, and give well-supported reasons in terms of standard QM for each of the experimental observations. In addition, in each case these explanations show why some of the often-quoted "mysteries" about these experiments are not really new or different, but rather stem from the principal mystery of QM, namely the non-local character of quantum entanglement. How is that not explaining the experiments?

Do you not agree that one of the principal questions these experiments were designed to address is whether or not the delayed choice violates causality? Do you not see why the explanations we have put forth demonstrate that the experiments can be explained in a consistent manner that does not involve causality violation?

No, I don't think anyone worries about causality violation, as that clearly is impossible.

And the "non-local character" of QM doesn't just require entanglement based experiments, the bog standard wheeler delayed choice experiment doesn't have any entanglement.

QM is spectacularly non-intuitive/non-classical, and any attempt to deny this via simplistic "explanations" that aren't explaining anything is not really useful.

You might be happy to say this is easy if you just accept non-local connection between photons or whatever you mean, but the whole point is that there is no known explanation of that non-local connection.

 

[SpectraCat]

@unusualname
If your point is that these experiments don't provide any further insight into the nature of quantum entanglement, i.e. WHY or HOW the entangled states behave the way they do, then fine, I guess I can agree with that. But that was not a goal of the experiments when they were conceived an carried out. They were conceived to address foundational issues in quantum mechanics, such as whether uncertainty or complementarity is "more fundamental" (that was the quantum eraser), or whether or not it was possible to violate causality by changing the results of an experiment after the photons had already supposedly passed a "decision point". The value of analyses like the ones Cthugha and I presented is that they show that those questions really don't even arise when you simply follow the photons through the apparatus, taking proper care to deal with quantum entanglement correctly. Thus we are left with only one "mystery" that we already knew we had, namely why quantum entanglement can lead to non-local effects.

 

[unusualname]

@unusualname
If your point is that these experiments don't provide any further insight into the nature of quantum entanglement, i.e. WHY or HOW the entangled states behave the way they do, then fine, I guess I can agree with that. But that was not a goal of the experiments when they were conceived an carried out. They were conceived to address foundational issues in quantum mechanics, such as whether uncertainty or complementarity is "more fundamental" (that was the quantum eraser), or whether or not it was possible to violate causality by changing the results of an experiment after the photons had already supposedly passed a "decision point". The value of analyses like the ones Cthugha and I presented is that they show that those questions really don't even arise when you simply follow the photons through the apparatus, taking proper care to deal with quantum entanglement correctly. Thus we are left with only one "mystery" that we already knew we had, namely why quantum entanglement can lead to non-local effects.

Yeah, but I didn't see the point of analysing the DCQE from any stance that didn't emphasize its spectacular non-classical nature, especially since people were annoyingly linking to the phase analysis (over the last year) to support views that seemed pre-quantum (Not you or Cthugha's fault, but the point was that the phase analysis mistakenly enables people to conclude there is nothing "mysterious here", move on)

Anyway, as long as we aren't suggesting Maxwell's equations or similar can completely explain the delayed choice experiments then I suppose we can agree.

 

[SpectraCat]

No, I don't think anyone worries about causality violation, as that clearly is impossible.

And the "non-local character" of QM doesn't just require entanglement based experiments, the bog standard wheeler delayed choice experiment doesn't have any entanglement.

Yes, it most certainly does involve entanglement. It doesn't involve entangled photon *pairs*, but it still involves entanglement, as you would appreciate if you had even skimmed over my analysis before dismissing it. The first polarizing beam splitter causes linearly polarized photon to become spatially entangled with the two paths for the different polarization state through the interferometer. Note that this is not my personal interpretation, but is the mainstream understanding of how these kinds of polarizers work .. Stern-Gerlach magnets work in a similar way to entangle atoms with particular spin projections with spatial paths through the apparatus. Then later, in the closed configuration, that spatial entanglement is converted into a polarization entanglement, which then leads to the observed interference patterns.

QM is spectacularly non-intuitive/non-classical, and any attempt to deny this via simplistic "explanations" that aren't explaining anything is not really useful.

You might be happy to say this is easy if you just accept non-local connection between photons or whatever you mean, but the whole point is that there is no known explanation of that non-local connection.

So what was the point of these experiments? Do you think they were simply intended as complicated demonstrations of the "spectacularly non-intuitive" nature of QM? They certainly were not .. they were intended to verify non-intuitive predictions experimentally, and thus help us *better understand* how entangled systems behave. Therefore I think the kind of simplifying analyses that Cthugha and I have advocated do in fact bring value to understanding that last point.

What I don't understand about your posts is your obdurate insistence that understanding these experiments is somehow unachievable, particularly in the face of completely correct examples to the contrary. Furthermore, one of the reasons I posted the analysis of the Aspect delayed choice experiment was that you ASKED for it specifically in your post.

I should probably note that, although I came up with that explanation on my own, it is probably not the first time someone has explained it in those terms.

 

[unusualname]

Yes, it most certainly does involve entanglement. It doesn't involve entangled photon *pairs*, but it still involves entanglement, as you would appreciate if you had even skimmed over my analysis before dismissing it. The first polarizing beam splitter causes linearly polarized photon to become spatially entangled with the two paths for the different polarization state through the interferometer. Note that this is not my personal interpretation, but is the mainstream understanding of how these kinds of polarizers work .. Stern-Gerlach magnets work in a similar way to entangle atoms with particular spin projections with spatial paths through the apparatus. Then later, in the closed configuration, that spatial entanglement is converted into a polarization entanglement, which then leads to the observed interference patterns.

lol, yes but the entire universe is entangled! I'm talking about experiments that produce entangled pairs to then purposefully exploit this specific entangledness

So what was the point of these experiments? Do you think they were simply intended as complicated demonstrations of the "spectacularly non-intuitive" nature of QM? They certainly were not .. they were intended to verify non-intuitive predictions experimentally, and thus help us *better understand* how entangled systems behave. Therefore I think the kind of simplifying analyses that Cthugha and I have advocated do in fact bring value to understanding that last point.

What I don't understand about your posts is your obdurate insistence that understanding these experiments is somehow unachievable, particularly in the face of completely correct examples to the contrary. Furthermore, one of the reasons I posted the analysis of the Aspect delayed choice experiment was that you ASKED for it specifically in your post.

I should probably note that, although I came up with that explanation on my own, it is probably not the first time someone has explained it in those terms.

Yes, there is no way to understand these experiments at the moment, there is shut up and calculate or there is commitment to one of the religious interpretations

 

[SpectraCat]

Yeah, but I didn't see the point of analysing the DCQE from any stance that didn't emphasize its spectacular non-classical nature, especially since people were annoyingly linking to the phase analysis (over the last year) to support views that seemed pre-quantum (Not you or Cthugha's fault, but the point was that the phase analysis mistakenly enables people to conclude there is nothing "mysterious here", move on)

Ok .. I can understand that last part .. I definitely wouldn't want someone to mistakenly use this sort of analysis as a justification for a local realist picture of the experiments, or some kind of pseudo-scientific claptrap, because it most certainly doesn't justify such interpretations.

Anyway, as long as we aren't suggesting Maxwell's equations or similar can completely explain the delayed choice experiments then I suppose we can agree.

Good.

 

[SpectraCat]

lol, yes but the entire universe is entangled!

Hmm .. not really. Macroscopic systems are subject to rapid decoherence, and are better characterized by mixed states rather than coherent superpositions. With mixed states, there is none of the phase coherence that gives rise to all of these neat effects that we have been discussing.

I'm talking about experiments that produce entangled pairs to then purposefully exploit this specific entangledness

In that case, yes .. it was one of the goals of the Aspect delayed choice experiment to use a single, non-entangled photon, as in Wheeler's original gedanken. However there is one significant difference remaining between Aspect's experiment and Wheeler's gedanken, and that is that Aspect used polarizing beamsplitters, while the Aspect gedanken just used a normal Mach-Zender interferometer, with "normal" beamsplitters (e.g. half-silvered mirrors, or just glass plates at the proper angle). Thus in the Wheeler gedanken, there is not a clear spatial separation of the different polarization components, but rather photons of both polarizations can travel down either arm. I don't see any reason why this would make a difference in the results, and it certainly makes the experiment easier to do, but it is worth noting that the difference exists I guess.

Yes, there is no way to understand these experiments at the moment, there is shut up and calculate or there is commitment to one of the religious interpretations

Well, I don't agree completely with that (obviously) ... while I do agree that the deeper questions about the nature of entanglement remain to be answered, I think that if we simply accept the demonstrated phenomenology of entanglement, we can gain insight and even predictive power about the how photons will behave in these sorts of experiments.

 

[unusualname]

Right so now we have the sole disagreement that the entire universe is entangled.

Hmm, with gazillions of degrees of freedom the entanglement is probably not statistically noticeable, but don't mean it don't exist.

It's very hard to produce pure entangled states in all the noise of the universe, once they decohere they're entangled with all that noise, even you and me SpectraCat, we're entangled don't you know (in a statistically meaningless way)