Delayed choice experiment (split from Question: Does the Double Slit Experiment prove Free Will?)

[swansont]

10 hours ago, bangstrom said:

The interpretation is that if which-path information is somehow available, you don't get interference. If which-path is not available or has been destroyed you do get interference. If you do the test in duplicate with entangled particles where one test comes first without which-path information and another comes later  with which-path information, you don't get interference in either.

Yes, that’s the interpretation. Why do you keep harping on about polarization? 

[bangstrom]

7 hours ago, studiot said:

Have you no diagram ?

I mean “behind” in the usual sense of the word. The light passes through the double slit and then directly into the BBO crystal.

The diagram is complicated, as its explanation, but both can be found in thousands of places on the Internet. Wikipedia is a good place to start. Look for “delayed-choice quantum eraser” and follow the article about half way down to where it says “The experiment of Kim et al. (1999)”. That is where you can find the diagram.

5 hours ago, swansont said:

Yes, that’s the interpretation. Why do you keep harping on about polarization? 

Every interpretation I can find is about which-path information but which-path information has nothing to do with the results. It is all about polarization. All the hounds are on the wrong trail.

[studiot]

30 minutes ago, bangstrom said:

I mean “behind” in the usual sense of the word. The light passes through the double slit and then directly into the BBO crystal.

The diagram is complicated, as its explanation, but both can be found in thousands of places on the Internet. Wikipedia is a good place to start. Look for “delayed-choice quantum eraser” and follow the article about half way down to where it says “The experiment of Kim et al. (1999)”. That is where you can find the diagram.

Every interpretation I can find is about which-path information but which-path information has nothing to do with the results. It is all about polarization. All the hounds are on the wrong trail.

You keep referring me to other websites for basic information needed to understand your proposal.

This is contrary to the rules here.

References to other websites etc is OK for those who understand the proposal and wish to delve further.

Thank you for telling me where the crystal cells are placed.

That could have been said 50 posts ago when I was asking what was happening before the slits (since behind is after).

 

RE polarisation.

It could be that is because that is one of the few proprties of phtons that can be entangled.

[swansont]

1 hour ago, bangstrom said:

Every interpretation I can find is about which-path information but which-path information has nothing to do with the results. It is all about polarization. All the hounds are on the wrong trail.

I must say you’re doing a poor job of explaining this. Especially since, as I said, you can do the experiment in such a way that you don’t change the polarization in the double-slit. You need to explain how that happens.

 

[bangstrom]

4 hours ago, studiot said:

You keep referring me to other websites for basic information needed to understand your proposal.

This is contrary to the rules here.

References to other websites etc is OK for those who understand the proposal and wish to delve further.

I have no idea how much anyone here understands about delayed-choice quantum eraser experiments. Specifically, Kim et al’s experiment which has become the main reference point for discussions about the topic. From the responses I have read so far, the familiarity about the topic here is little to nothing. I could be totally wrong about that but the experiment has been well discussed and explained better than I could ever do by persons elsewhere and with ample illustrations.

It would be an enormous effort for me to reinvent the wheel and explain Kim’s experiment when so many others have already gone to the trouble. That is why I find it necessary to refer to other websites for explanations. Unfortunately, nearly all the explanations elsewhere about the quantum eraser end with, ‘We don’t know how it works.’ I assumed the Kim et al experiment would be common knowledge to someone on a quantum thread and need little explanation.

I find that Kim’s experiment can be best understood as a “mostly” classical experiment that can be broken down into individual experiments involving known properties of polarized light. That is my intention and I find it more revealing than the usual attempts to understand the experiments.

If anyone is familiar with the usual explanations, they resemble trying to follow the paths of four balls “photons” through the bumpers and flickers of a pin ball machine.

In the quantum-eraser experiment, after passing through two slits, the BBO crystal, two more birefringent crystals, two color filters, and a prism, the light emerges into the main body of the experiment as two different beams of light that are both linear and circularly polarized. This is much the same mechanism as with a modern style 3D movie projector that uses two beams of light and combinations of both linear and circular polarizations. Some interpreters of the experiment recognize that the light is linearly polarized but they don’t consider that it is circularly polarized as well and that may be a key to a better understanding.

5 hours ago, studiot said:

Thank you for telling me where the crystal cells are placed.

That could have been said 50 posts ago when I was asking what was happening before the slits (since behind is after).

You asked me what was before the slits and I said there was “nothing.” You didn’t ask what was behind the slits. But if you recall, I did mention some of what was behind the slits long ago. There is a BBO crystal and two more birefringent crystals and there is a lot more behind that. The BBO crystal is where the action takes place. Remember ? the part about spontaneous parametric down conversion SPDC.

If anyone is curious about quantum physics they should have some familiarity with Kim’s experiment. Judging by the titles of articles of all kinds of woo, even people outside of physics may not know the name of Kim’s experiment but they are aware that such an experiment has been done and they are quite familiar with its implications.

5 hours ago, studiot said:

RE polarisation.

It could be that is because that is one of the few proprties of phtons that can be entangled.

The entire body of quantum identities can be entangled but we can only choose to discover one and entanglement is lost. A single observation destroys entanglement.

 

4 hours ago, swansont said:

I must say you’re doing a poor job of explaining this. Especially since, as I said, you can do the experiment in such a way that you don’t change the polarization in the double-slit. You need to explain how that happens.

 

You must have read that into my description because I never said anything about the double slit destroying polarization.

Polarization takes place after the light passes through the double slit when the light passes either through a polarizing filter or, in Kim’s experiment, when the light passes through the BBO birefringent crystal. The BBO crystal is behind the double slit.

[swansont]

9 hours ago, bangstrom said:

You must have read that into my description because I never said anything about the double slit destroying polarization.

I didn’t, either

9 hours ago, bangstrom said:

Polarization takes place after the light passes through the double slit when the light passes either through a polarizing filter or, in Kim’s experiment, when the light passes through the BBO birefringent crystal. The BBO crystal is behind the double slit.

In that experiment. But that’s not the only way to do it. You can polarize it before, and then not do anything to the positions of the polarizers while obtaining which-path information. As I have pointed out before.

If you want to show that polarization is the culprit, you would need to explain how that is possible.

[Markus Hanke]

9 hours ago, bangstrom said:

From the responses I have read so far, the familiarity about the topic here is little to nothing.

I don’t think this is a useful comment - you cannot know to what degree people here are really familiar with quantum physics. Some of us have studied this stuff in depth and for a long time, and know that there’s a much wider context to be considered than a single, specific experimental setup. 

[bangstrom]

1 hour ago, swansont said:

In that experiment. But that’s not the only way to do it. You can polarize it before, and then not do anything to the positions of the polarizers while obtaining which-path information. As I have pointed out before.

If you want to show that polarization is the culprit, you would need to explain how that is possible.

I am sure the polarizers could be placed in front of the double slits but it would require a bit of micro-manipulation to position the polarizers just right because the slits are so tiny. That must be why it is never done that way. The diffraction pattern spreads widely behind the slits so it is much easier to position the polarizers to the rear. Plus you can see what you are doing.

It should make no difference if the polarizers are in front of the slits or behind. I think that is what you are saying. The polarization should remain the same and the which-path information should remain the same. The question is, Why should we say the which path information information destroys the diffraction pattern when Fresnel and Arago have a perfectly good explanation for why this should happen.

On the other hand, the which-path explanation sounds more than a little like new-age crystal woo so I have my doubts. The part that needs explaining is how which-path information can destroy interference.

I have personally tried the experiment with circular polarizers (behind the slits) and found that circular polarizers do not destroy interference but they do provide which-path information just as linear polarizers do. This suggests that the culprit is linear polarization rather than which-path information.

I haven’t found any information in the literature about what happens if one substitutes circular polarizers for linear polarizers but I have found some anecdotal reports that circular polarizers destroy interference so the matter is in doubt but I haven’t found any indication in Fresnel and Arago to indicate that it shouldn’t work contrary to the which-path theory.

 

1 hour ago, Markus Hanke said:

I don’t think this is a useful comment - you cannot know to what degree people here are really familiar with quantum physics. Some of us have studied this stuff in depth and for a long time, and know that there’s a much wider context to be considered than a single, specific experimental setup. 

I don’t doubt there are several here that know more about quantum physics than I but my comment is in reference to “a single specific experimental setup.” I am just surprised that the physical setup is not better known.

[studiot]

11 hours ago, bangstrom said:

16 hours ago, studiot said:

RE polarisation.

It could be that is because that is one of the few proprties of phtons that can be entangled.

The entire body of quantum identities can be entangled but we can only choose to discover one and entanglement is lost. A single observation destroys entanglement.

How would you entangle the momentum of two photons, of identical frequency ?

Edited October 30, 2021 by studiot

[swansont]

48 minutes ago, bangstrom said:

I am sure the polarizers could be placed in front of the double slits but it would require a bit of micro-manipulation to position the polarizers just right because the slits are so tiny. That must be why it is never done that way.

But it has been done that way. Walborn, Terra Cunha, Padua, and Monken PRA 65, 033818, 2002

https://laser.physics.sunysb.edu/_amarch/eraser/index.html

 

48 minutes ago, bangstrom said:

 

It should make no difference if the polarizers are in front of the slits or behind. I think that is what you are saying. The polarization should remain the same and the which-path information should remain the same.

But it doesn't. The entangled partner allows you to know which path, without doing anything to the photon going through the double slit.

48 minutes ago, bangstrom said:

The question is, Why should we say the which path information information destroys the diffraction pattern when Fresnel and Arago have a perfectly good explanation for why this should happen.

The proposal that "the two beams interfere so rapidly and in such a random manner that the light is no longer coherent."? 

Show that this is the case, then. Nothing has changed with the light, so why do the beams "interfere more rapidly"?

48 minutes ago, bangstrom said:

On the other hand, the which-path explanation sounds more than a little like new-age crystal woo so I have my doubts.

Oh, well, that's just science, right? Oh, wait, no, that’s the fallacy of argument by personal incredulity 

48 minutes ago, bangstrom said:

The part that needs explaining is how which-path information can destroy interference.

 

If the photon only goes through one slit, how can you have interference?

48 minutes ago, bangstrom said:

I have personally tried the experiment with circular polarizers (behind the slits) and found that circular polarizers do not destroy interference but they do provide which-path information just as linear polarizers do. This suggests that the culprit is linear polarization rather than which-path information.

How are they showing which path?  Where is the linear polarization in the double slit beams in the experiment I've linked to?

48 minutes ago, bangstrom said:

I haven’t found any information in the literature about what happens if one substitutes circular polarizers for linear polarizers but I have found some anecdotal reports that circular polarizers destroy interference so the matter is in doubt but I haven’t found any indication in Fresnel and Arago to indicate that it shouldn’t work contrary to the which-path theory.

Again, the experiment I've discussed uses circular polarization and shows interference.

48 minutes ago, bangstrom said:

I don’t doubt there are several here that know more about quantum physics than I but my comment is in reference to “a single specific experimental setup.” I am just surprised that the physical setup is not better known.

That's because there are multiple ways of doing the experiment, which is often the case for complex experiments, where each step might have multiple options. There are even multiple ways to do two-beam interference, without the additional parts for quantum erasure, and for delayed choice.

[b_alsept@alsept]

On 10/19/2021 at 3:14 PM, bangstrom said:

There is an interference pattern when there are no detectors of which-way information but the interference pattern is lost when the detectors are present. This makes it suspicious that the detectors are causing the loss of interference.

I agree with this. Photons that are detected anywhere along the trajectory will not make it to the screen and therefore  not contribute to the pattern. The more photons you detect (remove from the experiment) the more the pattern goes away.

5 hours ago, swansont said:

If the photon only goes through one slit, how can you have interference?

You can have single slit interference which by the way comes from the overlapping of two single edge fringe patterns. Coherent photons through a double slit create the convolution of all four edges.

[bangstrom]

8 hours ago, studiot said:

How would you entangle the momentum of two photons, of identical frequency ?

Momentum is observer and reference frame dependent and not part of a particle's identity and only the identity can be entangled. Identity includes such things as charge, spin, or polarity.

8 hours ago, swansont said:

But it has been done that way. Walborn, Terra Cunha, Padua, and Monken PRA 65, 033818, 2002

https://laser.physics.sunysb.edu/_amarch/eraser/index.html

 

 

The experiment you cited is new to me but I like the choice. It is much simpler that what I had in mind and it confirms my view that circularly polarized light makes which-path information available while continuing to produce an interference pattern.

8 hours ago, swansont said:

The proposal that "the two beams interfere so rapidly and in such a random manner that the light is no longer coherent."? 

Show that this is the case, then. Nothing has changed with the light, so why do the beams "interfere more rapidly"?

The explanation about rapid interference refers to Fresnel and Arago' first law of polarization and their explanation of how linearly polarized beams interact. It does not include circularly polarized light which is quite different. Beams of circularly polarized light do not interfere destructively as do linearly polarized light.

8 hours ago, swansont said:

If the photon only goes through one slit, how can you have interference?

When light waves passes by an obstruction, they fan out. They diffract. A single slit has two edges and the diffraction patterns from each edge interfere to produce a diffraction pattern which is characteristic of light as a wave.

8 hours ago, swansont said:

How are they showing which path?  Where is the linear polarization in the double slit beams in the experiment I've linked to?

As described by Skully and Druhl, marking the two light beams with polarization makes which-path information available and destroys interference. Marking the light paths with circular polarization does the same without destroying interference contrary to Skully and Druhl but not Fresnel and Arago.. 

The linear polarization on the p path of your experiment is lost in passage through the two quarter wave polarizers and circular polarization is imparted. Since the beams are circularly polarized even though orthogonal, they can produce interference.   

[swansont]

50 minutes ago, bangstrom said:

The experiment you cited is new to me but I like the choice. It is much simpler that what I had in mind and it confirms my view that circularly polarized light makes which-path information available while continuing to produce an interference pattern.

When the which-path is available you don’t have an interference pattern.

 

50 minutes ago, bangstrom said:

The explanation about rapid interference refers to Fresnel and Arago' first law of polarization and their explanation of how linearly polarized beams interact. It does not include circularly polarized light which is quite different. Beams of circularly polarized light do not interfere destructively as do linearly polarized light.

Which is it? If you have an interference pattern, as you acknowledge above, you have destructive interference.

 

50 minutes ago, bangstrom said:

When light waves passes by an obstruction, they fan out. They diffract. A single slit has two edges and the diffraction patterns from each edge interfere to produce a diffraction pattern which is characteristic of light as a wave.

Since this does not address what I said, I will ask again: how do you have interference?

50 minutes ago, bangstrom said:

The explanation about rapid interference refers to Fresnel and Arago' first law of polarization and their explanation of how linearly polarized beams interact.

The 1st law is “Two orthogonal, coherent linearly polarized waves cannot interfere.”

Nothing about interfering rapidly. And since we do this without linearly polarized light, why do you continue to bring it up?

[bangstrom]

3 hours ago, b_alsept@alsept said:

You can have single slit interference which by the way comes from the overlapping of two single edge fringe patterns. Coherent photons through a double slit create the convolution of all four edges

 That's right but only if the photons are acting as waves... which is always.

[bangstrom]

1 hour ago, swansont said:

When the which-path is available you don’t have an interference pattern.

When the two beams are marked with linear polarization, which-path information is available and you don’t have interference.

When the two beams are marked with circular polarization, which-path information is available and you do have interference. `The two polarizations do not act the same.

[studiot]

1 hour ago, bangstrom said:

Momentum is observer and reference frame dependent and not part of a particle's identity and only the identity can be entangled. Identity includes such things as charge, spin, or polarity.

Please define identity.

Are you stating that you can entangle photons that are in different frames ?

Surely both photons must be in the same frame to entangle them.

And if they are in the same frame they must be in the same frame and therefore subject to the same transformations, relative to any other frame.

I simply asked because you originally stated quite clearly.

22 hours ago, bangstrom said:

The entire body of quantum identities can be entangled

Are you suggesting that say position and momentum are not quantum properties or quantum identities (whatever they may be). ?

[bangstrom]

1 hour ago, swansont said:

Which is it? If you have an interference pattern, as you acknowledge above, you have destructive interference.

That’s right but with linear polarized light the interference is so rapid and random that it is no longer apparent. Also, two linear polarizers at orthogonal angles block light but circular polarizers do not.

1 hour ago, swansont said:

Since this does not address what I said, I will ask again: how do you have interference?
 

When you have waves from one edge of a single slit and waves from the other edge and they overlap, the two waves interfere. In the case of a single quantum of light energy, the waves are said to be “probability” waves and the interference builds over time- one spot at a time.

1 hour ago, swansont said:

The 1st law is “Two orthogonal, coherent linearly polarized waves cannot interfere.”

Nothing about interfering rapidly. And since we do this without linearly polarized light, why do you continue to bring it up?

The part about linearly polarized light interfering so rapidly and randomly that interference is no longer apparent comes from Fresnel and Arago’s original paper where they introduce their three laws of polarity.

The loss of interference does not happen with circularly polarized light which counters Skully and Druhl’s theory that the availability of which-path information destroys interference. In which case, which-path information is a bogus explanation. That leaves polarity as the most likely explanation for the loss of interference.

[studiot]

30 minutes ago, bangstrom said:

When you have waves from one edge of a single slit and waves from the other edge and they overlap, the two waves interfere. In the case of a single quantum of light energy, the waves are said to be “probability” waves and the interference builds over time- one spot at a time.

Probability of what exactly ?

The probabilities associated with say electrons in molecules is quite different from the probabilities associated with photon generation.

10 hours ago, swansont said:

But it has been done that way. Walborn, Terra Cunha, Padua, and Monken PRA 65, 033818, 2002

https://laser.physics.sunysb.edu/_amarch/eraser/index.html

Thank you for this link +1

 

43 minutes ago, bangstrom said:

When the two beams are marked with linear polarization, which-path information is available and you don’t have interference.

When the two beams are marked with circular polarization, which-path information is available and you do have interference. `The two polarizations do not act the same.

This theoretical statement seems to be counter to the actual experimental results reported by Walborn in swansont's link.

[swansont]

1 hour ago, bangstrom said:

When the two beams are marked with linear polarization, which-path information is available and you don’t have interference.

When the two beams are marked with circular polarization, which-path information is available and you do have interference. `The two polarizations do not act the same.

Please, please, please read the description of the experiment I posted, because this is flat-out wrong, it’s been rebutted,  and it’s getting tiresome to keep reading it.

 

1 hour ago, bangstrom said:

That’s right but with linear polarized light the interference is so rapid and random that it is no longer apparent. Also, two linear polarizers at orthogonal angles block light but circular polarizers do not.

Your last justification of this claim didn’t actually support it, so where are you getting this information? And the experiment is using circular polarization, so this doesn’t apply anyway.

 

1 hour ago, bangstrom said:

When you have waves from one edge of a single slit and waves from the other edge and they overlap, the two waves interfere. In the case of a single quantum of light energy, the waves are said to be “probability” waves and the interference builds over time- one spot at a time.

Diffraction and interference are not the same, and this has been pointed out before. With one source, there is no interference. 

1 hour ago, bangstrom said:

The part about linearly polarized light interfering so rapidly and randomly that interference is no longer apparent comes from Fresnel and Arago’s original paper where they introduce their three laws of polarity.

Then give a proper citation. Though the odds are excellent that it’s an explanation that has since been discredited. 

[bangstrom]

1 hour ago, studiot said:

Please define identity.

Q:Surely both photons must be in the same frame to entangle them.

A: Not necessarily. But for experimental purposes, they must originate from the same spot if we are to recognize them as entangled. In other words, if we have a single particle, we have no way of knowing if it is entangled elsewhere or not.

Q: And if they are in the same frame they must be in the same frame and therefore subject to the same transformations, relative to any other frame.

A: Entangled particles have no “location.” We can’t say particle A is here and B is there. The locations are said to be "indefinite" until observed. In other words, a single unobserved particle can entangle spontaneously with another particle anywhere on the same light cone. And they are subject to the conditions of their local reference frame.

Q: I simply asked because you originally stated quite clearly.

  22 hours ago, bangstrom said:

The entire body of quantum identities can be entangled

Are you suggesting that say position and momentum are not quantum properties or quantum identities (whatever they may be). ?

A: Position and momentum are much the same as location. You can’t assign position or momentum to entangled particles until you can identify which particle is where and their location is indefinite until you observe the particle.

 

 

 

 

 

 

 

[swansont]

9 minutes ago, bangstrom said:

Entangled particles have no “location.” We can’t say particle A is here and B is there

Particles in ion traps have been entangled.

”With such 'traps', atomic ions can be stored nearly indefinitely and can be localized in space to within a few nanometres”

(captured text from “ion trap entanglement” search; Blatt, R., Wineland, D. Entangled states of trapped atomic ions. Nature 453, 1008–1015 (2008). https://doi.org/10.1038/nature07125

[Markus Hanke]

12 hours ago, bangstrom said:

That's right but only if the photons are acting as waves... which is always.

Not when they are hitting the screen - they behave like particles there. They also behave like particles in other circumstances, such as eg the photoelectric effect.

11 hours ago, bangstrom said:

That leaves polarity as the most likely explanation for the loss of interference.

Apart from this being inconsistent with the specific experiment linked to by swansont, it also runs counter to double-slit type experiments done with quantum objects other than light, which don’t exhibit the property of polarisation.

As already pointed out numerous times, the observed effects are independent of the nature of the quantum object - they happen with photons, but also with electrons, C60 molecules, or any other quantum object. The common denominator is always the availability of which-path information.

[bangstrom]

18 hours ago, swansont said:

Please, please, please read the description of the experiment I posted, because this is flat-out wrong, it’s been rebutted,  and it’s getting tiresome to keep reading it.

I still suspect the experiment has a explanation in the polarity. The light reaching the moving detector Ds is both linear and circular because quarter wave polarizers QWP’s have a linear polarizer followed by a circular polarizer so the light reaching the moving detector is both linear and circular and beyond just circular as mentioned in the description.

The description doesn’t explain the thinking behind the use of the polarizers or their orientations. That is likely to be found elsewhere but the information in the article is inadequate.

18 hours ago, swansont said:

Diffraction and interference are not the same, and this has been pointed out before. With one source, there is no interference.

Diffraction is interference with bands and all.

[studiot]

18 minutes ago, bangstrom said:

Diffraction is interference with bands and all.

Oh dear oh dear oh dear

 

Have you taken nothing from this thread ?

[CharonY]

!

Moderator Note

It seems obvious that OP keeps ignoring the explanation provided to them. As such there is no apparent way forward for a proper discussion. Thread locked.