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

[studiot]

1 hour ago, bangstrom said:

Quote from wiki-

“ Significance"

"This result is similar to that of the double-slit experiment, since interference is observed when it is not known from which slit the photon originates, while no interference is observed when the path is known.”

 

Wiki is not always accurate.

Photons do not 'originate' from slits.

 

[bangstrom]

1 hour ago, studiot said:

 

Wiki is not always accurate.

Photons do not 'originate' from slits.

 

The question is, Why is there no interference?

[swansont]

2 hours ago, studiot said:

 

Wiki is not always accurate.

Photons do not 'originate' from slits.

 

But the experiment behaves as if they do

4 hours ago, bangstrom said:

If you prefer, here is something more Socratic.

You posted a picture of two pages from a book. On page 358 “Polarized Light” it states:

"(1) Two beams of light plane-polarized in mutually perpendicular planes do not produce interference fringes under any condition."

Why do they not produce interference?

 

Because the polarizations do not cancel each other. You get more light, with both polarization states present.

 

 

1 hour ago, bangstrom said:

The question is, Why is there no interference?

It behaves like a point source, where there is no opportunity for interference.

[studiot]

47 minutes ago, swansont said:

But the experiment behaves as if they do

I don't agree.

If that were the case then you could remove everything before the slit barrier and 'photons' would still 'originate' from the slits.

[swansont]

14 minutes ago, studiot said:

I don't agree.

If that were the case then you could remove everything before the slit barrier and 'photons' would still 'originate' from the slits.

Well, the only thing before the slits in the double-slit experiment is the photon source, so I'm not understanding your objection.

[studiot]

19 minutes ago, swansont said:

Well, the only thing before the slits in the double-slit experiment is the photon source, so I'm not understanding your objection.

When I said everything I included the source.

If you are going to propose a model where photon 'bullets' are fired through the slots then you have to demonstrate the complete path from start of the bullet to impact with the detector.

The Wiki article was just poorly worded. The objectional word is originate, not appear or 'behaves as if'.

WE all do it and bangstrom has already pointed out that my textbook has also done it in talking about polarisation.

However with smart alec way every response is being presented, I don't feel inclined to continue developing the discussion further.

[swansont]

1 hour ago, studiot said:

When I said everything I included the source.

And part of what I don't understand. You need a photon source.

 

1 hour ago, studiot said:

If you are going to propose a model where photon 'bullets' are fired through the slots then you have to demonstrate the complete path from start of the bullet to impact with the detector.

I proposed no such model.

1 hour ago, studiot said:

The Wiki article was just poorly worded. The objectional word is originate, not appear or 'behaves as if'.

Which is consistent with Huygens' principle, an early model of double-slit interference

 

[studiot]

15 minutes ago, swansont said:

I proposed no such model.

I didn't say you did.

The model was proposed (and dismissed) by the OP, then reintroduced by the OP in the form of a quote from a Wiki article.

16 minutes ago, swansont said:

 

And part of what I don't understand. You need a photon source.

Do photons not originate at a photon source?

Can they actually originate anywhere else ?

I repeat a slit is not a photon source. If it were you could remove any other photon source from the experiment.

I also note that most sources have some form of conditioning device(s) to focus or spread or otherwise produce the desired light distribution pattern.

20 minutes ago, swansont said:

Which is consistent with Huygens' principle, an early model of double-slit interference

Which is consistent with the wave model, not the photon model and does indeed make the space between the slit walls into new sources.

But it does not say that photons constantly disappear and reappear as new photons as the light progresses.

But I'm sure you know all this.

[swansont]

1 hour ago, studiot said:

I didn't say you did.

You used "you" in a response to a quote of my statement. Forgive me for drawing the obvious conclusion.

 

1 hour ago, studiot said:

The model was proposed (and dismissed) by the OP, then reintroduced by the OP in the form of a quote from a Wiki article.

Do photons not originate at a photon source?

Can they actually originate anywhere else ?

I repeat a slit is not a photon source. If it were you could remove any other photon source from the experiment.

I also note that most sources have some form of conditioning device(s) to focus or spread or otherwise produce the desired light distribution pattern.

Which is consistent with the wave model, not the photon model and does indeed make the space between the slit walls into new sources.

But it does not say that photons constantly disappear and reappear as new photons as the light progresses.

But I'm sure you know all this.

 

I didn't say that a slit is a photon source, I said the double-slit experiment behaves as if it were.  You continue to "rebut" statements I did not make.

If you want to explain how two point sources of light will not interfere, feel free to do so, but absent that I think you have misconstrued my statements and/or are reading too much into them.

[studiot]

49 minutes ago, swansont said:

You used "you" in a response to a quote of my statement. Forgive me for drawing the obvious conclusion.

 

 

I didn't say that a slit is a photon source, I said the double-slit experiment behaves as if it were.  You continue to "rebut" statements I did not make.

If you want to explain how two point sources of light will not interfere, feel free to do so, but absent that I think you have misconstrued my statements and/or are reading too much into them.

I'm sorry for that misunderstanding. Please substitute the impersonal 'one' for 'you'.

I am also aware that appearances can be deceptive and that images in mirrors are virtual and so on.

But all of that is a digression form, the emission of one single photon and how or if that might be described as a plane wave over the entire approach space to the slit wall.

The OP keeps mixing up what happens with many photons (ie a full wave) and a single photon,

and keeps introducing esoteric experiments that only occur under highly specialized conditions not as special cases, but as general theory.

Further the OP says he favours the wave theory but does not seem able to correctly describe the wave or what usually actually happens so I have been trying to start at the simplest and build up to more complicated effects one change at a time.

 

 

 

[bangstrom]

45 minutes ago, studiot said:

 

The OP keeps mixing up what happens with many photons (ie a full wave) and a single photon,

and keeps introducing esoteric experiments that only occur under highly specialized conditions not as special cases, but as general theory.

Further the OP says he favours the wave theory but does not seem able to correctly describe the wave or what usually actually happens so I have been trying to start at the simplest and build up to more complicated effects one change at a time.

The question remains unchanged and it pertains to the Fresnel-Arago statement (1) from your book and I favor the wave theory as described by Fresnel and Arago as the most valid of the two possibilities that I listed but I am asking about your answer or that of anyone else who might have an opinion.

Here is a repeat of the question. 

You posted a picture of two pages from a book. On page 358 “Polarized Light” it states:

"(1) Two beams of light plane-polarized in mutually perpendicular planes do not produce interference fringes under any condition."

Why do they not produce interference?

 

[swansont]

1 hour ago, bangstrom said:

The question remains unchanged and it pertains to the Fresnel-Arago statement (1) from your book and I favor the wave theory as described by Fresnel and Arago as the most valid of the two possibilities that I listed but I am asking about your answer or that of anyone else who might have an opinion.

Here is a repeat of the question. 

You posted a picture of two pages from a book. On page 358 “Polarized Light” it states:

"(1) Two beams of light plane-polarized in mutually perpendicular planes do not produce interference fringes under any condition."

Why do they not produce interference?

 

Here is a repeat of the answer I posted ~7 hours ago:

Because the polarizations do not cancel each other. You get more light, with both polarization states present.

added: it might help to think about what's happening with the electric field

[studiot]

4 hours ago, bangstrom said:

Why do they not produce interference?

 

3 hours ago, swansont said:

Because the polarizations do not cancel each other. You get more light, with both polarization states present.

added: it might help to think about what's happening with the electric field

 

Jus to add to swansont's answer.

@bangstromWhy do you not answer my questions ?

 

Fresnel - Arago's work was between 1800 and about 1817.

At this time interference and diffraction were known and partly studied phenomena.
This was enough to lead to a simple scalar wave theory, but not enough to explain polarisation, which was known but less well understood.

Basically results were all empirical at that time.

An explanation of polarisation and its actions did not arrive until Maxwell published his vector theory of light in 1865.

As swansont says, this was based on the vector electric field being the leading actor.
It should be noted that this is not the vector associated with the scalar wave theory of Huygens.

The electric field can be plane polarised (do you understand what this means  - please answer this time).

Since in 3 dimensions two planes can exist at right angles and a vector in one plane has zero component at right angles to its own plane and therefore cannot affect a vector in the other plane at right angles,
Two such vectors cannot interfere.

I was going to draw some diagrams to show how this polarisation works and how it can lead to circular polarisation but you may have seen such diagrams ?

Polarisation is therefore a good way to obtain light as a plane wave, as needed for the slits experiment.

[bangstrom]

7 hours ago, swansont said:

Here is a repeat of the answer I posted ~7 hours ago:

Because the polarizations do not cancel each other. You get more light, with both polarization states present.

added: it might help to think about what's happening with the electric field

That explanation works for me.

3 hours ago, studiot said:

@bangstromWhy do you not answer my questions ?

I did answer your questions in the post directly following the one where you posted the pictures from the book citing Fresnel and Arago.

In retrospect, I would change my answer about there being no such thing as planar light waves since the term “planar” includes light waves in parallel. So my answers are that there should be a Poisson dot in all three cases.

3 hours ago, studiot said:

Fresnel - Arago's work was between 1800 and about 1817.

Much was known about polarization in the 1800-1817 years beginning with the work of Enne Malus.

3 hours ago, studiot said:

The electric field can be plane polarised (do you understand what this means  - please answer this time).

Please read my answer this time- The implication is that an electric current can be used to slow the electrical plane of a transverse light wave as it travels through a transparent material to induce variable amounts of circular polarization. Or it can be used to rotate linearly polarized light. One gadget that can do this is called a Pockel’s cell. This has a number of practical uses such as modulating a light signal.

3 hours ago, studiot said:

Since in 3 dimensions two planes can exist at right angles and a vector in one plane has zero component at right angles to its own plane and therefore cannot affect a vector in the other plane at right angles,
Two such vectors cannot interfere.

 That answer together with "swansot's" works for me.

3 hours ago, studiot said:

I was going to draw some diagrams to show how this polarisation works and how it can lead to circular polarisation but you may have seen such diagrams ?

I have probably seen such diagrams but if you have a diagram showing how light passes through a nonlinear, birefringent crystal that would be helpful.

It is my observation that none of the conclusions offered so far about why orthogonally polarized light beams do not interfere find application to explain the workings of the double slit experiment. The explanation favored by all experimenters is the one proposed by Scully and Druhl that the availability which-way information destroys interference.

Here is a quote from wiki which is the typical of explanations found elsewhere.

"The quantum eraser experiment is a variation of Thomas Young's classic double-slit experiment. It establishes that when action is taken to determine which of 2 slits a photon has passed through, the photon cannot interfere with itself. When a stream of photons is marked in this way, then the interference fringes characteristic of the Young experiment will not be seen. The experiment also creates situations in which a photon that has been "marked" to reveal through which slit it has passed can later be "unmarked." A photon that has been "marked" cannot interfere with itself and will not produce fringe patterns, but a photon that has been "marked" and then "unmarked" will interfere with itself and produce the fringes characteristic of Young's experiment.[1]"

The Scully-Druhl hypothesis is supported by observations but I don’t find that it explains what is happening at the physical level and it suggests that the knowledge of the experimenter is instrumental in the outcome of an experiment. That should be hard to accept but it has come to be the popular explanation.

[Markus Hanke]

7 hours ago, bangstrom said:

it suggests that the knowledge of the experimenter is instrumental in the outcome of an experiment

That’s not quite right - it’s about the availability of which-way information, not its being known. If such information is accessible, even just in principle, then no interference is seen. This is irrespective of whether or not there is a conscious observer who actually knows the information, so the observer does not alter the outcome of the experiment in a causal-mechanical sense.

Ultimately the issue is information-theoretic - some degrees of freedom of the entire (!) spacetime region representing the experimental setup become entangled with some degrees of freedom of the observer, wherein ‘observer’ just means any part of the environment external to the double slit (no sentience, sapience, or intentionality is necessary - just a screen is enough). It is precisely that - which degrees of freedom become entangled - that determine what ‘mix’ of particle and wave aspects of the quantum system becomes apparent to the observer. If you put your observer (=detector) directly into a slit, you end up with a different set of degrees of freedom as opposed to when the screen is far away, so it is not surprising that the pattern on the screen is not the same. You’re simply dealing with different information.

[studiot]

8 hours ago, bangstrom said:

Please read my answer this time- The implication is that an electric current can be used to slow the electrical plane of a transverse light wave as it travels through a transparent material to induce variable amounts of circular polarization. Or it can be used to rotate linearly polarized light. One gadget that can do this is called a Pockel’s cell. This has a number of practical uses such as modulating a light signal.

 

Like all your posts I have read all your answers and responded directly to them.

What you have written here is just plain rubbish

You cannot slow a light wave, or any part of it.

Electrical planes, whatever they are, do not travel through anything.

 

8 hours ago, bangstrom said:

Much was known about polarization in the 1800-1817 years beginning with the work of Enne Malus.

Yes, elementary polarisation phenomena have been known since antiquity.

But the important point, as I already said, was that knowledge at the time of Fresnel and Arago was insufficient to explain polarisation.

 

8 hours ago, bangstrom said:

It is my observation that none of the conclusions offered so far about why orthogonally polarized light beams do not interfere find application to explain the workings of the double slit experiment. The explanation favored by all experimenters is the one proposed by Scully and Druhl that the availability which-way information destroys interference.

I never said they did.

 

Here is your quote from Wiki again.

Quote

Wikipedia

A delayed-choice quantum eraser experiment, first performed by Yoon-Ho Kim, R. Yu, S. P. Kulik, Y. H. Shih and Marlan O. Scully,^[1] and reported in early 1999, is an elaboration on the quantum eraser experiment that incorporates concepts considered in Wheeler's delayed-choice experiment. The experiment was designed to investigate peculiar consequences of the well-known double-slit experiment in quantum mechanics, as well as the consequences of quantum entanglement.

The delayed-choice quantum eraser experiment investigates a paradox. If a photon manifests itself as though it had come by a single path to the detector, then "common sense" (which Wheeler and others challenge) says that it must have entered the double-slit device as a particle. If a photon manifests itself as though it had come by two indistinguishable paths, then it must have entered the double-slit device as a wave. If the experimental apparatus is changed while the photon is in mid‑flight, then the photon should reverse its original "decision" as to whether to be a wave or a particle. Wheeler pointed out that when these assumptions are applied to a device of interstellar dimensions, a last-minute decision made on Earth on how to observe a photon could alter a decision made millions or even billions of years ago.

While delayed-choice experiments have confirmed the seeming ability of measurements made on photons in the present to alter events occurring in the past, this requires a non-standard view of quantum mechanics. If a photon in flight is interpreted as being in a so-called "superposition of states", i.e. if it is interpreted as something that has the potentiality to manifest as a particle or wave, but during its time in flight is neither, then there is no time paradox. This is the standard view, and recent experiments have supported it.^{[clarification needed][2][3]}

All this stuff is based on assumptions there is no need to make.

And all the history of the development of our understanding of the phenomenon we call 'light' bears this out.

We have been through several cycles of

1) We have observed some usually common characteristic or property and hypothesised explanations/mechanisms for these properties within existing knowledge and theory.

2) Someone then discovers isolated incidents where light behaves differently in some way.

3) We are then faced with extending our theory or even radically revising it.

4) But light still continues to follow the previous set of behaviours so continues to act in that way if called upon to do so.

5) And we do not need to abandon the earlier models if they are satisfactory and easier to calculate for the circumstances they were originally conceived. So, for instance we still use rays in geometric optics for most work.

 

So this brings us to the Wiki statements which try to force us to make either-or choices about our hypotheses.

There is no need to invoke rewriting the past or make a wave/particle choice if we simply say that light is a complicated phenomenon which has the capability to interact in composite-like, component-separable-like, flow-like, particle-like, ray-like, wave-like and now sometimes some other manner yet to be fully understood.

In fact it is so complicated that I have probably missed some important behaviours off my list.

 

7 hours ago, bangstrom said:

I did answer your questions in the post directly following the one where you posted the pictures from the book citing Fresnel and Arago.

In retrospect, I would change my answer about there being no such thing as planar light waves since the term “planar” includes light waves in parallel. So my answers are that there should be a Poisson dot in all three cases.

You have indeed answered at least one of the many questions I asked, but you have not addressed all of the points in any of my post.

This is regretable since all my points have been either directly aimed at answering or discussing a point you had previously made or adding something I considered germaine to that discussion.

Since you clearly do not consider any of these of value I will leave it at that.

[swansont]

10 hours ago, bangstrom said:

It is my observation that none of the conclusions offered so far about why orthogonally polarized light beams do not interfere find application to explain the workings of the double slit experiment.

Because it’s not a polarization issue. If you’re using a common source of light for the double slit, you will have the same polarization. Orthogonal polarization doesn’t enter into the discussion. It’s a red herring.

[bangstrom]

12 hours ago, Markus Hanke said:

That’s not quite right - it’s about the availability of which-way information, not its being known. If such information is accessible, even just in principle, then no interference is seen. This is irrespective of whether or not there is a conscious observer who actually knows the information, so the observer does not alter the outcome of the experiment in a causal-mechanical sense.

We may be saying the same thing. If the experimental setup is such that it provides availability of which-way information, the results will show no interference. If the setup does not provide availability, there will be interference. The observer has nothing to do with the outcome.

The conventional explanation holds that the observer can alter the results but that is not my view. The logic behind the conventional view is that, if the experiment is run in duplicate so the outcomes should be both with and without interference, the results are found to be identical for both experiments depending upon which results the observer chooses to examine first. That is the problem.

12 hours ago, studiot said:

All this stuff is based on assumptions there is no need to make.

 

“All this stuff” refers to a quote I posted from wiki. And needless assumptions is also my position.

If I quote something from an outside source, that does not imply that I support it. I only do so to contrast my view with the usually more conventional.

12 hours ago, studiot said:

You have indeed answered at least one of the many questions I asked, but you have not addressed all of the points in any of my post.

This is regretable since all my points have been either directly aimed at answering or discussing a point you had previously made or adding something I considered germaine to that discussion.

Since you clearly do not consider any of these of value I will leave it at that.

 

Did I not answer your questions or did you not read my answers? Tell me what I did not answer.

12 hours ago, studiot said:

What you have written here is just plain rubbish

You cannot slow a light wave, or any part of it.

Electrical planes, whatever they are, do not travel through anything.

 

I am sure you know light is slowed while passing through an optically dense material. If that material is a nonlinear crystal such as calcite, the electro plane will be slowed more upon entry than the magnetic plane giving the wave the properties of a spiral. This is circular polarization.

10 hours ago, swansont said:

Because it’s not a polarization issue. If you’re using a common source of light for the double slit, you will have the same polarization. Orthogonal polarization doesn’t enter into the discussion. It’s a red herring.

If the light is a common source and it is passed through orthogonal polarizing filters, then it becomes a matter of polarization. The discussion is about why the two beams do not interfere.

[Markus Hanke]

10 hours ago, bangstrom said:

The observer has nothing to do with the outcome.

An ‘observer’ in quantum physics is any means by which some of the available information may be accessed. In this case, it is simply a screen at a certain location. The point is, an ‘observer’ does not need to be sentient or conscious. However, different types of observers may provide access to different information, so they do have a role to play in that sense. This isn’t a causal relationship though.

[swansont]

12 hours ago, bangstrom said:

If the light is a common source and it is passed through orthogonal polarizing filters, then it becomes a matter of polarization. The discussion is about why the two beams do not interfere.

That's not present in the standard double-slit experiment, which you had asked about. If you had orthogonal polarizations in the double-slit, you would not see interference. Which is probably why people don't do this, unless they're trying to show the effect of the polarization on interference.  

[bangstrom]

11 hours ago, Markus Hanke said:

An ‘observer’ in quantum physics is any means by which some of the available information may be accessed. In this case, it is simply a screen at a certain location. The point is, an ‘observer’ does not need to be sentient or conscious. However, different types of observers may provide access to different information, so they do have a role to play in that sense. This isn’t a causal relationship though.

I agree the relationship between an observation, mechanical or otherwise, and an experiment is not casual.

10 hours ago, swansont said:

That's not present in the standard double-slit experiment, which you had asked about. If you had orthogonal polarizations in the double-slit, you would not see interference. Which is probably why people don't do this, unless they're trying to show the effect of the polarization on interference.  

The double-slit quantum-eraser experiments do involve polarizations and that is the type of experiments in question. People use polarization in the experiments as a means of "marking" the which-path information without disturbing the light paths by other means of observation. They are trying to show the causal effect between the availability of which-path information and the experimental results as described by Skully and Druhl.

The theory is that an experiment potentially has multiple outcomes before it is observed. That is not my opinion.

[swansont]

59 minutes ago, bangstrom said:

The double-slit quantum-eraser experiments do involve polarizations and that is the type of experiments in question.

But when you say double-slit experiment, it implies the basic experiment - it does not imply quantum eraser. Similarly, a quantum eraser experiment does not imply delayed choice. These are all distinct experiments and should be properly identified.

 

59 minutes ago, bangstrom said:

People use polarization in the experiments as a means of "marking" the which-path information without disturbing the light paths by other means of observation. They are trying to show the causal effect between the availability of which-path information and the experimental results as described by Skully and Druhl.

The use of polarization is used to identify the path, but not in a way that compromises the interference, as shown by the fact that you get interference when you don’t know the path.

You can do which-path experiments where the path is identified by looking at an entangled photon created before the interference. So the which-path information has nothing to do with altering polarizations in the double-slit path.

 

[studiot]

I would like to hear bangstrom's complete description of an experiment where light of one polarisation is persuaded to pass through one slit and light of the other polarisation passes through the other slit.

 

[bangstrom]

3 hours ago, studiot said:

I would like to hear bangstrom's complete description of an experiment where light of one polarisation is persuaded to pass through one slit and light of the other polarisation passes through the other slit.

I have the impression that there is a lack of understanding about what the delayed-choice quantum-eraser is or specifically which one I have in mind so I will explain that part in a later post.

First, the simplest explanation is that the light passes through both slits as a wave and directly behind the slits there is a BBO crystal. A BBO crystal is a birefringent crystal with optical properties nearly identical to calcite. The light can take one of two possible paths. It can take a direct path known as the “p” path or the slightly angled path known as the “s” path.

Either path it takes, the light emerges from the crystal as two parallel beams that are oppositely polarized both linearly and circularly. When light passes through a birefringent crystal one wavelet at a time, it can either take the p path which gives it one combination of linear and circular polarization or it can take the s path and emerge with the opposite combination of polarizations. When linear and elliptical polarizations combine they are called elliptical polarization.

The two slightly divergent paths are taken as indicative of which slit the light went through as a particle but I think a more likely explanation is that they indicate which way the light went through the BBO crystal as a wave.

The actual explanation is much more complicated because the light that enters the BBO crystal is light from a high powered UV laser and this same UV light emerges from the other side. A miniscule amount of the original light emerges as a spectrum of shorter wavelengths including a lot of IR. And a small amount of the emerging spectrum is in the red range.

The light also passes through two more birefringent crystals and two color filters to remove all but the, by now, four beams of red light. It is the only the red light that is used for the experiment and for the later determination of which-path information.

The which-path determination is made on the red light that went through the BBO crystal rather than on the UV light that went through the slits so I find the latter interpretation of which-path information questionable.

I have mentioned the main experiment I have in mind several times as the Wheeler Delayed-Choice Quantum-Eraser experiment. There are a number of variations of this experiment from simple to complex but the one that is commonly referred as The Delayed-Choice Quantum Eraser is the Yoon-Ho Kim et al experiment (1999).

I am familiar with Kim’s original article but lately the only references I have been able to find are behind a pay wall. Wikipedia does a good job of describing the experiment and has illustrations from the original article. There are also loads of videos on YouTube about the same experiment.

I would also recommend the recent videos on YouTube by Don Lincoln of Fermi Labs for a nontechnical explanation of quantum eraser experiments given from the point of view of a particle physicist. (not my wave view)

I also have a simple working model of the experiment that nearly anyone can try with commonly found materials.

 

4 hours ago, swansont said:

 You can do which-path experiments where the path is identified by looking at an entangled photon created before the interference. So the which-path information has nothing to do with altering polarizations in the double-slit path.

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.

[studiot]

8 hours ago, bangstrom said:

I have the impression that there is a lack of understanding about what the delayed-choice quantum-eraser is or specifically which one I have in mind so I will explain that part in a later post.

First, the simplest explanation is that the light passes through both slits as a wave and directly behind the slits there is a BBO crystal. A BBO crystal is a birefringent crystal with optical properties nearly identical to calcite. The light can take one of two possible paths. It can take a direct path known as the “p” path or the slightly angled path known as the “s” path.

Either path it takes, the light emerges from the crystal as two parallel beams that are oppositely polarized both linearly and circularly. When light passes through a birefringent crystal one wavelet at a time, it can either take the p path which gives it one combination of linear and circular polarization or it can take the s path and emerge with the opposite combination of polarizations. When linear and elliptical polarizations combine they are called elliptical polarization.

The two slightly divergent paths are taken as indicative of which slit the light went through as a particle but I think a more likely explanation is that they indicate which way the light went through the BBO crystal as a wave.

The actual explanation is much more complicated because the light that enters the BBO crystal is light from a high powered UV laser and this same UV light emerges from the other side. A miniscule amount of the original light emerges as a spectrum of shorter wavelengths including a lot of IR. And a small amount of the emerging spectrum is in the red range.

The light also passes through two more birefringent crystals and two color filters to remove all but the, by now, four beams of red light. It is the only the red light that is used for the experiment and for the later determination of which-path information.

The which-path determination is made on the red light that went through the BBO crystal rather than on the UV light that went through the slits so I find the latter interpretation of which-path information questionable.

I have mentioned the main experiment I have in mind several times as the Wheeler Delayed-Choice Quantum-Eraser experiment. There are a number of variations of this experiment from simple to complex but the one that is commonly referred as The Delayed-Choice Quantum Eraser is the Yoon-Ho Kim et al experiment (1999).

I am familiar with Kim’s original article but lately the only references I have been able to find are behind a pay wall. Wikipedia does a good job of describing the experiment and has illustrations from the original article. There are also loads of videos on YouTube about the same experiment.

I would also recommend the recent videos on YouTube by Don Lincoln of Fermi Labs for a nontechnical explanation of quantum eraser experiments given from the point of view of a particle physicist. (not my wave view)

I also have a simple working model of the experiment that nearly anyone can try with commonly found materials.

 

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.

 

Firstly thank you for a better description, though it is still not complete enough for dullards like me.

Have you no diagram ?

I am for instance not sure of the meaning of behind ?