Feynman's double-slit experiment

[mn4j]

Well, this one is stochastic and they used their setup once with and once without gate, which reduces the time between consecutive photon detections by roughly 10. This cuts it for me. You will always say that the time delay chosen in any experiment does not match your conditions. If they wait a week you can tell them to wait a year. This is pointless.

I have already given you a paper in which fringe visibility vanished when the time between photons was varied. This should not happen if self-interference is at play.

Y. Kim, M.V. Chekhova, S.P. Kulik, Y. Shih, and M.H. Rubin, “First-order interference of nonclassical light emitted spontaneously at different times,” Physical Review A, vol. 61, Apr. 2000, p. 051803.

You will also remember, that in the famous Tonomura et al paper with electrons, fringe visibility faded when emission intensity was dropped.
A. Tonomura J. Endo, T. Matsuda, T. Kawasaki, and H. Ezawa, Am. J. Phys. 57(2), 117 (1989)

 

[Cthugha]

1) coherence has no meaning in the context of a single photon, just as you can not talk of the correlation of a single variable. A Fock state is NOT a single photon.

You are mixing up a single photon and a single detection. Sure, I can speculate about what a single detection means. However, experimentally I can only verify that a single photon is present by looking at an ensemble of single photon states (Fock states) and show that antibunching is present. In this scenario coherence is a sensible concept and a single photon state is a sensible concept. Talking about single photons from one detection alone is never sensible.

2) if self-interference is happening, phase-coherence between different photons should not affect fringe visibility but it does. (see the Sillitto and Wykes paper)

Hans de Vries already gave you a good answer about what is happening in that paper.

3) Add to that, the results of the Basano and Ottonello paper (L. Basano, and P. Ottonello, Am. J. Phys 68, 245 (2000)) in which two laser sources were used with photons from each only passing through a single slit and interference was obtained. Together they raise a serious question whether self-interference is ever happening at all.

I do not see any problem with that paper. Interference fringes in this experiment occur due to indistinguishable Feynman path amplitudes, too. Due to the fixed phase relationship between the fields from the two lasers interference occurs. You just have two synchronized sources. This is roughly a similar situation to a laser. Every atom (or molecule or QD or whatever) in the active medium is a single emitter, but they are all synchronized and therefore you cannot distinguish, which atom indeed emitted the photon.

4) Then look (carefully) at the Santori paper. The results rule out the occurence of any self-interference. I know that the authors set out to measure two-photon interference but the stark absence of self-interference is telling.

But they even present the results of measuring the results of a self-interference experiment using a Michelson interferometer in figure 2b, which give a coherence time of 351 ps at most, so I do not get your point here.

All I am saying is, don't claim self-interference has been proven. It hasn't.

This is true, but trivial. Physics never proves anything in a positive manner. It produces models and rules out the ones, which are not empirically adequate. Then the easiest and most predictive model is chosen. That's why I dislike de Raedt. Although his efforts are mathematically consistent an "adaptive learning" beam splitter/interferometer does not have much predictive power and is in my opinion not the easiest model. While this is ok for the guys, who are interested in reading and publishing in Foundations of Physics and other magazines with a very theoretical and mathematical focus, nobody in "real" physics (that means someone, who has to get funding somehow ;) ) has much time to worry about that.

I have already given you a paper in which fringe visibility vanished when the time between photons was varied. This should not happen if self-interference is at play.

Y. Kim, M.V. Chekhova, S.P. Kulik, Y. Shih, and M.H. Rubin, “First-order interference of nonclassical light emitted spontaneously at different times,” Physical Review A, vol. 61, Apr. 2000, p. 051803.

Sorry if I repeat, but you misinterpret thiss experiment. You do not have an interferometer present, so you do not expect any interference effect for large delays between two pump pulses becauses you have no indistinguishable Feynman amplitudes present. You produce the interference effect by putting two pump beams on a SPDC crystal. If these pump pulses follow shortly after each other you do not know, which of the two pulses created the down-converted photon and you again have indistinguishability of two Feynman amplitudes. If you enlarge the time delay it is clear, which pump pulse produced which down-converted photon. Then there is no indistinguishability and therefore no interference.

After rereading the last few posts I am not sure you get the meaning of self interference the way it is defined. It is misunderstood very often. The famous quote by Dirac about photons interfering only with themselves ("Each photon interferes only with itself. Interference between two different photons never occurs.") does not mean that it is impossible to have such situations like that one with the two pump pulses. This quote is only understood correctly if one also understands that all photons inside one coherence volume are indistinguishable. Dirac made this statement before the real birth of quantum optics and in a time, where there were no light sources present, which could produce large intensities and a long coherence time. So, to understand his quote, you must understand that Dirac does not mean the single quantized detection event when he talks about a photon, but all quantized excitations inside one coherence volume, which are indistinguishable and therefore not different photons in the sense of Dirac. So having different sources does not automatically imply you have two- or more-photon interference present, if there is a fixed phase relationship between the several em fields present.
Pure two-photon interference is a completely different process. In the easiest example of two-photon coherence in SPDC processes you have two fields present. Each of these fields has a rapidly varying phase and therefore a short coherence time and a small coherence volume. But although each field is fluctuating rapidly, these two strong fluctuations are synchronized with each other, so that the phase difference of the two fields is well defined even for large time delays, while each of the single phases is not, which enables one to test such things like quantum erasing, HOM-interference and all the other experiments using coincidence counting.

 

[Cyrus80772]

I herd the very act of observing this test changes the outcome and the results. Can somone please tell me what they mean by "observing" when they say this. I don't understand what they mean by this, Ivs seen a few short kid movies on this but they never explain that part.

 

[cesiumfrog]

[...]they used their setup once with and once without gate, which reduces the time between consecutive photon detections by roughly 10. This cuts it for me. You will always say that the time delay chosen in any experiment does not match your conditions. If they wait a week you can tell them to wait a year. This is pointless

All I am saying is, don't claim self-interference has been proven. It hasn't. Extraordinary claims require extraordinary evidence.

mn4j,

Am.J.Phys 41 p639 (1973) shows the double slit experiment done with electrons, at such low intensity that on average the apparatus has one electron per 200 metres.

I agree with Cthugha. You say it isn't enough to prove one particle is interfering at a time, but that the experiment must also wait sufficiently between particles for your hand-wavy affection of the slits to decay away. If such a thing existed, who would be able to say even that the affection of the slits really does decay away over time? (Why do you even ask us to search for evidence of a kind that wouldn't suffice for you to reject your hypothesis?)

Much like your opinion on Bell's theorem, you are looking for loopholes to disagree with textbook QM without actually providing a fleshed out alternative hypothesis to explain why the results nonetheless match the predictions of textbook QM up to now. In this case, if you actually could prove what you are claiming then it would be so extraordinary that we would all expect it to result in textbook-rewriting (it's not a conspiracy y'know) and a Nobel prize. So why bother arguing with us? Accept the predictions of the best verified theory in physics, or go publish the iron-clad results which contradict it.

 

[Hans de Vries]

mn4j,

Am.J.Phys 41 p639 (1973) shows the double slit experiment done with electrons, at such low intensity that on average the apparatus has one electron per 200 metres.

Correct, and of course there are the beautiful single electron
interference experiments of Tonomura:

http://www.hitachi.com/rd/research/em/doubleslit.html

Also: the transition current $\bar{\varphi}_i\gamma^\mu\varphi_f$: The self interference of an
electron between its initial and final state is a cornerstone of QED.

Regards, Hans

 

[mn4j]

Correct, and of course there are the beautiful single electron
interference experiments of Tonomura:

http://www.hitachi.com/rd/research/em/doubleslit.html

Regards, Hans

Tonomura et al observed a reduction in fringe contrast when they reduced the number of electrons reaching the bi-prism per second. The authors dismissed it as systematic error without testing, and concluded that fringe contrast stayed the same. It certainly raises the question, if the electrons are only interfering with themselves, why fringe contrast should change.

Secondly, you did not answer the question I asked above

Means that both slits were always open simultaneously in
the light cone frames of the detected photons. That, is if
you would "take a picture" of the slits from the place of
the impact, at the time of the impact, then you would see
both slits open due to the difference in propagation time.

Sillitto and Wykes made sure ONLY one path was open at a given time. Therefore it is impossible for a single photon to have gone through both paths simultaneously. Are you suggesting, that the reason interference persisted is because a single photon accessed both paths at different times (non-simultaneously)?

Are you saying that the photons knew that length of the other path without passing through it? Since only one path was open when the passed, for your bolded statement above to make sense, the photons must be aware of the length of the path through which they certainly did not pass.

The following are established in that experiment
1) Only one slit was open at any instant
2) Either a single photon passed through ONLY one slit

OR
3) The photon passed through both slits at different times.

Those are the only options. Clearly you are implying (3) is what happened. Could you explain what it means for a photon to pass through the slits at different times?

 

[cesiumfrog]

Tonomura et al observed a reduction in fringe contrast when they reduced the number of electrons reaching the bi-prism per second. The authors dismissed it as systematic error without testing, and concluded that fringe contrast stayed the same. It certainly raises the question, if the electrons are only interfering with themselves, why fringe contrast should change.

The experiment was performed at the electron arrival rate of approximately 10³ electron/s in the whole field of view so that the interference fringes could be formed in a reasonable time, say, 20min. The distance from the source to the screen is 1.5 m, while the average interval of successive electrons is 150km. In addition, the length of the electron wave packet is as short as ~ 1 $\mu$m. Therefore, there is very little chance for two electrons to be present simultaneously between the source and the detector, and much less chance for two wave packets to overlap.
...
A series of similar experiments was carried out for different electron intensities ranging from 5000 to 200 elecrons/s. The contrast of the fringes obtained remains the same within experimental error of 10%. At the smaller intensity, the error often becomes large due to long exposure time, since the error originates mainly from the drift of the biprism filament.

The experimenters answered that very question: they did not correct for ordinary drift of their apparatus over very long time periods. If any doubt was justified, they would have characterised this drift (by high or medium intensity runs being repeated over long periods of time) to subtracted its effect (when combining long periods of low intensity data), but naturally in any measurement it will still remain impossible for the uncertainty ever to reach exactly zero (though finite uncertainty may inevitably constitute wiggle room for some crank to claim the whole theory is outright wrong.. not as if the mainstream community should care, since they already presume the theory is just a quantifiably outstanding approximation of that domain of nature). Can you present subsequent data from this experiment that is inconsistent with the conclusions of Tonomura, et al.?

Could you explain what it means for a photon to pass through the slits at different times?

That the light reaching the screen (at position X and time C) is a superposition of the two indistinguishable possibilities of either light from one slit (at time A) or light from the other slit (at time B, even from a different source).

 

[Hans de Vries]

Tonomura et al observed a reduction in fringe contrast when they reduced the number of electrons reaching the bi-prism per second. The authors dismissed it as systematic error without testing, and concluded that fringe contrast stayed the same. It certainly raises the question, if the electrons are only interfering with themselves, why fringe contrast should change.

Secondly, you did not answer the question I asked above

Are you saying that the photons knew that length of the other path without passing through it? Since only one path was open when the passed, for your bolded statement above to make sense, the photons must be aware of the length of the path through which they certainly did not pass.

The following are established in that experiment
1) Only one slit was open at any instant
2) Either a single photon passed through ONLY one slit

OR
3) The photon passed through both slits at different times.

Those are the only options. Clearly you are implying (3) is what happened. Could you explain what it means for a photon to pass through the slits at different times?

I see that cesiumfrog has more than adequately answered your post.
(although the c-word should be avoided)

You have to realize that (self) interference is fundamental property of
the propagation of light and matter fields. Light would not refract and
reflect the way it does without self interference.

(self) interference is what makes Huygens' principle such a useful tool.
Single photons without self interference, as you propose, would not
refract in a lens or prism, they would need other photons in order to
show light like behavior.Regards, Hans

 

[mn4j]

You are mixing up a single photon and a single detection. Sure, I can speculate about what a single detection means. However, experimentally I can only verify that a single photon is present by looking at an ensemble of single photon states (Fock states) and show that antibunching is present. In this scenario coherence is a sensible concept and a single photon state is a sensible concept. Talking about single photons from one detection alone is never sensible.

I have come to the realization that we are not even talking about the same thing. I'm talking about single photons and you are talking about single photon states. I wonder why you would think those are relevant. It's like talking about "human race" in discussions where the focus is on individuals. So there is no point discussing this further with you.

Physics never proves anything in a positive manner. It produces models and rules out the ones, which are not empirically adequate. Then the easiest and most predictive model is chosen.

I guess by this you mean the more empirically adequate and predictive model in this case is the one which is utterly unable to predict individual events, and in which even though only a single slit is open at a time, a photon can still pass through both simultaneously.

Good luck making sense with that. As Feynman said "I think I can safely say that nobody understands quantum mechanics."

That's why I dislike de Raedt. Although his efforts are mathematically consistent an "adaptive learning" beam splitter/interferometer does not have much predictive power and is in my opinion not the easiest model. While this is ok for the guys, who are interested in reading and publishing in Foundations of Physics and other magazines with a very theoretical and mathematical focus, nobody in "real" physics (that means someone, who has to get funding somehow ;) ) has much time to worry about that.

And what is the predictive power of Bell's inequalities that has convinced you that it must be correct?

After rereading the last few posts I am not sure you get the meaning of self interference the way it is defined. It is misunderstood very often. The famous quote by Dirac about photons interfering only with themselves ("Each photon interferes only with itself. Interference between two different photons never occurs.") does not mean that it is impossible to have such situations like that one with the two pump pulses. This quote is only understood correctly if one also understands that all photons inside one coherence volume are indistinguishable.

The timing of Dirac's statement does not excuse the fact that he was wrong. Multi-photon interference has been demonstrated. Here again, you are talking about the "human race" when the discussion is about "individuals". Because of this you have completely failed to appreciate the difference between a photon and an ensemble of identical photons.

 

[Cthugha]

I have come to the realization that we are not even talking about the same thing. I'm talking about single photons and you are talking about single photon states. I wonder why you would think those are relevant. It's like talking about "human race" in discussions where the focus is on individuals. So there is no point discussing this further with you.

The only way to realize single photons is by means of single photon states. In any other beam of light - no matter, whether coherent, thermal or whatever - you can never model the situation by considering a model of "ball particle" like photons. Indistinguishability is at the heart of quantum mechanics. I repeat: photons in qm formulation are the product of two field operators. As soon as it is possible that the product of two field operators relating to different emitters does not cancel, the model of photons as simple particles goes wrong.

I guess by this you mean the more empirically adequate and predictive model in this case is the one which is utterly unable to predict individual events, and in which even though only a single slit is open at a time, a photon can still pass through both simultaneously.

As I said: A photon is NOT a ball particle. Two fields pass through the slits. Due to the different distances from the two slits to several points on the screen or detector, there is no problem. De Raedt also is not able to predict individual events. He can just simulate them afterwards after the results are known. What he tries, is pretty simple. He tries to avoid any usage of fields and claims that the phase information, which the fields carry is instead carried by ball particle photons, which change their environment just in such a manner that the results are the same as in a field formalism and then - for example -consecutive photons act differently at a beam splitter due to this interaction. Although avoiding the usage of field phases, which do not directly correspond to a real physical entity, this model is not convincing. It is well known that these phase effects cancel out if you superpose a huge amount of noise fields with random phase relationship. In self interference such a huge amount of fields present additionally to some signal state at one of the ports of a beam splitter in a Mach-Zehnder interferometer does not change the results because the fields cancel out. In de Raedts model this result is not intuitive. One signal photon will change the state of the beam splitter, so that the next signal photon will take one well defined exit port, no matter how large the time delay between them is. However the huge amounts of noise photons, which are the equivalent of the noise fields in the field model, are supposed to leave the state of the beam splitter unaltered. How should the beam splitter "know", which of the photons are noise and which are not? That does not sound sensible.

Good luck making sense with that. As Feynman said "I think I can safely say that nobody understands quantum mechanics."

The majority of the physics community agrees that it indeed does make perfect sense - despite the fact that most of them indeed understand quantum theory.

And what is the predictive power of Bell's inequalities that has convinced you that it must be correct?

Why Bell? You do not even need entanglement for this discussion. In the cases of self interference, entangled photons are not even of interest. This is the plain quantum theory of optical coherence. I suppose you also think the work in that field Glauber got his Nobel prize for is wrong as well?

The timing of Dirac's statement does not excuse the fact that he was wrong. Multi-photon interference has been demonstrated. Here again, you are talking about the "human race" when the discussion is about "individuals". Because of this you have completely failed to appreciate the difference between a photon and an ensemble of identical photons.

A discussion of what you call a single photon is not possible in any situation, where coherence properties need to be taken into account because either:

-the photon emission processes are not statistically independent of each other (like for thermal light) and the nth order intensity correlation functions do not factorize in lower order correlation functions and therefore the whole physics, which determines the detection of one of these photons, can only be described by taking all photons into account.

- the photons are emitted statistically independent of each other (like in laser light), but the coherence increases strongly. This leads to indistinguishability of all photons inside one coherence volume and to lots of interference effects, which are also not explained in a ball like model of independent photons.

 

[DB Katzin]

Actually, it is misleading to claim that path knowledge definitely disturbs the interference. At the very least, the jury is still out but here are some experiments which prove otherwise:
* R. Sillitto, and C. Wykes, Phys. Lett. A 39, 333 (1972): two slits with only one open at a time. Interference fringes clearly observed
* E. Fonseca, P. S. Ribeiro, S. Padua, and C. Monken, Phys. Rev. A 60, 1530 (1999): complete path knowledge was obtained for all photons and interference persisted
* L. Basano, and P. Ottonello, Am. J. Phys 68, 245 (2000). Two laser sources, photons from each can only pass through a single slit. Path knowledge complete yet interference obtained.
* C. Santori, et al., Nature 419, 594 (2002): Complete path knowledge, interference obtained

Oh, and the above experiments together with a hand full of others rule out self-interaction which is the current dogma. For example:
* Yu. Dontsov, and A. Baz, Sov. Phys - JETP 25, 1 (1967). Interference disappeared when the photon flux was drastically reduced by placing neutral density filters before the slits but not by placing filters after the slits.
* Y.-H. Kim, et al., Phys. Rev. A 61(R), 051803 (2000). By making sure only one photon was in the system at a given time and controlling the interval between successive photons, the appearance/disappearance of interference was heavily dependent on the time interval.

The doctrine of self-interference is dependent on the stipulation that there be only one photon/electron passing through the slit array in a time interval long enough to allow interaction if there were more than one. What you are saying here is not just that path knowledge may not degrade interference patterns, but something much more significant: The original thought experiment conclusion and apparent confirmation by subsequent experimentation was all entirely in error. This mysterious finding which prompts sensational explanations like self-interference is no mystery at all, just sloppy technique. Is this what you are saying?

 

[mn4j]

The doctrine of self-interference is dependent on the stipulation that there be only one photon/electron passing through the slit array in a time interval long enough to allow interaction if there were more than one. What you are saying here is not just that path knowledge may not degrade interference patterns, but something much more significant: The original thought experiment conclusion and apparent confirmation by subsequent experimentation was all entirely in error. This mysterious finding which prompts sensational explanations like self-interference is no mystery at all, just sloppy technique. Is this what you are saying?

That is correct!

I am saying: The claims (1) that path knowledge necessarily disturbs the interference pattern and (2) that photons interfere with themselves, are unjustified and boil down to "sensational explanations" by those with a penchant for mysticism. The experimental record clearly rules out (1) and raises serious questions about (2).

I have provided reference to experiments in which complete path knowledge was available for each photon in the pattern, to which the opposing view responded by claiming some kind of special status to multi-photon interference, essentially conceding that path knowledge only affects single-photon interference.

I have also provided references to experiments in which the time lag between photons/electrons had an effect on the interference pattern, which it should not if the photons were interfering only with themselves, to which the opposing view has responded by saying the experiments are wrong.

I have also provided an experiment in which only a single slit was open at a time, to which the opposing view responded by saying, although only one slit was open at a time, the single photon must have passed through both slits anyway (because they said so, it couldn't not have).

It reduces to the fact that once mysticism has invaded physics, there is nothing that can not be explained with mysticism, no matter how well meaning the individual -- that is what is happening here. So the issue is not with the experiments, but with the attempt to introduce mysticism in every explanation.

I have provided references to deterministic simulations which reproduce several interferometer and double slit results, contrary to Feynman's earlier claim that it was impossible to explain these observations deterministically. Yet the opposing view responds by saying it's all just useless mathematics. Yet the same individuals, probably believe Feynman's claim on face value, not backed by any mathematics or experiment. The same probably accept Bell's inequalities as valid, based only on the mathematics of it (for there has never been an experimental validation of Bell's inequalities and violation of the inequality experimentally only shows the inadequacy of the inequality).

 

[Doc Al]

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