if someone asked you to prove that electromagnetic waves travel in a vacuum, what would you say?

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What do physicists mean when they say photons have a "path"?

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• Thread starter vanhees71
• Start appointment Mar 3, 2022
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[Moderator's notation: thread spun off from previous thread due to topic/level change.]

This [Ed.: the claim that photons take a "path"] is a misconception of breakthrough theory already for massive particles. It's fifty-fifty more severely misleading for massless quanta of spin ##\geq 1##, which practice not even allow the definition of a position observable itself. Particularly in the hither discussed specifically quantum properties of entanglement any reference to paths must lead to confusion, specially for the delayed-selection quantum erasure experiment discussed right now.

Answers and Replies

This is a misconception of breakthrough theory already for massive particles. It's even more severely misleading for massless quanta of spin ##\geq 1##, which do not fifty-fifty permit the definition of a position observable itself. Especially in the hither discussed specifically quantum properties of entanglement any reference to paths must lead to defoliation, particularly for the delayed-pick quantum erasure experiment discussed right now.

In that location is nada misleading in what I accept said, because it matches exactly mostly accustomed scientific discipline (what we are supposed to advance here). As I have already referenced, the acme scientific team of Walborn et al in 2001 used the term "path" 38 times in a single paper to describe how a photon travels. Maybe y'all should inform them that photons don't have paths, then they can correct the newspaper you lot referenced. In the meantime, I ask you to provide any substantiation to support the thought that photons - or any quantum particle - lacks a path. (And please, I already mentioned that their path is non sharply defined - whatsoever more or less than any quantum observable.)

This may shock you: the low-cal from your CRT - what you are observing now to read this post- arrived at your eye shortly subsequently emission from an LED. To go there, it went on a "path" from the "source" CRT to your "destination" heart, and there are no other words in any language to depict this better.

And so no position observable? And all the same photons are regularly found - exactly where they are expected to be! By the scientists who are performing experiments on entangled photons. How near you return to the subject of this intermediate (I) thread: paths of entangled photons, not whether photons have paths.

And then no position observable? And all the same photons are regularly institute - exactly where they are expected to be!

Notation, though, that the "position observable" in question in such experiments is the one for the photon detector, not for the photon itself. Yes, of course, if a photon detector clicks, we know a photon was detected at the position of the detector (which is non a single point but a reasonably small finite region of space). That is not at all inconsistent with the fact that there is no position observable for the photon itself. Information technology simply ways that, if you actually want to be rigorous, you take to be more than careful in formulating exactly how you model, in the math, what the photon detector is doing.

the light from your CRT - what you are observing now to read this post- arrived at your middle presently afterwards emission from an LED. To get in that location, it went on a "path" from the "source" CRT to your "destination" middle

Actually, the last statement hither is non necessary either to draw or to model mathematically what is going on in this scenario. And 1 of the full general guidelines in QM is that we should be careful virtually such statements. (And note that this would apply to an electron besides as to a photon. The primal is that in the scenario being described, we are not measuring what happens in between the source and detector.)

Note, though, that the "position observable" in question in such experiments is the 1 for the photon detector, not for the photon itself. Yes, of grade, if a photon detector clicks, nosotros know a photon was detected at the position of the detector (which is not a unmarried point just a reasonably small finite region of space). That is not at all inconsistent with the fact that there is no position observable for the photon itself. It just means that, if you really want to be rigorous, you have to exist more careful in formulating exactly how you model, in the math, what the photon detector is doing.

Yes, and that'due south crucial to empathize what'southward understood as "path of photons", especially in this specific experiment with entangled photon pairs. "Which-path data" means that by some means, in this case using the quarter-moving ridge plates in the slits, the photon is prepared in such a manner that it is possible to obtain the information through which slit it went. In this case it'due south by the polarization state (left- or right-handed polarized is uniquely "entangled" with the "which-mode information"). As a effect a such prepared ensemble of photons does not show a double-slit interference pattern on the screen. It's also important to note that the which-manner data is non "read out" by whatsoever measurement hither, such that the entanglement with the other photon is not destroyed. This enables the "erasure of the which-fashion information" by postselection using this other photon of the pair, splitting the ensemble in 2 sub-ensembles, each showing an appropriately shifted double-slit interference blueprint.

To know through which slit an private photon would accept gone, you'd have to determine whether it's 50- or R-polarized, simply and so the entanglement with the other photon and thus the possiblity of "erasure of which-mode data" is destroyed. The "path" of the photon, i.eastward., through which slit it went is not due to the measurement of a "photon position" (which does not exist) only due to interaction with the quarter-wave plates in the slits, and these define a "position" in terms of "which slit".

Really, the last argument here is not necessary either to draw or to model mathematically what is going on in this scenario. And one of the general guidelines in QM is that we should exist careful nigh such statements. (And note that this would apply to an electron as well every bit to a photon. The primal is that in the scenario being described, we are not measuring what happens in betwixt the source and detector.)

Of course a photon had a path to your middle. I tin can't believe this is even a topic of any kind of fence. As I mentioned several times, it is not necessarily a specific path - and not with a specific source point or destination point. But Nosotros Exercise IN FACT MEASURE that the photon took a path - in contradiction to your statement. And guess what, that general (and classical!) path is predicted in advance and confirmed with entangled photons in thousands of experiments (besides the administrative one I referenced, which has so far been ignored). Aye, the existence of a path IS necessary to depict the experiment. So I would challenge @vanhees71 (or you) to show me any authoritative source that says otherwise.

Not but exercise particles motility along (quantum) paths when not observed: they likewise have mass, accuse, momentum, spin and other observables. @vanhees71 may as well exist telling the OP that teaching these quantum observables are misleading. He identifies our ii entangled particles past referring to their momentums p1 and p2, and implies that is proper. Instead: I can just equally rationally identify those photons equally taking paths P1 and P2 (or having frequency f1 and f2, etc). That is no more or less correct, and again I challenge you or vanhees71 to show me any authoritative source that says otherwise. On the other hand, about every newspaper on entangled particles reference the photons as either bespeak or idler, which is a clarification of the path taken (and NOT the initial momentum, equally it should be if he were correct).

If my Walborn reference and its usage of path in the same context equally I use it isn't enough, so I will supply as many more than as is necessary. I'm not trying to be contrarian, but your statements are every bit discipline to challenge equally anyone else's. So... where is ANY published experiment that says entangled photons don't take a path to a detector? Every photon detected in any entangled experiment is proof of just that... just every bit every ray of calorie-free from the lord's day is.

I enquire that this portion of the discussion be split off from this thread. [Ed.: it now has been, to this thread.] Seriously, does anyone recollect this ridiculous discussion is assisting the OP @fluidfcs to sympathize my answers to his questions (which I answer in post #xx)? Of class non, and anyone else reading might be wondering about their understanding as well. If this give-and-take belongs anywhere, it is in Quantum Interpretations/Foundations as there are valid interpretations that in fact assert particles have specific paths at all times. I don't follow those interpretations personally, only: if nosotros are going to dissect the semantics of the word "path", I guess we'd better consider that too.

I think information technology's a common misunderstanding hither. Information technology is but very important, particularly in this context, not to confuse photons with something like particles. A improve intuitive picture is the electromagnetic field, and this it is for light since Maxwell! Of form you lot have a light source, but light is not a particle moving on a path but an electromagnetic wave field propagating in infinite (if you have a point-like source it's a spherical moving ridge). Your eye is, for the purpose of this discussion, a detector, and this detector registers "lite" basically via the photoeffect. The localization of this "registration effect" is due to your eye, information technology's not measuring the position of an electromagnetic wave, for which "position" doesn't brand much sense. It'south rather a field, i.e., at each position you have an electromagnetic field, and the intensity of the light is given by the energy density of this em. field every bit a function of space and fourth dimension.

At present a photon is a specific land of the quantized electromagnetic field and as such it does non have a position. It doesn't even have an observable referring to position. All there is, is the probability to register the photon at a place defined past the detector (e.k., a CCD camera, where a pixel gets excited by the photon with some probability with is proportional to the energy density of the em. field).

The trouble with the naive photon-particle motion-picture show, as if it were a little bullet going a specific path is going wrong, particularly when information technology comes to the here discussed question of specifically wave-like properties like the interference fringes due to diffraction at a double slit or even the very quantum-specific properties described by entangled two-photon states.

It's also very clear what the Walborn et al mean by "path" in their newspaper, and information technology's clearly meant in the sense of quantum optics and not in a naive classical-particle like sense of some pop-sci writeups!

1. I think it'due south a common misunderstanding hither. Information technology is only very important, particularly in this context, not to confuse photons with something like particles.

2. It's likewise very clear what the Walborn et al mean by "path" in their paper...

1. This is wrong too. Photons are quantum particles, and are well thought of equally such when discussing entangled photons where the number is two. I am not denying that photons can be thought of as "excitations of the electromagnetic field" - or "specific states of the quantized electromagnetic field"- or waves for that matter. But a photon tin be chosen a particle as a matter of convenience, just as protons and electrons can. That is because many cases, quantum objects act equally if they are classical objects.

Particle: It's a give-and-take! Farther, it is quite convenient to talk of the position of a photon even if it lacks a precise observable. Again, I can predict with swell precision when/where a detector click will occur due to the presence of a photon. Just equally I can refer to your "heed" and discuss that, even though it is the "brain" where the heed is consider to reside. That's because it's useful to do so. Yet your heed has no position observable either. When someone asks if yous have made upwards your heed, practice yous tell them that you don't have a heed? No, because we sympathize the limits of the word's usage and accept that for the sake of convenience.

@vanhees71: And sorry, there is NO ONE Best Manner to refer to and teach quantum ideas. I don't care if you teach or not, y'all tin't tell people to read an unabridged textbook in club to answer a basic post question. Our answers should reflect the level of understanding of the person who asks. I shudder to think of what you might answer if someone here asked the distance to the sunday (PS: ane good respond is 93 million miles).

2. Yes, thank you for finally acknowledging the obvious. What they mean is EXACTLY the same matter equally was meant in usage in this thread. And how everyone uses the term. So I repeat: entangled photons travel on paths from a source PDC crystal to detectors A and B. And Walborn agrees.

1. This is incorrect too. Photons are quantum particles, and are well thought of every bit such when discussing entangled photons where the number is two. I am not denying that photons tin can exist idea of equally "excitations of the electromagnetic field" - or "specific states of the quantized electromagnetic field"- or waves for that matter. Just a photon can be called a particle as a matter of convenience, simply as protons and electrons tin. That is because many cases, breakthrough objects act as if they are classical objects.

Particularly here they exercise definitely non act every bit if they were classical objects, and photons are the least particle like of everything we call a particle. They don't fifty-fifty admit a position appreciable and thus thinking in terms of paths of pointlike particles is even worse than when doing this for massive particles, where indeed under some circumstances that's non too incorrect.

Particle: It's a give-and-take! Farther, it is quite user-friendly to talk of the position of a photon even if it lacks a precise observable. Again, I tin can predict with peachy precision when/where a detector click will occur due to the presence of a photon. Merely as I can refer to your "mind" and discuss that, even though it is the "brain" where the mind is consider to reside. That's because it'south useful to do so. Yet your mind has no position observable either. When someone asks if you have made upwardly your listen, do yous tell them that y'all don't take a listen? No, considering we understand the limits of the discussion'southward usage and accept that for the sake of convenience.

It's not convenient if you use words in a situation, where it leads to misconceptions and misunderstandings. Here we discuss near physics and non much more complicated philosophical issues about, what "mind" might be and what it might have to do with the physics and chemistry of our brains.

@vanhees71: And sorry, there is NO ONE BEST WAY to refer to and teach quantum ideas. I don't care if you teach or not, you lot can't tell people to read an unabridged textbook in order to answer a basic post question. Our answers should reflect the level of understanding of the person who asks. I shudder to think of what you might answer if someone here asked the altitude to the sun (PS: one skilful answer is 93 meg miles).

I don't claim that I take a good way to teach QT. All I say is that it is for sure a bad way to teach QT, using wrong concepts.

two. Yes, thank you for finally acknowledging the obvious. What they mean is EXACTLY the same thing equally was meant in usage in this thread. And how everyone uses the term. So I repeat: entangled photons travel on paths from a source PDC crystal to detectors A and B. And Walborn agrees.

Yous may repeat it equally often as you lot want, but it does not become right just repeating information technology.

Of course a photon had a path to your eye. I tin can't believe this is even a topic of any kind of debate.

It's a topic of fence because, as far as I can tell, you are making claims that become across what QM actually says. And then one of the states must be wrong. If information technology's me, I'd similar to find that out. If information technology's you, I'd like you to find that out. Either style, in that location is definitely something to discuss.

As I mentioned several times, information technology is not necessarily a specific path - and not with a specific source indicate or destination indicate. Merely We DO IN FACT MEASURE that the photon took a path - in contradiction to your statement

How do we mensurate that the photon took a path? Please be specific. Note that just saying, well, we know the source emitted a photon and we detect a photon at detector A, and the source and detector both have well-defined positions, is not sufficient. That'southward not a measurement of a path.

Also note that phenomena similar, for case, beingness able to bespeak a light amplification by stimulated emission of radiation arrow at a specific spot on a screen are not relevant, because a light amplification by stimulated emission of radiation arrow does non emit single photons, and we are talking here about experiments involving unmarried photons (or pairs of entangled photons). The betoken in dispute here is non whether classical light rays have paths, merely whether single photons practice.

How do we measure that the photon took a path? Delight be specific. Annotation that just saying, well, nosotros know the source emitted a photon and we detect a photon at detector A, and the source and detector both accept well-divers positions, is not sufficient. That's not a measurement of a path.

That'southward the of import point: According to QED, and that's finally the theory which is today considered valid to draw the quantized electromagnetic field and thus also photons as specific asymptotic free ane-photon Fock states. The photons themselves accept no position observable. All you tin calculate are the probabilities for detecting a photon at a identify determined by the detector given the state of the electromagnetic field.

Besides annotation that phenomena like, for case, being able to point a laser pointer at a specific spot on a screen are not relevant, because a laser pointer does not emit unmarried photons, and we are talking here almost experiments involving single photons (or pairs of entangled photons). The signal in dispute here is not whether classical light rays have paths, but whether single photons practice.

The laser field is described past a coherent Gaussian axle. FAPP almost ever you can draw it past the corresponding classical electromagnetic field. I think here nobody doubts that there is no specific position of "a photon" within this beam. Of course it's intensity is pretty narrow in the transverse direction merely in longitudinal direction it's totally delocalized.

The problem merely comes when people remember of single photons not in terms of waves just equally if they were localized "massless particles", as Einstein thought in 1905. Einstein himself was the commencement to acknowledge that this is not the terminal answer. Perhaps mod QED is also not the concluding answer but at least it's a much more consistent and accurate clarification than the old "wave-particle-duality quantum mechanics" of earlier 1926.

one. Information technology'due south a topic of debate because, as far as I can tell, you are making claims that go beyond what QM actually says. So i of usa must be wrong. ...

2. How exercise we measure that the photon took a path? Please be specific. Note that just saying, well, we know the source emitted a photon and we detect a photon at detector A, and the source and detector both accept well-defined positions, is not sufficient. That's not a measurement of a path.

Also annotation that phenomena like, for example, being able to point a laser arrow at a specific spot on a screen are not relevant, because a laser arrow does not emit unmarried photons, and we are talking hither about experiments involving single photons (or pairs of entangled photons). The signal in dispute here is not whether classical light rays have paths, but whether single photons do.

i. I will repeat that this discussion has absolutely cypher to do with the OP. It should therefore be split up away, preferably to the Interpretations subforum. At all-time, this is a gentlemanly semantics discussion; and at worst it is distracting contest of wills that tin accept no winner.

@PeterDonis: if yous aren't willing to provide a hard reference contradicting my reference and commentary below, what does that really say about this word anyhow? Same question to @vanhees71... you both are acting as if your words lonely are authority when challenged.

------------------------------

two. I have already explained how we know light takes a path, and provided referential support for same. And I accept repeatedly specified that ENTANGLED PHOTONS (not a light amplification by stimulated emission of radiation pointer) take paths that are adamant in accelerate from a source crystal, where they originate, to a destination detector, where they are destroyed as office of the detection process.

On the other manus: y'all turn down to provide any reference for the (absurd) thought that entangled photons lack a path. And yet, bank check out this paper in which the path length deviation is controlled to inside a one-half-wavelength of the detected photons after traveling more than 8 kilometers. That's a ratio of ane:12 billion for the path length for individually detected photons. Hmmm, that's a path!

https://arxiv.org/abs/quant-ph/9806043

This is the entangled photon path, which originates as I described above in Geneva, follows a carefully laid out path and ends an entire city away, at the precise location Tittel et al selected:

"For our Franson-type test of Bell inequalities [xvi], we produce energy-time entangled photons past parametric downconversion (Fig. ane). Light from a semiconductor laser with an external cavity (x mW at 655 nm, ∆ν <10MHz) passes through a dispersion prism P to separate out the residual infrared fluorescence light and is focused into a KNbO 3 crystal. The crystal is oriented to ensure degenerate collinear type I phasematching for betoken and idler photons at 1310 nm [17]. Behind the crystal, the pump calorie-free is separated out past a filter F (RG 1000) while the passing down-converted photons are focused (lens L) into one input port of a standard iii-dB fiber coupler. Therefore one-half of the pairs are divide and exit the source by unlike output fibers. Using a telecommunications cobweb network, the photons are and then analysed past all-fiber interferometers located 10.9 km apart from one another in the small villages of Bellevue and Bernex, respectively. The source, located in Geneva, was 4.v km abroad from the showtime analyser and 7.iii km from the second, with connecting fibers of 8.ane and 9.three km length, resp., every bit indicated in Fig. 1. Our interferometers utilize both the Michelson configuration and accept a long and a short arm. In club to compensate all birefringence effects in the arms (i.due east. to stabilize the polarization), we employ so called Faraday mirrors (FM) to reflect the light [18]. At the input ports, we use optical circulators (C). These devices guide the light from the source to the interferometer, just, thanks to the non-reciprocal nature of the Faraday consequence, guide the light reflected dorsum from the interferometer to some other fiber, serving as second output port. The output ports of each interferometer are connected to photon counters [19]. Nosotros label the "direct" port every bit "+", the one continued to the circulator "-". To command and modify the phases ( δ 1, δ 2), the temperature of the interferometers can be varied.

Since the arm length difference is v orders of magnitude larger than the single photon coherence length, there is no unmarried photon interference. Nevertheless, the path divergence in both interferometers is precisely the aforementioned, with a sub-wavelengths accurateness. Moreover, this imbalance is two orders of magnitude smaller than the coherence length of the pump light amplification by stimulated emission of radiation. Hence, an entangled state can be produced where either both photons pass through the brusque arms or both employ the long arms. Noninterfering possibilities (the photons pass through different arms) can be discarded using a loftier resolution coincidence technique."

And please take specific note of the fiber path, including the coiled portions, equally that is adamant by the scientists and the photons are not free to get anywhere else. Other than the PATH it actually takes, of form.
@vanhees71 said: "...in the here discussed specifically quantum properties of entanglement, any reference to paths must lead to defoliation". Perhaps you lot should explicate to this "confused" top team that it is non useful to think of these entangled particles equally having a path. Because they did exactly the opposite of what you say.

And you asked: "How do we measure that the photon took a path? Please be specific." Then there'due south my verbatim quoted answer, I don't call back it gets any more specific than this solid gold reference. So... what ya got that says photons don't accept paths? A quoted reference would exist appropriate here.

---------------------------

Lastly: I will concede the betoken, if yous tin explain the following well-nigh entangled photons:

a. I say they have a path, which length can exist measured within tolerance (eight.1 km +/- 655 nm in the reference).
b. I say they have a frequency/wavelength, which tin can exist measured within tolerance (1310 nm +/- in the reference).
c. I say they accept a spin, which can exist measured within tolerance (per figure 2 in the reference).

Tell me: what is different virtually a.path equally opposed to b.frequency and c.spin? Photons take paths, accept a frequency, and have spin. How is maxim any of this misleading in whatever way?

-DrC

This is a matter of semantics.

DrC, you are calling "a path" a drove of two ( or a finite number at most) of spatial points, whereas Peter Donis and Vanhees are calling "a path" something like a "continuous curve".

You both are right in your assertions (within your unlike meanings of "path").

In other words, we know these particular photons took a detail path because we confined them to that item path using a cobweb optic cable.

That's a perfectly good respond to the question of how we know that those particular photons take a path--we forced them to have one with a physical device.

Simply I understood y'all to be claiming that all photons--even photons traveling through costless space, non confined past any physical device like a fiber optic cable--have a path. That is the merits I was disputing.

Take an arbitrary standard textbook nigh quantum optics similar Scully and Zubairy for the standard modern handling of what photons are, namely specific states of the quantized em. field. Likewise in a fiber you don't take classical light particles that are localized or taking a path. A fiber is rather discribed as a wave guide.

This is a thing of semantics.

DrC, yous are calling "a path" a collection of 2 ( or a finite number at nigh) of spatial points, whereas Peter Donis and Vanhees are calling "a path" something like a "continuous curve".

Yous both are correct in your assertions (within your unlike meanings of "path").

Information technology certainly can exist semantics, as I mentioned before ("At best, this is a gentlemanly semantics discussion"). Give thanks yous for your confirming comment.

A comment I would add: This thread was intended to answer a questions from the OP nearly what happens when you modify an entangled photon path. I take answered that question, while @vanhees71 (saying use of the words "path" and "particle" are poor pedagogy tools) and @PeterDonis ("saying I am non following generally accepted science") took the give-and-take in a direction that has no bearing at all on that question.

To @vanhees71:
If someone refers to the speed of lite c: it doesn't brand to say that photons are not particles, or that particles don't have velocities, or that the speed of low-cal is c simply in a vacuum, or that the best manner to teach quantum physics is to read a 664 page volume entitled "Quantum Optics" (and intended for graduate students) - when a thread is marked Beginner or Intermediate. Just answer the question using language that the person will probable understand.

To @PeterDonis:
What are we doing here, when the give-and-take becomes one of semantics? If y'all read my posts, it should be obvious that my usage of the word "path" was correct in every example related to the questions of the OP. I repeatedly referenced *entangled* particles; and I repeatedly specified that such photons travel in most classical paths (every bit described in nifty detail) but those paths don't take a well-defined source or destination - and that the nature of their trajectories is open for interpretation (eastward.thousand. Feynman: path integrals).

But I am disappointed that you did not at any point ask vanhees71 to provide a reference (as I did) when I challenged him. (And delight, a general reference to a textbook is a far cry from a PF reference.) Is my only recourse in this situation to report the post? I didn't do that, because you lot as moderator were participating, and in fact said the debate nearly "path" was meaningful. There is absolutely no justification for asking for a reference from me, when he (and you for that matter) refuse to reciprocate.

To @vanhees71:
If someone refers to the speed of light c: it doesn't make to say that photons are not particles, or that particles don't take velocities, or that the speed of light is c only in a vacuum, or that the all-time way to teach quantum physics is to read a 664 folio book entitled "Quantum Eyes" (and intended for graduate students) - when a thread is marked Beginner or Intermediate. Just answer the question using language that the person will likely understand.

What has ##c## to practice with the question, whether photons are particles or quanta. I quoted a textbook, because you told me to practise then. Any standard textbook describes photons every bit specific Fock states of the quantized electromagnetic field, and it's not simply semantics nosotros are discussing hither, because to answer the questions of the OP to refer to paths in the sense of old quantum mechanics as if photons were localizable particles is utterly misleading.

To @PeterDonis:
What are nosotros doing here, when the discussion becomes one of semantics? If y'all read my posts, information technology should be obvious that my usage of the word "path" was correct in every example related to the questions of the OP. I repeatedly referenced *entangled* particles; and I repeatedly specified that such photons travel in near classical paths (as described in great detail) but those paths don't have a well-defined source or destination - and that the nature of their trajectories is open for interpretation (eastward.g. Feynman: path integrals).

But I am disappointed that you did not at any point ask vanhees71 to provide a reference (as I did) when I challenged him. (And delight, a full general reference to a textbook is a far cry from a PF reference.) Is my only recourse in this situation to written report the post? I didn't do that, because you equally moderator were participating, and in fact said the debate almost "path" was meaningful. There is absolutely no justification for request for a reference from me, when he (and you for that matter) refuse to reciprocate.

I gave a reference. What's the problem?

1. What has ##c## to do with the question...

ii. I gave a reference. What's the trouble?

1. I don't call up yous understand the concept of "analogy".

2. A reference to a 664 page text does not qualify as a reference. I provide exact quotes from references. If you are and so correct (when actually you lot are so wrong), where's a specific quote? And again, please cease quoting yourself unless it is to peer reviewed paper. Notation that textbooks are not always consider PF suitable references anyway.

Please note that I am moving further discussion to the Advisor'southward lounge.

Since when are textbooks no valid references? Information technology's such a bones and fundamental result that it'south in whatever modernistic textbook on the subject. What else should I cite? If you want a more specific shorter quote, take the volume

J. Garrison and R. Chiao, Quantum optics, Oxford University
Press, New York (2008), https://doi.org/10.1093/acprof:
oso/9780198508861.001.0001

and but read Chpt. 1, particularly Sects. 1.iii. and i.4.

It is articulate that either yous tin can use semiclassical theory (e.g., for the leading-social club treatment of the photoelectric consequence and Compton handful), where the em. field is treated as a classical field and the charged particles quantum-mechanically or you demand full breakthrough electrodynamics (including effective models for the in-medium example to describe the standard optical elements similar lenses, mirrors, beam splitters, polarizers, etc.).

The naive Einsteinian particle picture of 1905 is not adequate except in a very heuristic sense for the photo or Compton effects. It'southward for sure inadequate to empathize experiments involving entangled states, particularly the hither discussed quantum-erasure experiment.

A actual verbatim quote from any specific citation would exist a expert beginning, after probably 5 repeated requests for aforementioned.

How about: "entangled photons don't have a path" quoted from ??? Or "photons should never be referred to as anything like a quantum particle" from source ???

I quoted enough textbooks almost the very foundations of quantum electrodynamics and quantum optics. If you lot want original papers on these simple subjects, maybe yous find them in these textbooks.

Edit: Mayhap a bit too desperate in the conclusion to carelessness the word "photon" entirely from the physics literature, merely very clear to the signal apropos the physical problems we talk over hither (and information technology's NOT pure semantics!)

Lamb, W.E. Anti-photon. Appl. Phys. B 60, 77–84 (1995). https://doi.org/x.1007/BF01135846

At the risk of getting dragged into a semantic debate. Office of the trouble is whether experimental descriptions must necessarily be changed to remain in line with the relevant theory.

To an experimental physicist there must clearly be a "path" along which light may exist detected. The equipment is no doubt set upwardly in specific locations along a path in space in club to successfully detect the photon. The theory predicts the probabilistic detection along this path without the photons in any sense really following the path.

The simplest experiment would exist to direct a laser across the lab. Then ready upwardly detectors at random throughout the lab. All the detection events may take place along a directly line through the lab.

The theorist may say that the photons do not themselves in any meaningful style travel forth that path. Just, experimentally the gear up of possible detection events forms, without dubiety, a path through the lab.

No! The experimentalists are very clear in describing their apparati. Nowhere is a path of a betoken particle needed in describing the here discussed quantum-eraser experiment. To the opposite it's all using the correct QED clarification, and it's demonstrating very specific breakthrough-fieldtheoretical properties of entangled photon pairs, in that location's nothing particle-like in this phenomenology.

I stressed more than one time that of course the detection events of single photons are localized, but not because the photon has a position appreciable only considering the detector and the atom/molecule interacting with the electromagnetic field and giving rise to the measured signal, which means to detect the photon, has one. All you can calculate with the theory are probabilities for detecting a photon (or several photons in coincidence as in the quantum-eraser experients for entangled photon pairs) at the place of the detector.

Of course your laser-axle case is perfectly well described with the standard theory. Take a laser pointer and use some dust to make it visible as a "beam". The laser light is described well by a coherent state (a socalled "Gaussian beam" in the paraxial approximation; for details see Garrison and Chiao, Chpt. 7). That predicts indeed precisely what you say: It's almost entirely along a straight line, but this must not be misinterpreted as a stream of classical particles running along this beam. To the contrary, we have a very clear manifestation of an electromagnetic-wave phenomenon here!

Nevertheless, I would like to run across proof that papers past reputable experimental physicists never point, implicitly or explicitly, a photon "path". In diagrams, for case.

@DrChinese claims they do. I don't know, although I'yard tempted to believe him on this bespeak.

At the risk of getting dragged into a semantic debate. Role of the problem is whether experimental descriptions must necessarily be changed to remain in line with the relevant theory.

To an experimental physicist there must clearly exist a "path" forth which light may exist detected. The equipment is no doubt ready up in specific locations along a path in space in order to successfully discover the photon. The theory predicts the probabilistic detection along this path without the photons in whatever sense really following the path.

The simplest experiment would exist to direct a laser across the lab. And then set up detectors at random throughout the lab. All the detection events may take identify forth a straight line through the lab.

The theorist may say that the photons practice not themselves in any meaningful way travel along that path. But, experimentally the prepare of possible detection events forms, without doubt, a path through the lab.

That is what I meant past "a finite number of spatial points", referred to equally "a path" by DrC ( and experimental physicists really), whereas Peter Donis and Vanhees wouldn't call it "a path".

I quoted plenty textbooks about the very foundations of quantum electrodynamics and quantum optics. If you want original papers on these simple subjects, perhaps you notice them in these textbooks.

Edit: Maybe a bit too drastic in the decision to abandon the word "photon" entirely from the physics literature, but very clear to the betoken apropos the physical issues nosotros discuss here (and it's NOT pure semantics!)

Lamb, W.E. Anti-photon. Appl. Phys. B sixty, 77–84 (1995). https://doi.org/10.1007/BF01135846

Showtime, you seem to take completely forgotten that nosotros were primarily discussing whether entangled photon accept paths (they practise!). I accept challenged you on that... where'due south the reference?

2d, "Anti-Photon" from Lamb? Of course he is an authority on light, that'south no issue (I already had this link). Merely his viewpoint hardly represents scientific consensus... why else would he write this article? And he clearly states information technology is his stance, that he prefers the usage of the word "low-cal" and "radiation" over photons (which by definition are low-cal quanta). The existence of Fock states can be considered solid bear witness for grouping photons equally particles of spin one. For anyone interested, here is a link to the total paper itself (no paywall):

http://www-3.unipv.it/fis/tamq/Anti-photon.pdf

The post-obit shows that at that place is ongoing debate about whether or not photons should exist considered particles. Note that these are non intended as references themselves, just showing that it is substantially a affair of opinion how best to refer to photons (particle, or not).

https://physics.stackexchange.com/q...-what-westward-e-lamb-means-in-his-newspaper-anti-photon
https://scienceblogs.com/principles/2013/07/12/photons-are-hither-to-stay-deal-with-it

If Lamb needed in 1995 to write an commodity to limited his displeasure that since 1926, photons are grouped equally particle but don't actually be... well, his opinion is just as controversial today, and certainly not universally accepted. Hither is a suitable reference from one 2001 paper (I got 13.five million hits on "photon spin 1 particle"):

Kim et al, 2001
https://arxiv.org/abs/quant-ph/0103168

"It is articulate that grooming of maximally entangled ii particle (2-photon) entangled states, or Bong states, is an important discipline in modernistic experimental quantum optics. Past far the most efficient source of obtaining two particle entanglement is spontaneous parametric downward conversion (SPDC). ... is the optical path length experienced by the o-polarized photon from the output face of the crystal..."

To the extent this give-and-take is about semantics, clearly the usage of "photon path" and "photons are particles of spin 1" (along with their variations) is extremely pervasive amid scientific papers. To the extent that this word is almost whether entangled photons accept paths (I remember nosotros already settled that) or whether photons should be considered particles: I would say the same thing - information technology's pervasive among scientific papers.

That is what I meant by "a finite number of spatial points", referred to every bit "a path" by DrC ( and experimental physicists really), whereas Peter Donis and Vanhees wouldn't telephone call it "a path".

Information technology's pretty much like the "path" due to a charged particle in a cloud chamber, i.due east., information technology's rather a track left due to the interaction with the vapour molecules. There is no contradiction to QT here too. Information technology has been described in a famous paper by Mott (I think in 1929).

No! The experimentalists are very clear in describing their apparati. Nowhere is a path of a betoken particle needed...

Hahaha, tell that to anyone setting up a Bong examination. They draw a classical path decision process with mirrors, axle splitters, polarizers, filters - you know, the devices that have been around for hundreds of years. That's how the experiment is made to piece of work, they create the photon path themselves. Add in some fiber, and you can be sure the photon has precisely the desired path length and get precisely where the scientist desires. Every single such test is proof that entangled photons take a path. And it is usually called that explicitly. No other term is equally useful, for obvious reasons. There basically is no amend discussion than "path" to describe where a photon goes!

And where have I ever said a photon is a indicate particle? Its location tin be and is constrained to small spacetime volumes all the fourth dimension (as I previously demonstrated in mail service #32. But they do not have precise sources, nor exercise they have precise ending destinations.

Of course, for convenience, I DO consider them equally point particles for most purposes. I would estimate most every reader does besides.

First, you seem to accept completely forgotten that we were primarily discussing whether entangled photon take paths (they exercise!). I have challenged you on that... where's the reference?

It's but impossible to describe these entangled photons in terms of trajectories (or paths) every bit you claim. In all papers and books nosotros have discussed then far the authors employ the quantum-theoretical clarification in terms of bras and kets or, equivalently, creation and annhilation operators, because that'southward the but mode this situation can be described. What additional reference are you lot looking for?

2nd, "Anti-Photon" from Lamb? Of course he is an authority on lite, that'south no result (I already had this link). But his viewpoint hardly represents scientific consensus... why else would he write this article? And he clearly states it is his opinion, that he prefers the usage of the word "lite" and "radiation" over photons (which by definition are low-cal quanta). The existence of Fock states tin be considered solid evidence for grouping photons as particles of spin i. For anyone interested, hither is a link to the full newspaper itself (no paywall):

http://www-3.unipv.it/fis/tamq/Anti-photon.pdf

Every bit I said, I find this likewise also desperate, because the notion of "photon" is well-established and present everywhere understood in terms of mod QED, except in popular-scientific discipline writing, where an outdated naive particle picture is still propagated. Imho this is tlarable just to a certain extent. To explain the photo upshot of the Compton upshot it'southward semi-ok-ish, considering it'southward describing the phenomenology non too wrongly. I remember this narrative is however totally misguiding, when information technology comes to phenomena, where the field-quantization of the em. field is unavoidable, and entangled photon pairs for sure are an case.http://www-3.unipv.it/fis/tamq/Anti-photon.pdf

The post-obit shows that there is ongoing debate about whether or not photons should exist considered particles. Note that these are not intended equally references themselves, just showing that it is essentially a matter of opinion how best to refer to photons (particle, or non).

https://physics.stackexchange.com/q...-what-w-e-lamb-ways-in-his-paper-anti-photon
https://scienceblogs.com/principles/2013/07/12/photons-are-here-to-stay-bargain-with-it

If Lamb needed in 1995 to write an article to express his displeasure that since 1926, photons are grouped as particle but don't actually exist... well, his opinion is just as controversial today, and certainly non universally accepted. Here is a suitable reference from ane 2001 paper (I got xiii.5 million hits on "photon spin 1 particle"):

"It is clear that preparation of maximally entangled 2 particle (two-photon) entangled states, or Bell states, is an of import subject field in modern experimental quantum optics. Past far the near efficient source of obtaining two particle entanglement is spontaneous parametric downwards conversion (SPDC). ... is the optical path length experienced by the o-polarized photon from the output face of the crystal..."

Which paper is this? If information technology'southward a scientific paper, I'yard pretty certain that "path" must exist meant not in a naive classical-particle-like way. Parametric downconversion tin only exist understood in a QFT way (non-linear breakthrough eyes).

To the extent this word is near semantics, clearly the usage of "photon path" and "photons are particles of spin ane" (along with their variations) is extremely pervasive among scientific papers. To the extent that this give-and-take is about whether entangled photons take paths (I think nosotros already settled that) or whether photons should be considered particles: I would say the same thing - it's pervasive among scientific papers.

Nobody denies that photons are "massless spin-one particles", only they are understood as described necessarily every bit a quantized Abelian approximate field, and that'south why, as Lamb writes in his (obviously still controversial essay), the classical limit tin can not be a point-particle description but classical Maxwell theory. Of course this slang is ubiquitous in the scientific literature. The more of import it is to explicate to beginners, what'south really understood by this slang!

Nevertheless, I would like to see proof that papers by reputable experimental physicists never bespeak, implicitly or explicitly, a photon "path". In diagrams, for example.

@DrChinese claims they do. I don't know, although I'1000 tempted to believe him on this bespeak.

LOL how many practice you lot want to encounter? Hmmm, I wonder what arxiv says?

"Photon" & "path" in the Abstruse, 1691:

https://arxiv.org/search/advanced?a...cts=show&size=200&gild=-announced_date_first

"Photon path" exactly in the abstract (89):
https://arxiv.org/search/advanced?a...cts=evidence&size=200&order=-announced_date_first

"Photon path" exactly anywhere in the text (10,400):
https://www.google.com/search?q="ph...ACAAUeIAdABkgEBM5gBAKABAcABAQ&sclient=gws-wiz

On the other hand:

"excitations of the quantized electromagnetic field" exactly anywhere in the text (137*, all papers and posts past @vanhees71):
https://world wide web.google.com/search?q="ex...gB6gaSAQM0LjWYAQCgAQHIAQnAAQE&sclient=gws-wiz

Number of arxiv hits on "Photon" in the Abstract: 64,980
Number of arxiv hits on "excitations of the quantized electromagnetic field" in the Abstract: 0

Usually, voting doesn't brand it so in science. But in this case, it actually does. It'south "Photons" and "Photon Path" by a landslide!

--------------------------------------
*Just kidding - actually 1300, or nigh 1/8 of the number mentioning photon path. But 1260 (97%) of these references plow around and switch to the usage of photon for most of the article.

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