Photons, particles and wavepackets

[alvaros]

"Each photon then interferes only with itself. Interference between two different photons never occurs."

If a photon is a pointlike particle, as some ( all ? ) of you say, the probability of being two photons at the same time at the slits is 0.

 

[jostpuur]

It is relevant. Statistics always sounds so frightning and irrelevant to practical applications since it seems to contradict intuition. But statistics is the reason why it makes no sense thinking of two photons as two little black spots surfing on a wave. With a certain probability amplitude you have a two-photon state in your wave functional and you can't decide which photon's which.

So when source A and source B both create one photon, the Bose statistics forces these photons to get in superposition, so that each of the photons immediately has amplitude for starting at both sources?

If this is the explanation for the interference of independently emitted photons, that certainly is a convincing proof for the Bose statistics.

Shortly, their electromagnetic fields do interfere.

But I don't see how this expectation value of the electromagnetic field was directly related to probability densities of individual photons.

 

[Anonym]

If a photon is a pointlike particle, as some ( all ? ) of you say, the probability of being two photons at the same time at the slits is 0.

The referred paper by A.Zeilinger et al., PRL 96, pp 240502 (2006) defines clearly the “size” of the photons (the longitudinal and the transverse coherence length).

Can you please give a reference to some other experimental work that meets your "high standards"?

Sorry, the list will be too long. In your area of research consider roughly everything done by A.Tonomura et al, A.Zeilinger et al, A.Aspect et al.

Regards, Dany.

 

[Demystifier]

Why you consider EM fields and not potentials?

Actually, both satisfy a linear equation, so both interfere. But you are right, potential is more closely related to a wave function, so it is better to speak about potentials. In fact, when I said EM field, I actually meant potential.

 

[Demystifier]

But I don't see how this expectation value of the electromagnetic field was directly related to probability densities of individual photons.

It is not. As I said, see post #10.

By the way, given a 1-photon state, how would you calculate the probability density of photon positions? (One encounters the same problem as with the KG equation.)

 

[Anonym]

potential is more closely related to a wave function, so it is better to speak about potentials.

And to the SR and to the reality (Aharonov-Bohm).

Regards, Dany.

 

[Demystifier]

And to the SR and to the reality (Aharonov-Bohm).

And to canonical formulation of field theory (both classical and quantum), and to spin of the photon, ...

 

[Anonym]

And to canonical formulation of field theory (both classical and quantum), and to spin of the photon, ...

Sure.

Regards, Dany.

 

[ZapperZ]

The referred paper by A.Zeilinger et al., PRL 96, pp 240502 (2006) defines clearly the “size” of the photons (the longitudinal and the transverse coherence length).

Can you point out exactly where he defined this in that paper?

Zz.

 

[OOO]

The referred paper by A.Zeilinger et al., PRL 96, pp 240502 (2006) defines clearly the “size” of the photons (the longitudinal and the transverse coherence length).

By definition a photon has no finite size. A photon is a plane wave with a definite wave vector and thus infinite extension or, to be more precise, an n-photon state corresponds to a certain probability amplitude distribution of gauge field amplitudes for said plane wave.

Of course you can always try to invent your own language but that will only add to the confusion about quantum field theory.

 

[Anonym]

Can you point out exactly where he defined this in that paper?

“They produced pulses at approx. 76 MHz repetition rate with centre wavelengths of 788.5 +/- 0.4 nm and 788.5 +/- 0.4 nm, r.m.s. bandwidths of 2.9 +/- 0.1 nm and 3.2 +/- 0.1 nm and r.m.s. pulse widths of 49.3 +/- 0.3 fs and 46.8 +/- 0.3 fs. The laser pulses were synchronized via electronic feedback loops up to a relative timing jitter of 260 +/- 30 fs…, (Fig.2): … All photons were coupled into single mode fibers (SMF) to guarantee optimal spatial mode overlap.”

In spite that I have some background in the coherent integration and the pulse compression, I am sure that f95toli may provide much more detailed demonstration.

Regards, Dany.

P.S. Zz, I would like to attract your attention that sometimes your attitude is similar to that of the individual here that find burning books the natural idea.

 

[rewebster]

good thread, good topic (once again)--too bad there aren't more 'easier' ways to have face-to-face discussions (like the ones in the history books)

 

[ZapperZ]

“They produced pulses at approx. 76 MHz repetition rate with centre wavelengths of 788.5 +/- 0.4 nm and 788.5 +/- 0.4 nm, r.m.s. bandwidths of 2.9 +/- 0.1 nm and 3.2 +/- 0.1 nm and r.m.s. pulse widths of 49.3 +/- 0.3 fs and 46.8 +/- 0.3 fs. The laser pulses were synchronized via electronic feedback loops up to a relative timing jitter of 260 +/- 30 fs…, (Fig.2): … All photons were coupled into single mode fibers (SMF) to guarantee optimal spatial mode overlap.”

In spite that I have some background in the coherent integration and the pulse compression, I am sure that f95toli may provide much more detailed demonstration.

Regards, Dany.

P.S. Zz, I would like to attract your attention that sometimes your attitude is similar to that of the individual here that find burning books the natural idea.

Er.. first of all, what attitude? I asked you to clarify where exactly in that paper there's a mention of the photon size, and you're giving me grief about that? Puhleeze! I will, however, refrain from describing YOUR attitude.

Secondly, none of what you quoted has anything to do with a photon size. The "pulse width" is not the size, nor the bandwidth. Step into any pulsed laser source and ask those people if the "pulse width" that they measure, either via a streak camera or other technique, is the size of a photon. It also happens that I deal with almost the same set-up with the same Ti-Sapphire laser that is also mode-locked to a master oscillator.

So I still want to know where exactly in this paper is there any size of a photon? And don't attempt to derail this by throwing out insults at me as your red herrings.

Zz.

 

[OOO]

P.S. Zz, I would like to attract your attention that sometimes your attitude is similar to that of the individual here that find burning books the natural idea.

Apparently, sarcasm is one of those things you do not understand.

 

[rewebster]

or take with a sense of humor

 

[jostpuur]

Can anyone confirm that I got this right?

So when source A and source B both create one photon, the Bose statistics forces these photons to get in superposition, so that each of the photons immediately has amplitude for starting at both sources?

If this is the explanation for the interference of independently emitted photons, that certainly is a convincing proof for the Bose statistics.

It makes sense to me now, and I think I'll believe it unless somebody explains what could be wrong with it. But it would be nicer to be sure. It is difficult to feel confident, when the popular explanations of QM don't bother with this paradox at all

 

[rewebster]

Can anyone confirm that I got this right?



It makes sense to me now, and I think I'll believe it unless somebody explains what could be wrong with it. But it would be nicer to be sure. It is difficult to feel confident, when the popular explanations of QM don't bother with this paradox at all

If I'm reading all this right---what you're REALLY looking for is, "What IS the strongest evidence so far?"

is that about 'right'?

 

[jostpuur]

I somehow missed OOO's #30 post in this thread. Now I think I agree with OOO now with this thing about photons starting in superposition of being at the two sources.

If I'm reading all this right---what you're REALLY looking for is, "What IS the strongest evidence so far?"

is that about 'right'?

hmhm... I'm not sure. It was not my intention to seek evidence for Bose statistics, but I just made the remark, that this interference phenomena turned out to be evidence for it, and it surprised me.

 

[rewebster]

I somehow missed OOO's #30 post in this thread. Now I think I agree with OOO now with this thing about photons starting in superposition of being at the two sources.




hmhm... I'm not sure. It was not my intention to seek evidence for Bose statistics, but I just made the remark, that this interference phenomena turned out to be evidence for it, and it surprised me.

I didn't mean 'Bose statistics'----I was leaning toward the idea of your 'topic'

 

[Anonym]

none of what you quoted has anything to do with a photon size.

f95toli, I would appreciate it greatly obtaining your comment on our debate.

like the ones in the history books.

Do you know the story I mentioned in my post #106 in the “cat in a box paradox” session?

I am not sure about his name (I think P.Wood; my first book on ED at high school was written by him). He was in the middle of the measurements. The performance severely degraded due to dust on the internal surfaces and the tubes were about 11-14 m long. Project! He took the cat, put him inside, thus to show the completely deterministic way out. After 10 min he continues the measurements.

Regards, Dany.

 

[OOO]

I somehow missed OOO's #30 post in this thread. Now I think I agree with OOO now with this thing about photons starting in superposition of being at the two sources.




hmhm... I'm not sure. It was not my intention to seek evidence for Bose statistics, but I just made the remark, that this interference phenomena turned out to be evidence for it, and it surprised me.

Bose statistics is one thing we should always be aware of. But you could also do the experiment with single photons (on average), but of course a single photon wave function is also one aspect of bose statistics.

So let me put it that way: suppose you were sure somehow that a single photon you detect on the screen comes from exactly one of your sources then it is impossible for it to interfere in the sense of the double slit since it must have gone through one slit only. Since in classical terms non-interference means that both sources are not synchronized enough, we could say that knowing for sure were a single photon was emitted from means desynchronization of the sources.

The other way round this means that we have only synchronized the two sources sufficiently if and only if we can't be sure were a photon detected on the screen came from.

You probably might say that a photon always comes from one of the sources, even if they are synchronized but this is truly not the case in a quantum mechanical system. Actually you have to add some measurement device to the sources in order to detect where a photon is emitted from, and this measurement device destroys your coherence.

 

[jostpuur]

My final word on this is, that merely saying "you cannot know where the photon came from" doesn't make the whole point clear, pedagogically. As I said in my original response to the cesiumfrog, there can be several reasons for why we don't know something, and they are not always related to the quantum mechanics itself. I understood originally that we cannot know from which source the photon comes from, but so what, I don't know what you are doing behind your computer either, and that doesn't mean that you are in superposition of doing several things. The symmetry of the wave function, according to the Bose statistics, makes the explanation complete.

 

[rewebster]

Do you know the story I mentioned in my post #106 in the “cat in a box paradox” session?


Regards, Dany.

no, sorry, I hadn't read that thread yet (read the part leading up to your post just now though)

my, my, my--'interpretation' is such an all encompassing word (even when it comes to 'data')

 

[OOO]

My final word on this is, that merely saying "you cannot know where the photon came from" doesn't make the whole point clear, pedagogically. As I said in my original response to the cesiumfrog, there can be several reasons for why we don't know something, and they are not always related to the quantum mechanics itself. I understood originally that we cannot know from which source the photon comes from, but so what, I don't know what you are doing behind your computer either, and that doesn't mean that you are in superposition of doing several things. The symmetry of the wave function, according to the Bose statistics, makes the explanation complete.

Sure you are right, there are several reasons for why we don't know what we'd like to know. For the last 80 or so years quantum mechanics has appeared "as if" its uncertainties were something substantially different from not knowing what somebody does behind his computer. But is it really ?

"Pedagogically" I like to think about these things as some kind of analogy with Maxwell's demon: although in classical physics it might be considered possible to control a system as much to violate the second law of thermodynamics, it is practically impossible so that you come nowhere near violating the second law of thermodynamics. But do not take this analogy too literally. It will probably be easy for one of you to point out the flaw in this argument. As I said "Pedagogically"...

 

[mn4j]

So let me put it that way: suppose you were sure somehow that a single photon you detect on the screen comes from exactly one of your sources then it is impossible for it to interfere in the sense of the double slit since it must have gone through one slit only. Since in classical terms non-interference means that both sources are not synchronized enough, we could say that knowing for sure were a single photon was emitted from means desynchronization of the sources.

How then do you explain "interference" patterns from single slits?

You probably might say that a photon always comes from one of the sources, even if they are synchronized but this is truly not the case in a quantum mechanical system. Actually you have to add some measurement device to the sources in order to detect where a photon is emitted from, and this measurement device destroys your coherence.

I'm sorry but this makes absolutely no sense to me. Just because you don't know the source of a photon does not mean it does not come from a particular source. Probability is a measure of the quality of information our mind has about reality, not a reflection of reality. If you don't know where the photon come from, the probability is 0.5,0.5 It doesn't mean half the photon came from one source andhalf from the other. (see http://bayes.wustl.edu/etj/articles/prob.in.qm.pdf)

 

[OOO]

How then do you explain "interference" patterns from single slits?

I think you got me wrong. It's become a fairly long thread now so it's forgivable that you don't have the thread start in mind anymore. But jostpuur initially talked about putting a wall congruent to the plane between both slits so as to assure that no photon crosses the plane. If you have just one slit then of course you will have interference according to the circumstances of a one-slit experiment. Is that what you meant ?

 

[OOO]

Probability is a measure of the quality of information our mind has about reality, not a reflection of reality.

As far as quantum mechanics is concerned I guess most people would say you're wrong and probability amplitude in quantum mechanics does indeed reflect some physical reality independent of the observer. If this was simply probability in the sense of the maximum entropy compatible with observation then how would you explain the complex probability amplitude ? If you have some solution to this problem, I'd be grateful if you explained it to me.

 

[mn4j]

As far as quantum mechanics is concerned I guess most people would say you're wrong and probability amplitude in quantum mechanics does indeed reflect some physical reality independent of the observer. If this was simply probability in the sense of the maximum entropy compatible with observation then how would you explain the complex probability amplitude ? If you have some solution to this problem, I'd be grateful if you explained it to me.

Could you be kind to define the word "probability". Say the word slowly "probable...ity". There is no physical entity as a probability amplitude. Call it something else, so long as it is a probability amplitude, it is not a real "thing". The fact that the probable outcome of an experiment matches the physical outcome when dealing with an ensemble system does not mean the two are the same.

If you throw a die a single time, only one phase will show, yet you can talk of a probability for each phase showing (6 values). It does not mean the expectation value, which is a probability distribution (what you call a wavefunction or probability amplidue) is a real thing. It is a mathematical description of our uncertainty. The only situation in which it gets close to anything real is when you throw an infinitely large number of dice. You will find that the observed histogram matches the wavefunction of a single die. Even though each die only shows a single face. In other words, the wavefunction of a single particle, is a good mathematical description of how an ensemble of particles will behave, rather than a physical description of an individual particle.

That is why the double slit pattern appears one speck at a time but a large number of particles builds up the pattern. Interpreting the wavefunction as though it were the physical nature of individual particles is not science but metaphysics. That is why nobody has ever explained why the wavefunction collapses at point A and not point B

The first sentence of the wikipedia article on the subject of "probability amplitude" describes it quite well.

Have you ever seen a quantum explanation of a single-slit experiment?

 

[OOO]

Could you be kind to define the word "probability". Say the word slowly "probable...ity". There is no physical entity as a probability amplitude. Call it something else, so long as it is a probability amplitude, it is not a real "thing". The fact that the probable outcome of an experiment matches the physical outcome when dealing with an ensemble system does not mean the two are the same.

If you throw a die a single time, only one phase will show, yet you can talk of a probability for each phase showing (6 values). It does not mean the expectation value, which is a probability distribution (what you call a wavefunction or probability amplidue) is a real thing. It is a mathematical description of our uncertainty. The only situation in which it gets close to anything real is when you throw an infinitely large number of dice. You will find that the observed histogram matches the wavefunction of a single die. Even though each die only shows a single face. In other words, the wavefunction of a single particle, is a good mathematical description of how an ensemble of particles will behave, rather than a physical description of an individual particle.

That is why the double slit pattern appears one speck at a time but a large number of particles builds up the pattern. Interpreting the wavefunction as though it were the physical nature of individual particles is not science but metaphysics. That is why nobody has ever explained why the wavefunction collapses at point A and not point B

The first sentence of the wikipedia article on the subject of "probability amplitude" describes it quite well.

Have you ever seen a quantum explanation of a single-slit experiment?

I did know what probability is before you told me. Only I don't know what you're trying to tell me. Quantum mechanics describes physics by means of probability amplitudes. Period.

If you got some more clever way to describe nature but by using probability amplitudes then feel free to tell us.

By the way, what do you precisely mean by "quantum explanation of a one-slit experiment" ? Is that something different than a "quantum explanation of a one-slit-and-a-bump-on-the-edge-experiment" ?

 

[mn4j]

I did know what probability is before you told me. Only I don't know what you're trying to tell me. Quantum mechanics describes physics by means of probability amplitudes. Period.

Yes, quantum mechanics is a mathematical model of the behavior of nature. Yes you can solve a lot of problems in nature by using probability amplitudes. But it doesn't mean the probability amplitudes are physical entities. That is what I'm trying to tell you. Probability means just that -- probability: a mathematical tool for doing inference from incomplete information. Period.

If you got some more clever way to describe nature but by using probability amplitudes then feel free to tell us.

No I don't. But questioning the establishment is a step in the right direction. All progress in physics starts with a critical look at the establishment. If we are expected to form new theories before questioning existing ones, there will never be progress. Besides I don't question QM. It is a great mathematical tool. I question the prevailing physical interpretation of the mathematics. The fact that the mathematics works does not validate any interpretation of it.

By the way, what do you precisely mean by "quantum explanation of a one-slit experiment" ? Is that something different than a "quantum explanation of a one-slit-and-a-bump-on-the-edge-experiment" ?

However you choose to call it, do you have any pointers to a quantum explanation of single-slit experiments with photons/electrons etc.

 

[jostpuur]

Just because you don't know the source of a photon does not mean it does not come from a particular source.

I agree to some extent, and understand the confusion.

merely saying "you cannot know where the photon came from" doesn't make the whole point clear, pedagogically ...

... The symmetry of the wave function, according to the Bose statistics, makes the explanation complete.

If you are interested to understand why independently emitted photons interfere, which you seemingly do not yet understand because you don't yet seem to believe in the probability concept of QM, you can find the explanation in my post #37

This thread was about one particular kind of experiment, its outcome, and its explanation. There is no point in leading the discussion to the foundations of the QM.

 

[OOO]

Yes, quantum mechanics is a mathematical model of the behavior of nature. Yes you can solve a lot of problems in nature by using probability amplitudes. But it doesn't mean the probability amplitudes are physical entities. That is what I'm trying to tell you. Probability means just that -- probability: a mathematical tool for doing inference from incomplete information. Period.

No I don't. But questioning the establishment is a step in the right direction. All progress in physics starts with a critical look at the establishment. If we are expected to form new theories before questioning existing ones, there will never be progress. Besides I don't question QM. It is a great mathematical tool. I question the prevailing physical interpretation of the mathematics. The fact that the mathematics works does not validate any interpretation of it.

However you choose to call it, do you have any pointers to a quantum explanation of single-slit experiments with photons/electrons etc.

I guess I didn't claim anywhere that a probability amplitude is a sort of physical entity. But on the other hand I believe it is a valid point of view to think of the wave function as a physical entity the square of which is "accidentally" a measure for the probability for what happens in a certain context.

In this sense, one day, we could probably find the reason why there is such parallelism between the actual physical field (the wave function) and the fact that it can trigger a stochastic process with well defined transition probabilities. (we need not extend the discussion to the relation between the terms "physical field" and "wave function", I know about gauge invariance and probably there are some other issues with it, but I think that's not relevant here)

So I think we are not so far apart regarding our skepticism against quantum mechanics. It seems you're tending more towards particles as the fundamental reality whereas I'd prefer the field and collapse viewpoint. But arguing about these views is futile as long as nobody of us has a mathematical formulation that expresses one of these views clearly.

It's fine to question the establishment but one always has to keep in mind that every alternative must be at least as powerful in explaining reality as the established theories.

 

[Demystifier]

I guess I didn't claim anywhere that a probability amplitude is a sort of physical entity. But on the other hand I believe it is a valid point of view to think of the wave function as a physical entity the square of which is "accidentally" a measure for the probability for what happens in a certain context.

In this sense, one day, we could probably find the reason why there is such parallelism between the actual physical field (the wave function) and the fact that it can trigger a stochastic process with well defined transition probabilities.

We already know (a possible) reason for that: Bohmian mechanics.

 

[OOO]

We already know (a possible) reason for that: Bohmian mechanics.

I like Bohmian mechanics but I think it is a bit optimistic to say that it solves the problems discussed here. I guess that was the reason why you put "possible" in parentheses.

For example I know of no consistent relativistic formulation of Bohmian mechanics. So how do you explain particle creation and annihilation with it. And by the way it doesn't explain the collapse of the "guidance field" either. As far as I know it just serves to demonstrate that quantum mechanics does not contradict permanent localization of particles.

 

[Demystifier]

For example I know of no consistent relativistic formulation of Bohmian mechanics. So how do you explain particle creation and annihilation with it. And by the way it doesn't explain the collapse of the "guidance field" either. As far as I know it just serves to demonstrate that quantum mechanics does not contradict permanent localization of particles.

Bohmian mechanics DOES explain the EFFECTIVE collapse of the guidance field, provided that environment induced decoherence is also taken into account.

Concerning relativistic formulation and particle creation/destruction, there are several inequivalent approaches, so the things are not yet settled. In my opinion, the most promising approach is the one pushed forward in
http://xxx.lanl.gov/abs/0705.3542
See also Refs. [16] and [3] for other approaches.