Photons, particles and wavepackets

[Demystifier]

If this is reality, then I can accept it, but how is this not in contradiction with what Dirac is saying? Different photons don't interfere!

See my post #10.
Shortly, their electromagnetic fields do interfere.

 

[f95toli]

About these experiments. Are the two light sources really physically two different light sources, or do they use one source, split the beam, and then call it two sources?

They use two sources. The paper is freely available on the arXiv
http://www.arxiv.org/abs/quant-ph/0603048

 

[Anonym]

There are a few experiments where photons coming from different sources have been shown to interfere. See e.g,. Kaltenback et al PRL 96, pp 240502 (2006) which also gives are good background to the topics… The paper is freely available on the arXiv.

The referred paper is the engineering/applied physics achievement trivial from the theoretical POV. Only Fig. 3(d) is relevant to the OP question. The paper doesn’t contain theoretical background to the topics.

See my post #10. Shortly, their electromagnetic fields do interfere.

Why you consider EM fields and not potentials?

Regards, Dany.

 

[f95toli]

The referred paper is the engineering/applied physics achievement trivial from the theoretical POV. Only Fig. 3(d) is relevant to the OP question. The paper doesn’t contain theoretical background to the topics.

Regards, Dany.

The main result in the paper is the HOM dip seen in figure 3a.
And I never claimed that the paper gave a comprehensive theoretical background to the topic; only that they give some background to the topic and put their experiment in a context. The relevant theory can be found in their list of references.

Also, their "trivial" achievement is extremely impressive from an experimental point of view. Can you please give a reference to some other experimental work that meets your "high standards"?

 

[Anonym]

The main result in the paper is the HOM dip seen in figure 3a.
And I never claimed that the paper gave a comprehensive theoretical background to the topic; only that they give some background to the topic and put their experiment in a context. The relevant theory can be found in their list of references.

Also, their "trivial" achievement is extremely impressive from an experimental point of view. Can you please give a reference to some other experimental work that meets your "high standards"?

You should not consider my post above in the negative spirit and I qualify the paper as achievement and not the “trivial” achievement. However, the authors might add a few paragraphs in the introduction to make the paper readable for the students also.

You also should agree that for any theoretician the paper do not contain any surprise or new information. It is my problem that I still don’t know what the consistent orthonormal basis that describes adequately that experiment is and I blame for that only myself. However that experiment don’t help me.

Regards, Dany.

P.S. Also the purpose of my post was to make clear to everybody participated in this sessions what we are talking about.

 

[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)