Is the nonlocality of a single photon an indisputable fact?

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Discussion Overview

The discussion revolves around the nonlocality of a single photon as presented in a 2015 paper by M. Fuwa et al. The participants explore the implications of the experimental findings on the nature of the wavefunction, interpretations of quantum mechanics, and the validity of the results in the context of existing theories.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants reference the 2015 paper as evidence of nonlocality in single photons, questioning whether this has been verified subsequently.
  • There is a discussion about the meaning of "nonlocality," with some defining it as a violation of Bell inequalities demonstrated in the experiment.
  • Participants express differing views on the interpretation of the wavefunction, with some suggesting it could be considered a "probability amplitude" rather than a traditional wavefunction.
  • One participant argues that the experiment shows correlations between measurements made by Alice and Bob, implying a real existence of the wavefunction, while others counter that such correlations do not imply influence due to spacelike separation.
  • Concerns are raised about the uniqueness of the Fuwa experiment, with some noting that similar violations of Bell inequalities have been observed in previous experiments, particularly those involving two-photon states.
  • Some participants clarify that in the context of single-photon states, only one detector (either Alice or Bob) can register the photon, challenging the notion that a single photon can be split or influence both measurements simultaneously.
  • There is a debate about the applicability of the Bohm interpretation in light of the experimental results, with some expressing skepticism about its compatibility with relativistic quantum field theory.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of the Fuwa experiment, with multiple competing interpretations of the results and the nature of the wavefunction being discussed. The discussion remains unresolved regarding the verification and implications of the findings.

Contextual Notes

Participants note that the interpretations of the wavefunction and the implications of nonlocality depend heavily on the chosen framework of quantum mechanics, and there are unresolved questions about the nature of single photons and their detection.

ilper
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A 2015 paper in Nature Communications by M.Fuwa et al
arxiv [1412.7790] Experimental Proof of Nonlocal Wavefunction Collapse for a Single Particle Using Homodyne Measurement
Authors:Maria Fuwa, Shuntaro Takeda, Marcin Zwierz, Howard M. Wiseman, Akira Furusawa
states that a single photon is nonlocal. Is this verified later or not?
If verified what does it mean about the wavefunction? Is it real, existing in both arms of a Michelson Zehnder interferometer like in Bohm interpretation?
 
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ilper said:
A 2015 paper in Nature Communications by M.Fuwa et al
arxiv [1412.7790] Experimental Proof of Nonlocal Wavefunction Collapse for a Single Particle Using Homodyne Measurement

The actual link to the arxiv paper is here:

https://arxiv.org/abs/1412.7790

For future reference, please provide links to references.
 
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ilper said:
Is this verified later or not?

"Nonlocality" here means "violation of the Bell inequalities or the equivalent". The experiment described in the paper itself shows this happening. What other "verification" are you asking for?
 
ilper said:
what does it mean about the wavefunction?

That depends on which interpretation you adopt. Nothing in this experiment changes anything about QM interpretations. It's just another experimental verification of violation of Bell-type inequalities, with more loopholes closed.
 
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A photon doesn't have a wave function, at least not in the usual sense since there's no position operator for the electromagnetic field.
 
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Ok let's name it probability amplitude. Is it ok?
 
PeterDonis said:
"Nonlocality" here means "violation of the Bell inequalities or the equivalent". The experiment described in the paper itself shows this happening. What other "verification" are you asking for?
Of course I mean that just one experiment of one group is not enough.
 
PeterDonis said:
That depends on which interpretation you adopt. Nothing in this experiment changes anything about QM interpretations. It's just another experimental verification of violation of Bell-type inequalities, with more loopholes closed.
I can not understand!
The experiment is like = one single photons probability amplitude is is split in two by one beam splitter and takes to Alice and Bob which are separated spacelike. Alice measures her amplitude (nevermind how) and later Bob measures his. After receiving the results of Alice, Bob finds correlations (his results are influenced by Alice).
So for me (I can not see why one can think contrary) Alice influences Bobs amplitude, which means that it is real existing thing like electromagnetic fields for example, isn't it? Then how to accept Copenhagen interpretation that is just probability of nothing real existing? For me only Bohm interpretation of some variants may be consistent with this experiment (if it is verified).
 
ilper said:
I can not understand!
The experiment is like = one single photons probability amplitude is is split in two by one beam splitter and takes to Alice and Bob which are separated spacelike. Alice measures her amplitude (nevermind how) and later Bob measures his. After receiving the results of Alice, Bob finds correlations (his results are influenced by Alice).
For spacelike separation, neither measurement absolutely precedes the other, so there can be no "influence". There is only correlation.
 
  • #10
ilper said:
I can not understand!
The experiment is like = one single photons probability amplitude is is split in two by one beam splitter and takes to Alice and Bob which are separated spacelike. Alice measures her amplitude (nevermind how) and later Bob measures his. After receiving the results of Alice, Bob finds correlations (his results are influenced by Alice).
So for me (I can not see why one can think contrary) Alice influences Bobs amplitude, which means that it is real existing thing like electromagnetic fields for example, isn't it? Then how to accept Copenhagen interpretation that is just probability of nothing real existing? For me only Bohm interpretation of some variants may be consistent with this experiment (if it is verified).
There is no wave function for photons! There's not even a position observable. That's because the photon is a single-quantum Fock state of a massless spin-1- field. There's nothing split by a beam splitter or any other optical element. A photon cannot be split. There are many famous experiments which demonstrate this distinct feature of a true single-photon state. If you have a single-photon Fock state either A or B register a photon (indeed what's described by any state of the em. field is the detection probability at a given position and time, where the position is determined by the measurement device like a photo plate or CCD cam). Two space-like separated registration events cannot be causally connected according to standard relativistic QFT (microcausality/locality of interactions). I don't know, what any Bohmian interpretation changes about this mathematical fact. I also doubt that there is a viable Bohmian interpretation for relativistic QFT including photons/QED.
 
  • #11
ilper said:
The experiment is like = one single photons probability amplitude is is split in two by one beam splitter and takes to Alice and Bob which are separated spacelike. Alice measures her amplitude (nevermind how) and later Bob measures his. After receiving the results of Alice, Bob finds correlations (his results are influenced by Alice).

All of which has already been done in many previous experiments that showed violations of the Bell inequalities. The most famous one is probably the Aspect experiment in 1982. None of this is unique to this experiment.

ilper said:
Of course I mean that just one experiment of one group is not enough.

As noted just now, there have been many similar experiments. This is certainly not even close to being the only experiment that shows violations of the Bell inequalities with measurements of photons.
 
  • #12
Just to be clear: The usual Bell experiments are done with two-photon states, not single-photon states. As I said above, in the case of a one-photon state the photon can only be detected by one, Alice or Bob. There's no way that A sees part and B seed another part of the one photon. That's the most simple feature of the electromagnetic field which necessarily requires field quantization.

That's also emphasized in the above mentioned experimental paper. The applied language is a bit unfortunate in my opinion, but it can be easily "translated" to proper quantum-field theoretical language ;-)).
 
  • #13
PeterDonis said:
All of which has already been done in many previous experiments that showed violations of the Bell inequalities. The most famous one is probably the Aspect experiment in 1982. None of this is unique to this experiment.
As noted just now, there have been many similar experiments. This is certainly not even close to being the only experiment that shows violations of the Bell inequalities with measurements of photons.
Aspect and other experiments are with two entangled photons. Surely there are photons in the labs of Bob and Alice simultaneously. Soon after Aspect followed other such exp. With two entangled photons there are thousands experiments and hence it is verified.
But in Fuwa it goes about a single photon. It turns out that the photon (or something - wavefunction or probability amplitude on what both make measurements) reaches both sides! And 5 years there is no (as far as I know) confirming exp.
 
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  • #14
ilper said:
Aspect and other experiments are with two entangled photons.

Yes, and the "split single photon" part of what is described in the paper also appears to be with two entangled photons created from a single photon, which is the same way entangled pairs of photons for experiments like the Aspect experiment are created.

ilper said:
in Fuwa it goes about a single photon

Part of what is described in the paper is, yes.

ilper said:
It turns out that the photon (or something - wavefunction or probability amplitude on what both make measurements) reaches both sides!

The experiment never detects a single photon in two places; so nothing "reaches both sides" in that sense. As for probability amplitudes, those are always defined everywhere in spacetime for any particle, so it's meaningless to ask whether those "reach both sides".

All this experiment is doing is showing correlations between the photon measurements that violate single-photon analogues of the Bell inequalities. Note that the photon measurements being made are not just detections of the presence of a photon; they are more complicated measurements which, over many runs (many single photons), allow data to be collected that can be used to test whether the relevant inequalities are violated. Everything is entirely as expected according to QM, and, as I said, gives no new information that is relevant to any QM interpretations.

(Note also that the states used in the experiment are not exactly "single photon" states; as noted on p. 3 under "Bob's Tomography Results", there is some admixture of both a vacuum (zero photon) state and a two photon state.)
 
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  • #15
vanhees71 said:
Just to be clear: There's no way that A sees part and B seed another part of the one photon.
That is true for any other experiment but not this one. I think that exactly that is shown in the exp of Fuwa. A(lice) sees ´part´of something and B(ob) part of something. (I won´t argue if it is the photon, wavefunction or probability amplitude) but you can not do measurements on something which is not there. (like probability of Copenhagen interpretation).
 
  • #16
ilper said:
That is true for any other experiment but not this one. I think that exactly that is shown in the exp of Fuwa.

No, that is not what this experiment shows.

ilper said:
A(lice) sees ´part´of something and B(ob) part of something.

No, that is not what is happening in this experiment. Both Alice and Bob are collecting data from multiple photons. There is no case where both of them collect an individual data point from the same photon.
 
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  • #17
PeterDonis said:
Yes, and the "split single photon" part of what is described in the paper also appears to be with two entangled photons created from a single photon, which is the same way entangled pairs of photons for experiments like the Aspect experiment are created.
No I don't think so. That would be very trivial. To split the photon in two photons with half Energy is done by Aspect (with BBO cristal). If they (Fuwa) can split a photon in another way only that would be remarkable.
Here the point is the nonlocality of a single photon state. An idea put forward by L.Hardy, Tan, Collett and others.
 
  • #18
ilper said:
I don't think so.

On what basis? Can you give anything from the actual paper that supports your position?

ilper said:
the nonlocality of a single photon state. An idea put forward by L.Hardy, Tan, Collett and others.

And everyone who has ever worked with QM. As I have already said several times, nothing about this experiment is new from a QM perspective; it's exactly what would have been expected decades ago by anyone who understood QM.
 
  • #19
PeterDonis said:
No, that is not what this experiment shows.
No, that is not what is happening in this experiment. Both Alice and Bob are collecting data from multiple photons. There is no case where both of them collect an individual data point from the same photon.
Even if the photons are multiple surely there are great periods between the events. I don't think different events can correlate in any way.
They want to prove nonlocality of single photons - idea put forward by L.Hardy about 90-ties.
 
  • #20
ilper said:
Even if the photons are multiple surely there are great periods between the events.

In other words, you haven't bothered to actually read the paper to see. Go do that.
 
  • #21
PeterDonis said:
On what basis? Can you give anything from the actual paper that supports your position?
And everyone who has ever worked with QM. As I have already said several times, nothing about this experiment is new from a QM perspective; it's exactly what would have been expected decades ago by anyone who understood QM.
No, the idea of Hardy is very controversial. He offered a simpler experiment which was regarded as hidden multiparticle experiment by Zeilinger I think and many others.
 
  • #22
PeterDonis said:
In other words, you haven't bothered to actually read the paper to see. Go do that.
Really not. Have you? But I read the proposal of Hardy. (Fuwa is a variant) I assure you that there are single photons one after one.
A single photon state at both locations are mixed with other photons and the results are compared. They correlate which means that the photon (or something) is in both arms (nonlocality of single photon). Many photons are just for the statistics.
Are there not multiple photons in Aspect?
 
  • #23
ilper said:
the idea of Hardy is very controversial

What idea? I don't see anthing controversial at all in the experiment described in the paper. Some of the interpretation being put on it might be, but that's because all opinions about QM interpretations are controversial, since there is no generally accepted consensus on QM interpretation.
 
  • #24
ilper said:
Really not.

You haven't read the paper you yourself provided a reference to?

In that case, thread closed.
 
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