A Possibilities of Time-Independent Entangled Photons

[DrChinese]

1. Which version? The paper you cited is a survey of many experiments!

2. Note also that the paper never mentions temporal entanglement or temporally entangled states.

3. In a survey that has several subsections about entanglement swapping, this should make you question how established the notion of temporal entanglement is!

1. It is in the quoted portion in italics: Ma et al 2012 (which I previously referenced).

2. I said: "But it does not qualify as temporal entanglement."

3. Well, to be fair: The title of the summary is "Delayed-choice gedanken experiments and their realizations". The Megidish experiment does not feature Delayed Choice. I didn't expect to see it referenced.

 

[A. Neumaier]

So, considering one of the terms on the right hand side of equation (3) in the Megidish paper, would you object to a sketch like this?

$$U(t)|\phi^-\rangle^{0,2\tau}_{1,4}|\phi^-\rangle^{\tau,\tau}_{2,3}|\epsilon_\mathrm{ready}\rangle_{1,4}^{0,2\tau}|\epsilon_\mathrm{ready}\rangle_{2,3}^\tau = |\epsilon^-\rangle_{1,4}^{t,2\tau+t}|\epsilon^-\rangle_{2,3}^{\tau+t}$$where $\epsilon$ are environmental/detector degrees of freedom to permit a unitary description of detection.

No. The $\tau$-labelled states are temporal modes, hence have no interpretation as dynamical states.

 

[A. Neumaier]

1. It is in the quoted portion in italics: Ma et al 2012 (which I previously referenced).

Hopefully, this version of entanglement swapping is something we can all agree produces genuine Bell states.

I disagree. There cannot be a genuine Bell state involving photons 1 and 4 since according to Fig. 33 of the paper you quoted from, Victor's choice was made only when photons 1 and 4 already ceased to exist. But nonexistent entities cannot have dynamical states.

2. I said: "But it does not qualify as temporal entanglement."

OK, Then you changed the subject of the thread, which is temporal entanglement. Sorry for haven't missed this change of subject.

3. Well, to be fair: The title of the summary is "Delayed-choice gedanken experiments and their realizations". The Megidish experiment does not feature Delayed Choice.

The Bell projection is done after Photon 1 was measured. If instead Photons 2 and 3 had measured directly, the statistics would have been different (trivial). Thus there was a delayed choice, though, to make a difference to their citations [21,22] directly after their Fig.1 and hence something novel-sounding, only one of the choices was discussed and actualized, and the other suppressed.

 

[WernerQH]

There cannot be a genuine Bell state involving photons 1 and 4 since according to Fig. 33 of the paper you quoted from, Victor's choice was made only when photons 1 and 4 already ceased to exist. But nonexistent entities cannot have dynamical states.

I think it is a fundamental mistake to think (let alone speak) of photons as "entities". The electromagnetic field of course has dynamical states that show temporal correlations.

 

[A. Neumaier]

The electromagnetic field of course has dynamical states that show temporal correlations.

Of course. On the other hand, ''states'' involving temporal modes are clearly not states of the electromagnetic field.

I think it is a fundamental mistake to think (let alone speak) of photons as "entities".

In typical quantum optical arrangements, the state of the electromagnetic field contains between nonunitary interactions (creations and measurements) a definite number of photons (changing as 0, 2, 1, 3, 1, 0 in the arrangement depicted in Fig. 1. of Megidish et al.).

These photons can be unambiguously labeled by a label for the path they formally travel. This is the traditional terminology, universally used in quantum optics.

 

[WernerQH]

Of course. On the other hand, ''states'' involving temporal modes are clearly not states of the electromagnetic field.

I was talking about temporal correlations. I have no idea what special connotations "temporal modes" have for you.

These photons can be unambiguously labeled by a label for the path they formally travel. This is the traditional terminology, universally used in quantum optics.

The terminology is utterly familiar -- as familiar as the notorious "aether" was in Michelson's time. It was obvious to everyone that such an "entity" must exist. To me your discussion with @DrChinese sounds as pointless as the debate about how many angels can dance on a needle tip. You seem to have very restrictive ideas about the conditions under which these "entities" (photons) may be called entangled.

 

[DrChinese]

1. I disagree. There cannot be a genuine Bell state involving photons 1 and 4 since according to Fig. 33 of the paper you quoted from, Victor's choice was made only when photons 1 and 4 already ceased to exist. But nonexistent entities cannot have dynamical states.

2. OK, Then you changed the subject of the thread, which is temporal entanglement. Sorry for haven't missed this change of subject.

3. The Bell projection is done after Photon 1 was measured. If instead Photons 2 and 3 had measured directly, the statistics would have been different (trivial). Thus there was a delayed choice, though, to make a difference to their citations [21,22] directly after their Fig.1 and hence something novel-sounding, only one of the choices was discussed and actualized, and the other suppressed.

1. This is actually the point I was getting at from 2. Specifically, that for you, there is no difference between the case where 1 and 4 never co-exist , and the case where 1 and 4 never exist when the Bell State Measurement (BSM) is performed.


2. As I say, this was intended to tease out your viewpoint on the Delayed Choice Entanglement Swap. I want to make sure I have your perspective correct, so please help me along here: In your view (paraphrasing): "Photons 1 & 4 are never 'dynamically' entangled into a true Bell state. That's because photons cannot be entangled after they cease to exist." Your viewpoint would then demonstrate a consistent application of that idea between the Megidish 2012 paper (Never Co-exist) and the Ma 2012 paper (Delayed Choice).

Assuming that I have fairly represented your viewpoint: Your position is now quite distant from generally accepted physics. Basically, yours is a denial of the conclusions of all Delayed Choice Experiments - Entanglement Swapping, MZI variants, or whatever. They all create some kind of quantum state due to some future (or alternative) action being performed. Again, there are interpretations of QM that explain these phenomena in various manners, and if you are using your preferred interpretation (I assume the Thermal Interpretation) then that is an "out". But it is still not generally accepted.

3. I don't follow your comments here. Can you clarify? What do you mean by:

a. "If instead Photons 2 and 3 had measured directly"? Especially curious about "directly".
b. "only one of the choices was discussed and actualized, and the other suppressed"? Especially curious about "suppressed". All of the delayed choice experiments I cite show both the statistics for the swapping choice (creating a Bell state for 2 & 3) and the non-swapping choice (creating a Product state for 2 & 3).
c. Not entirely sure which experiment/references you mean "[21, 22]". I checked both papers and could not fit them to what you said.

 

[PeterDonis]

The terminology is utterly familiar -- as familiar as the notorious "aether" was in Michelson's time.

This is an unjustified comparison. The aether, by definition, was unobservable. The things @A. Neumaier was describing are not.

You seem to have very restrictive ideas about the conditions under which these "entities" (photons) may be called entangled.

No, he is just applying non-relativistic QM in a domain in which everyone, including @DrChinese, agrees that it works. And he is being precise about exactly what it says, and more to the point, what it doesn't say. In particular, it doesn't say that mathematical Bell states involving kets at different times occur anywhere within the dynamics; the dynamics of NRQM only include states with kets that all refer to the same time. So any talk about any states involving kets at different times, as in the papers under discussion, cannot be justified by NRQM. If there is a theoretical justification for such talk at all, it does not appear anywhere in any reference that has so far been given.

This is not a question of what words to use. If someone wants to call Bell states with kets at different times "entangled", that's a matter of words, not physics. But if someone wants to claim that such states are "real"--and @DrChinese has explicitly said in this thread that he is making that claim, and that he thinks the authors of the papers he has referenced are making that claim too--then they should be able to justify that claim with something more than just the assertion that, since the Bell states make the correct predictions, they must be real. But that claim, right now, is all we have.

 

[A. Neumaier]

1. This is actually the point I was getting at from 2. Specifically, that for you, there is no difference between the case where 1 and 4 never co-exist , and the case where 1 and 4 never exist when the Bell State Measurement (BSM) is performed.

In both cases, the system to which a state is assigned doesn't exist. Hence the assignment is scientifically meaningless. It is like assigning properties to a unicorn.

2. As I say, this was intended to tease out your viewpoint on the Delayed Choice Entanglement Swap. I want to make sure I have your perspective correct, so please help me along here: In your view (paraphrasing): "Photons 1 & 4 are never 'dynamically' entangled into a true Bell state. That's because photons cannot be entangled after they cease to exist." Your viewpoint would then demonstrate a consistent application of that idea between the Megidish 2012 paper (Never Co-exist) and the Ma 2012 paper (Delayed Choice).

See my comment above.

yours is a denial of the conclusions of all Delayed Choice Experiments - Entanglement Swapping, MZI variants, or whatever.

No. I deny nothing. I only emphasize that the interpretation given to these experiments is not backed by theory. If there were theory you would have pointed to it by now.

They all create some kind of quantum state due to some future (or alternative) action being performed.

They create statistics, not states! Their states are already determined by the standard quantum evolution of their experimental setup, and they all have definite times! No matter which as-if states are promoted by words without a theoretical justification.

Again, there are interpretations of QM that explain these phenomena in various manners, and if you are using your preferred interpretation (I assume the Thermal Interpretation) then that is an "out".

My claim that there is no backup by theory is interpretation independent.

a. "If instead Photons 2 and 3 had been measured directly"? Especially curious about "directly".

Without putting them through the half-silvered mirror = what you call the non-swapping choice.

b. "only one of the choices was discussed and actualized, and the other suppressed"? Especially curious about "suppressed".

suppressed = neither performed nor its possibility discussed, although they could have done it, as in the references [21,22] they refer to at the top of the left column under Figure 2:

Recently, entanglement swapping was demonstrated with a delayed choice, where all four photons were created simultaneously, but photons 1 and 4 were measured before a choice had been made whether to entangle them or not [21, 22].

They report about delayed choice experiment, and deliberately made their experiment different by not allowing a choice - that could have easily be made (and then would have been delayed) by a straightforward modification of their setup.

All of the delayed choice experiments I cite show both the statistics for the swapping choice (creating a Bell state for 2 & 3) and the non-swapping choice (creating a Product state for 2 & 3).

Yes, and the authors of the paper under discussion could have done so as well, then it would have been a delayed choice experiment. But they needed to be different (else their result could not have been sold as exciting...)

c. Not entirely sure which experiment/references you mean "[21, 22]". I checked both papers and could not fit them to what you said.

I hadn''t check the papers but simply assumed that their remark (quoted above) summarized correctly these references. The only point of mentioning [21,22] was to point out that they knew about delayed choice experiments similar to their own experiment, and that they could have designed their experiment as a delayed choice experiment, too, but had reasons not to do so.

 

[Morbert]

No. The $\tau$-labelled states are temporal modes, hence have no interpretation as dynamical states.

What I am wondering is if, using your terminology, a "fictitious state" is just an expansion of a "real state" in a time-bin basis, like equation (5) in this paper, and hence subject to dynamics as per usual. i.e.
$$U|\Psi\rangle = \sum_jc_jUA^\dagger_j|0\rangle$$where the lhs is "real" and the rhs is a linear combination of "fictitious" temporal modes.

 

[PeterDonis]

if, using your terminology, a "fictitious state" is just an expansion of a "real state" in a time-bin basis

No, it isn't, because any "real" state by the definition @A. Neumaier gave only has kets that all refer to the same time. There's no way to rearrange such a state to include kets that refer to different times.

 

[DrChinese]

No, he is just applying non-relativistic QM in a domain in which everyone, including @DrChinese, agrees that it works. ... any talk about any states involving kets at different times, as in the papers under discussion, cannot be justified by NRQM.

This is not a question of what words to use. If someone wants to call Bell states with kets at different times "entangled", that's a matter of words, not physics. But if someone wants to claim that such states are "real"--and @DrChinese has explicitly said in this thread that he is making that claim, and that he thinks the authors of the papers he has referenced are making that claim too--then they should be able to justify that claim with something more than just the assertion that, since the Bell states make the correct predictions, they must be real.

Of course there is no question about the predictions of NRQM, everyone agrees on these - fortunately!

And there is no question that every reference on Delayed Choice Entanglement Swapping refers to it in the same way, which is as I do. That's a *large* body of (wrong?) work. And they all follow the same NRQM. I'm pretty sure they well understand the Schrödinger equation and its time parameter.

Below, a diagram associated with the 2022 Nobel for Physics on entanglement phenomena (from the Royal Swedish Academy of Sciences*). Note that there is no mention that the 1 & 4 photons need to be measured at any particular time relative to the execution of the Entangler for them to become entangled. That's because there is no such requirement. In the words of Ma, Zotter, Kofler, Ursin, Jennewein, Brukner, and Zeilinger: "Using four photons, we can actively delay the choice of measurement – implemented via a high-speed tunable bipartite state analyzer and a quantum random number generator – on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one definite of two mutually exclusive quantum states in which either the photons are entangled (quantum correlations) or separable (classical correlations). This can also be viewed as 'quantum steering into the past'."

Is this a difference in semantics? I'm not particularly disagreeing with that assessment**. Certainly, Quantum Interpretations tend to come to the foreground in these discussions, even if we are not explicitly discussing those in this context.

*Known for the rapid speed at which they award prizes for novel work.
** And if it is, I don't think I am in the smaller boat.

 

[mattt]

Of course there is no question about the predictions of NRQM, everyone agrees on these - fortunately!

And there is no question that every reference on Delayed Choice Entanglement Swapping refers to it in the same way, which is as I do. That's a *large* body of (wrong?) work. And they all follow the same NRQM. I'm pretty sure they well understand the Schrödinger equation and its time parameter.

Below, a diagram associated with the 2022 Nobel for Physics on entanglement phenomena (from the Royal Swedish Academy of Sciences*). Note that there is no mention that the 1 & 4 photons need to be measured at any particular time relative to the execution of the Entangler for them to become entangled. That's because there is no such requirement. In the words of Ma, Zotter, Kofler, Ursin, Jennewein, Brukner, and Zeilinger: "Using four photons, we can actively delay the choice of measurement – implemented via a high-speed tunable bipartite state analyzer and a quantum random number generator – on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one definite of two mutually exclusive quantum states in which either the photons are entangled (quantum correlations) or separable (classical correlations). This can also be viewed as 'quantum steering into the past'."

Is this a difference in semantics? I'm not particularly disagreeing with that assessment**. Certainly, Quantum Interpretations tend to come to the foreground in these discussions, even if we are not explicitly discussing those in this context.


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*Known for the rapid speed at which they award prizes for novel work.
** And if it is, I don't think I am in the smaller boat.

Your interpretation of their wording (in the case I refer to) IS incompatible with NRQM. You are NOT using NRQM to explain the results, because your explanation invokes "states" that don't exist in NRQM.

 

[DrChinese]

Without putting them through the half-silvered mirror = what you call the non-swapping choice.

suppressed = neither performed nor its possibility discussed, although they could have done it, as in the references [21,22] they refer to at the top of the left column under Figure 2:

They report about delayed choice experiment, and deliberately made their experiment different by not allowing a choice - that could have easily be made (and then would have been delayed) by a straightforward modification of their setup.

I believe you have misread the results. The Ma (2012) and Megidish (2012) experiments both show the statistics when there is a choice made to swap, versus when the choice is made not to swap. A time delay for one path, or other method to distinguish the sources, is the difference. The statistics change as would be expected by theory. See table 1 (page 12) in the Ma experiment, and figure 3 (page 4) in the Megidish experiment. I can cite the specific numbers if you like.

 

[PeterDonis]

I'm pretty sure they well understand the Schrödinger equation and its time parameter.

We are going around in circles. Basically you are saying that (1) the authors of the paper are authorities, so we should accept without question whatever they say, and (2) your interpretation of what they say, which entails the claim you have made about Bell states involving photons that never coexist being "real", is obviously correct and we have no valid reason to question it.

At least three others in this thread (myself, @A. Neumaier and @mattt) do not accept either of those statements. So just continuing to repeat them and add more quotes from more papers that say the same things gets us nowhere. If you cannot actually address the substance of our counterarguments in any other way, this thread has run its course and we might as well close it.

 

[DrChinese]

Your interpretation of their wording (in the case I refer to) IS incompatible with NRQM. You are NOT using NRQM to explain the results, because your explanation invokes "states" that don't exist in NRQM.

You are around 20 years behind the community. How you present the 1927 Schrödinger equation (i.e. prior to the discovery of entanglement) does not change the consensus today. Because these entangled Bell states have been discussed for a long time in the manner I describe, and in the delayed choice math I have presented, going back at least to Peres (1999) and Jennewein et al (2002).

Again: all you need to do is present a recent contrary peer-reviewed reference to convince anyone (including me) otherwise. Doesn't it make you wonder why such a reference seems to be missing for your perspective, while apparently there are no shortage of references supporting what I say? (As included in the post you are replying to.)

My references are not proof of the correctness of what I claim. But they are proof that the physics community consensus is as I describe (and as I claim). They can all still be wrong, and you could still be right - no argument from me. So your simple acknowledgement of these facts is in order. You are the one out of sync with generally accepted physics.

 

[mattt]

You are around 20 years behind the community. How you present the 1927 Schrödinger equation (i.e. prior to the discovery of entanglement) does not change the consensus today. Because these entangled Bell states have been discussed for a long time in the manner I describe, and in the delayed choice math I have presented, going back at least to Peres (1999) and Jennewein et al (2002).

Again: all you need to do is present a recent contrary peer-reviewed reference to convince anyone (including me) otherwise. Doesn't it make you wonder why such a reference seems to be missing for your perspective, while apparently there are no shortage of references supporting what I say? (As included in the post you are replying to.)

My references are not proof of the correctness of what I claim. But they are proof that the physics community consensus is as I describe (and as I claim). They can all still be wrong, and you could still be right - no argument from me. So your simple acknowledgement of these facts is in order. You are the one out of sync with generally accepted physics.

Your answer is really strange. NRQM explains all these experiments just fine.

It is your wording what is not backed up by NRQM.

 

[DrChinese]

1. We are going around in circles. Basically you are saying that (1) the authors of the paper are authorities, so we should accept without question whatever they say, and (2) your interpretation of what they say, which entails the claim you have made about Bell states involving photons that never coexist being "real", is obviously correct and we have no valid reason to question it.

2. At least three others in this thread (myself, @A. Neumaier and @mattt) do not accept either of those statements. So just continuing to repeat them and add more quotes from more papers that say the same things gets us nowhere. If you cannot actually address the substance of our counterarguments in any other way, this thread has run its course and we might as well close it.

1. You have a reason to question it. And yes, the authors *are* authorities. But that does not make them automatically correct. You could acknowledge that your view is out of sync with those authorities without any inconsistency. Do you believe your own viewpoint? If so, accept that you are out of step with the community. You wouldn't be the first.

2. I stepped away from the Megidish paper precisely because you three questioned its results. I re-focused on the Ma paper because I thought that had more authoritative weight within the community, and might be easier to discuss. Apparently, no evidence is adequate here if it differs from what you believe - even the words of highly respected teams.

If you think those following this thread are NOT learning (or otherwise gaining) from the references and quotes I provide: then I don't object to any decision you make as a moderator about closing it.

But: I do think you are biased in your presentations in this thread in favor your own viewpoint (i.e. not as a moderator). I say that precisely because I have provided authoritative citations, and no one (including you) else has; and yet you question whether my perspective matches scientific consensus. Clearly, mine does; and yet you easily dismiss both quoted consensus and my presentation of same. Are you being objective?

While your experience here dwarfs mine, I have never seen that from any other moderator in any other thread - in QM or anywhere here. So I would ask you to re-consider the full value of the citations I have provided. If after that, you think the thread has no ongoing utility, then the decision to close is yours to make without objection from me.

 

[DrChinese]

Your answer is really strange. NRQM explains all these experiments just fine.

It is your wording what is not backed up by NRQM.

I am quoting papers by a Nobel prize winner, who is describing standard contemporary QM. Seriously, doesn't it seem strange that you aren't - and I am? What's ambiguous here? Delayed choice entanglement swapping:

"Using four photons, we can actively delay the choice of measurement – implemented via a high-speed tunable bipartite state analyzer and a quantum random number generator – on two of the photons into the time-like future of the registration of the other two photons. This effectively projects the two already registered photons onto one definite of two mutually exclusive quantum states in which either the photons are entangled (quantum correlations) or separable (classical correlations). This can also be viewed as 'quantum steering into the past'."

 

[mattt]

But DrChinese, your description of the situation in this thread is not accurate.

Peter, Arnold and myself are using the proper mathematics of NRQM to support everything we say.

You are not doing the same. You only present some fictional mathematical symbols, that supposedly represent a "state" that doesn't exist in NRQM, and pretend that everything is OK (an attitude that is quite surprising to me, by the way).

Another strange thing is your tendency to constantly misrepresent what other posters are writing. For example more than once you were trying to suggest that Arnold or I were disputing those experimental results. That was never true. Or that we said that NRQM could not explain them. Never true again.

This causes more repetition than necessary, because we then feel obligued to state that we never implied those things, and the thread goes on and on in circles, when it could have ended had you addressed the only thing we were asking for: "a theoretical, mathematical explanation of those states within the mathematics of NRQM".

Obviously there is a huge asymmetry in the solidness of the different reasonings here.

By the way, I say all this with all my respect to you and anyone else. I love to talk about Physics and Mathematics, but I think it is sad when very interesting threads turn at the end in a succession of pointless repetitions.

English is neither my first nor my second language, so I hope I didn't sound harsh or inappropiate.

 

[PeterDonis]

the authors *are* authorities

I disagree. There are no authorities in science. Everything in science stands or falls by whether it matches experimental results. It doesn't matter who says it or what credentials they have.

And when experiments can't decide the question, as in this case (and in any case involving QM interpretations--here it's basically realist vs. non-realist), there is no way to resolve the question. Certainly you can't resolve it by saying that a Nobel prize winner says X, therefore X is right. The best we can do is to express the different viewpoints as best we can. That has certainly been done in this thread, but it's all that can be done.

the 1927 Schrödinger equation (i.e. prior to the discovery of entanglement)

Um, what?

First, the Schrodinger equation is still the basis of NRQM today.

Second, while our understanding of entanglement has improved since the 1920s, the mathematical basis for analyzing it in NRQM is still the Schrodinger equation, which already contains all the necessary ingredients--all you need is an appropriate interaction Hamiltonian.

So I do not think your dismissive attitude here is at all justified.

you question whether my perspective matches scientific consensus

Science doesn't work on "consensus" any more than it works on authoritative pronouncements.

That said, the use of the Schrodinger equation in NRQM is "scientific consensus", as embodied in more textbooks and peer-reviewed papers than we have time to count or reference here.

Also, your "perspective" does not necessarily match the actual claims that are intended by the authors of the papers you cite. Every quote you have given is ambiguous, for reasons that have already been given multiple times in this thread. You are of course entitled to your opinion about what you think the authors mean, but you can't just help yourself to the claim that your opinion is obviously correct and no other interpretation of what they mean is possible.

Are you being objective?

Since we are discussing QM interpretations, there is no "objective" way to resolve our disagreement. So expecting objectivity from anyone in this discussion, including you, is expecting too much. That is an unavoidable aspect of any discussion in this subforum, and is one of the main reasons this subforum was split off from the main QM forum.

 

[PeterDonis]

I would ask you to re-consider the full value of the citations I have provided.

I've already given your citations plenty of consideration. My opinion has been stated.

If after that, you think the thread has no ongoing utility, then the decision to close is yours to make without objection from me.

I'm going to close the thread for moderation at this point so other mentors can take a look.

 

[jedishrfu]

Upon review, it seems this thread has run its course and so now is a good time to close it.

We have all exhausted the subject and its time to move on to other topics.

Thank you all for contributing here.

Jedi