Curious about an idea of a modified polariser to send signals with QE

[tade]

Then you don't really have an "I" level background in this subject, and it's hard for me to see how you would be able to follow any explanations more complicated than "well, that's just how it works". Which is basically what you've already been told.How the correlation changes with what?

oh no worries, i think i would be able to

as DrChinese said, "The only thing that changes when Alice acts is the CORRELATION", so it'd be Alice swapping the polarisers

 

[Motore]

oh no worries, i think i would be able to

Can you show some effort yourself?
At least check a textbook, post the math and then ask about that math.
You are responding very quickly and it seems to me you are not digesting the answers because your follow up questions are all pretty much the same.

 

[vanhees71]

If you have a Bell state, e.g., the spin-0 state,
$$|\Psi \rangle=\frac{1}{\sqrt{2}} (|HV \rangle-|VH \rangle),$$
then the single photons are precisely unpolarized, i.e.,
$$\hat{\rho}_A =\mathrm{Tr}_B |\Psi \rangle \langle \Psi|=\frac{1}{2} \hat{1}, \quad \hat{\rho}_B=\mathrm{Tr}_A |\Psi \rangle \langle \Psi|=\frac{1}{2} \hat{1},$$
and thus if Alice measures 48% H and 52% V, her polarization experiment is somehow inaccurate. As has been correctly stressed over and over again that cannot be used to transmit a signal, because all that B has are also precisely unpolarized photons. With a correctly working polarization measurement he'll simply get a random sequence of H and V polarized photons with the 50%:50% probabilities, no matter what A does with her photons. Only if both detectors work precisely you can figure out the correlations predicted by QED given the two-photon Bell state, and this can be done only by comparing A's and B's measurement protocols, which is possible only by exchange of this information, which is possible only with real-world signals, which propagate at most at the speed of light.

 

[tade]

Can you show some effort yourself?
At least check a textbook, post the math and then ask about that math.
You are responding very quickly and it seems to me you are not digesting the answers because your follow up questions are all pretty much the same.

Well, from my perspective, I am examining the answers and it seems to be pretty roundabout, you can pick anyone of my replies and I can explain it.

And you've been following the thread right, and I think that the main issue is that you're "skeptical" right

and anyway, also since you've been following, hope you don't mind if I ask you, what do you think are the changes which DrChinese is referring to in "The only thing that changes when Alice acts is the CORRELATION"

Because to be clear its just that I'm interested to know what they are.

 

[tade]

If you have a Bell state, e.g., the spin-0 state,
$$|\Psi \rangle=\frac{1}{\sqrt{2}} (|HV \rangle-|VH \rangle),$$
then the single photons are precisely unpolarized, i.e.,
$$\hat{\rho}_A =\mathrm{Tr}_B |\Psi \rangle \langle \Psi|=\frac{1}{2} \hat{1}, \quad \hat{\rho}_B=\mathrm{Tr}_A |\Psi \rangle \langle \Psi|=\frac{1}{2} \hat{1},$$
and thus if Alice measures 48% H and 52% V, her polarization experiment is somehow inaccurate. As has been correctly stressed over and over again that cannot be used to transmit a signal, because all that B has are also precisely unpolarized photons. With a correctly working polarization measurement he'll simply get a random sequence of H and V polarized photons with the 50%:50% probabilities, no matter what A does with her photons. Only if both detectors work precisely you can figure out the correlations predicted by QED given the two-photon Bell state, and this can be done only by comparing A's and B's measurement protocols, which is possible only by exchange of this information, which is possible only with real-world signals, which propagate at most at the speed of light.

oh sorry, to be crystal clear, is this explanation with the assumption that the 52/48 nudge is unphysical or impossible?

 

[tade]

I've very often expressed my opinion on this topic. For me it is very clear that there's no possibility to send signals faster than light using quantum entanglement. The reason is that this impossibility is implemented in relativistic quantum field theory by the socalled microcausality principle. In my scientific community, high-energy particle/nuclear physics, that's what's called "locality", and this excludes any causal connections between space-like separated events.

The observed long-ranged correlations between space-like separated measurements are due to the preparation of the system in the entangled state and not due to any "spooky action at a distance" of one measurement apparatus at position A on the part of the system at B or the measurement apparatus used at B.

Since thus relativistic local QFT realizes locality via the microcausality constraint on local observables, what one has to give up according to Bell's theorem is "realism", i.e., the assumption that all observables always take determined values, which are only appearing probabilistic because of our ignorance of some "hidden variables".

Whether there are non-local deterministic (realistic) relativistic models in accordance with the observations I don't know.

and I came across this article from Quanta Magazine titled "How Bell’s Theorem Proved ‘Spooky Action at a Distance’ Is Real", seems like they're really affirmative on it

https://www.quantamagazine.org/how-...spooky-action-at-a-distance-is-real-20210720/

 

[vanhees71]

It's a pop-sci article on a subject, which cannot be communicated on that level in all details appropriately, and given that constraint it did a good job. Generally I think, QUANTA is a pretty good pop-sci source, although quite often I don't understand their articles and only when reading the scientific papers I understand what they want to say (provided, I've enough expertise in the subject discussed in that papers ;-)).

The point is that, as soon as you simply accept that in Nature there is inherent randomness as described by the minimal statistical interpretation of the QFT formalism that what's described by entangled states of the kind we discuss here are correlations of observables on far-distantly measured parts of the entangled quantum system (e.g., two photons in an entangled state). These correlations are there, because of the preparation of the system in such an entangled state, and thus nothing in the QFT description needs a faster-than-light-signal propagation between the two spacelike separated measurement events (e.g., clicks of the far-distant photon detectors in our example). Within relativistic local (i.e., microcausal) QFT by construction such faster-than-light signals are excluded, and as long as QFT (in our example it's QED, the best-tested QT ever) can explain the phenomena, you have at least one theory describing these phenomena, and this implies that what has to be abandoned of Bell's class of realistic local hidden-variable theory is reality (i.e., the assumption that all observables always take determined values), because local QFT is by construction local (i.e., space-like separated events cannot be causally connected within this theory).

 

[DrChinese]

1. and also i was wondering about the maths of the situation at the moment when the wave encounters the molecules of the polariser, the interaction of the target particle with the polariser molecules

2. also the maths of the transition when and as the wavefunction is collapsing as it encounters and interacts with the molecules of the polariser...

3. and as DrChinese has said: "It is possible for Alice to have a polarizer which produces outcomes different than the usual 50-50. For obvious reasons, manufacturers of polarizing beam splitters work very hard to achieve as close to 50-50 as possible.", so that sounds pretty interesting, like, perhaps they could go in the opposite direction, and deliberately manufacture a wonky polariser, and try testing quantum entanglement with that

1. This has nothing to do with entanglement. And in some ways has nothing to do with quantum mechanics. The full details of how a PBS works requires a working knowledge of quantum optics, and explaining that is far outside the scope of any PF thread. Although the following reference is not a summary in any way, I think you would benefit from studying it. It is specifically about entangled particles encountering a polarizing beam splitter as a demonstration of the existence of photons (as a nonclassical field).

http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf2. The concept of collapse is interpretation dependent. There is no known quantum explanation of collapse that fits into what you are asking. If quantum collapse does occur, no one can pinpoint when or how it does experimentally. (Nor even theoretically, for that matter, since in principle collapse can be reversed.)3. As mentioned, no scientific manufacturer will intentionally manufacture "wonky" instrumentation. Any more than a toaster manufacturer will make toasters that don't cook your bread.

Keep in mind that in principle, you could yourself construct a black box device that is inefficient at performing the T and R operations of a PBS - and presents any combination of output percentages you care to work with. However, the problem is that none of this actually allows any meaningful exploration of quantum entanglement (as far as I can imagine, although I am far from the authority on such). As is always the case in the experimental world: scientists experiment to prove/disprove/quantify specific ideas they feel have value. They don't normally spend their days working on "interesting" speculations by non-professionals.

 

[tade]

Although the following reference is not a summary in any way, I think you would benefit from studying it. It is specifically about entangled particles encountering a polarizing beam splitter as a demonstration of the existence of photons (as a nonclassical field).

http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf

oh i see, because i was thinking about and hoping for literature on the maths and mechanics of photon-polariser interactions at the quantum-molecular level, to take a look at the limits of what the Born probabilities might possibly be at the moment of interaction

Keep in mind that in principle, you could yourself construct a black box device that is inefficient at performing the T and R operations of a PBS - and presents any combination of output percentages you care to work with. However, the problem is that none of this actually allows any meaningful exploration of quantum entanglement

hmmm, I was thinking about the possible meaningfulnesses of my OP example, though is it that its ruled-out like Nugatory saying in #49

 

[tade]

These correlations are there, because of the preparation of the system in such an entangled state, and thus nothing in the QFT description needs a faster-than-light-signal propagation between the two spacelike separated measurement events (e.g., clicks of the far-distant photon detectors in our example).

man that's really mind-bending

 

[Nugatory]

man that's really mind-bending

Do remember that quantum mechanics is not a theory about what’s going on, it’s a theory about measurement results. QM says “these are the outcomes we’ll see if we look”, not “this is what is happening”.

 

[tade]

Do remember that quantum mechanics is not a theory about what’s going on, it’s a theory about measurement results. QM says “these are the outcomes we’ll see if we look”, not “this is what is happening”.

yeah that's true, though I was also thinking that Bell's theorem was to decide the issue of locality and spooky action at a distance

by the way just wondering is it you in your profile pic, at first glance I thought it was something like Hugh Laurie playing a physicist lol

 

[vanhees71]

Bell's theorem is about the properties of local realistic theories, where locality has the standard meaning of relativistic physics, i.e., that there cannot be causal connections between space-like separated events, particularly there cannot be a causal influence of one measurement at another measurement, where the measurement events (e.g., the registration of two photons at far-distant places by detectors) are space-like separated. "Reality" means that the theory assumes that all observables take always determined values, which may be unknown due to the inobservabality of some "hidden variables".

Now by construction the usual relativistic QFTs are "local", i.e., all operators representing local observables commute at space-like distance between their arguments. Particularly any such local observable operator commutes with the Hamilton density at space-like separated arguments, which implies that there cannot be FTL signals of any kind. At the same time these QFTs are not "realistic" in Bell's sense, because as any QT there's no "dispersion free state", i.e., in any state you may prepare any quantum system in there are observables that do not take a determined value. E.g., if you prepare a photon to be horizontally polarized in wrt. to some direction, it's polarization state wrt. another non-collinear direction is indetermined.

Bell's inequality, derived from the assumption of a "local realistic HV theory", is predicted to be violated within QT (including its realization of a relativistic local QFT), and thus the question, whether there's a local realistic HV theory that explains all observable facts or whether this is the case for QT can be decided by experiment, and this has been done over the last 40 years with the unanimous result that QT delivers the correct description, while any local realistic HV theory fails to do so. That's the content of the work of the three Nobel Laureats of this year.

 

[tade]

Bell's inequality, derived from the assumption of a "local realistic HV theory", is predicted to be violated within QT (including its realization of a relativistic local QFT), and thus the question, whether there's a local realistic HV theory that explains all observable facts or whether this is the case for QT can be decided by experiment, and this has been done over the last 40 years with the unanimous result that QT delivers the correct description, while any local realistic HV theory fails to do so. That's the content of the work of the three Nobel Laureats of this year.

 

[vanhees71]

I'm not sure, whether this statement is really what Clauser (or any other physicist working with quantum theory) wants to say, because of course Clauser very well understands quantum mechanics (and for sure also QT in general, i.e., including relativistic QFT), because he has been able to use it to plan and do his seminal experiment in relation to Bell's theoretical proposal. I think there's not more to understand about a physical theory than what it predicts for real-world observations of Nature, and if you even can use it to plan and do a very dedicated experiment testing its very foundations, that's proof of a very deep understanding of its implications.

What he perhaps means is the difficulty to understand that there's "irreducible randomness" in nature, i.e., that the outcome of the measurement of an observable which is indetermined due to the state the measured system is prepared in is not in some way predetermined but just unknown and how it comes that when measuring an observable we get a well-defined outcome although there's nothing predetermining this outcome, but that's precisely what QT implies and what this year's Nobel laureats (and many more) have unambigously demonstrated with their experiments. So it's not so much a lack of understanding of quantum theory as a scientific theory but the difficulty to accept this "irreducible randomness" implied by its formalism.

It's in the nature (pun intended) of the natural sciences that its methods tells us how nature behaves, even if this behavior is against our intuition based on our very limited experience of the "everyday world". For many people in the renaissance it was "incomprehensible" how it could be that the Earth moves around the Sun, which contradicts our experience that we and the Earth "are at rest" and the Sun moves in a very regular way.

 

[tade]

I'm not sure, whether this statement is really what Clauser (or any other physicist working with quantum theory) wants to say, because of course Clauser very well understands quantum mechanics (and for sure also QT in general, i.e., including relativistic QFT), because he has been able to use it to plan and do his seminal experiment in relation to Bell's theoretical proposal. I think there's not more to understand about a physical theory than what it predicts for real-world observations of Nature, and if you even can use it to plan and do a very dedicated experiment testing its very foundations, that's proof of a very deep understanding of its implications.

What he perhaps means is the difficulty to understand that there's "irreducible randomness" in nature, i.e., that the outcome of the measurement of an observable which is indetermined due to the state the measured system is prepared in is not in some way predetermined but just unknown and how it comes that when measuring an observable we get a well-defined outcome although there's nothing predetermining this outcome, but that's precisely what QT implies and what this year's Nobel laureats (and many more) have unambigously demonstrated with their experiments. So it's not so much a lack of understanding of quantum theory as a scientific theory but the difficulty to accept this "irreducible randomness" implied by its formalism.

It's in the nature (pun intended) of the natural sciences that its methods tells us how nature behaves, even if this behavior is against our intuition based on our very limited experience of the "everyday world". For many people in the renaissance it was "incomprehensible" how it could be that the Earth moves around the Sun, which contradicts our experience that we and the Earth "are at rest" and the Sun moves in a very regular way.

I think what Clauser means is that he finds it hard to comprehend that spooky action at a distance is real

 

[PeterDonis]

I think what Clauser means is that he finds it to comprehend that spooky action at a distance is real

Such statements don't say anything useful, because they're about the definitions of words rather than physics.

The physics is that the Bell inequalities are violated in experiments on entangled particles.

If you define "spooky action at a distance" to mean "the Bell inequalities are violated", then of course "spooky action at a distance" is "real" because the Bell inequality violations are real; they're right there in the data.

But then saying "spooky action at a distance is real" is just another way of saying "the Bell inequalities are violated". It conveys no additional information. But it has much greater potential for confusion because people are much more likely to read other connotations into "spooky action at a distance" than they are into "Bell inequality violations", and to be led into claims that are not justified by the physics, or to misunderstand what the physics actually says and does not say.

 

[tade]

Such statements don't say anything useful, because they're about the definitions of words rather than physics.

The physics is that the Bell inequalities are violated in experiments on entangled particles.

If you define "spooky action at a distance" to mean "the Bell inequalities are violated", then of course "spooky action at a distance" is "real" because the Bell inequality violations are real; they're right there in the data.

But then saying "spooky action at a distance is real" is just another way of saying "the Bell inequalities are violated". It conveys no additional information. But it has much greater potential for confusion because people are much more likely to read other connotations into "spooky action at a distance" than they are into "Bell inequality violations", and to be led into claims that are not justified by the physics, or to misunderstand what the physics actually says and does not say.

hmm I was thinking that the view is that the Bell inequalities are violated due to spooky action at a distance

 

[PeterDonis]

I was thinking that the view is that the Bell inequalities are violated due to spooky action at a distance

But that would require "spooky action at a distance" to be something other than "Bell inequality violations". What other something would that be? Look through the physics literature, and you will not find any such thing. You will find lots of vague talk, but nothing in the actual physics. The actual physics is just that the Bell inequalities are violated in measurements on entangled particles. That's it.

 

[tade]

But that would require "spooky action at a distance" to be something other than "Bell inequality violations". What other something would that be? Look through the physics literature, and you will not find any such thing. You will find lots of vague talk, but nothing in the actual physics. The actual physics is just that the Bell inequalities are violated in measurements on entangled particles. That's it.

Could that other something be the entanglement itself, even though no scientist has as of yet been able to delve deeper into how it might work

 

[PeterDonis]

Could that other something be the entanglement itself

Then you have just defined "spooky action at a distance" as "entanglement". Still doesn't give any useful information that we didn't already have.

 

[tade]

Then you have just defined "spooky action at a distance" as "entanglement". Still doesn't give any useful information that we didn't already have.

hmm well I was just thinking that Clauser still can't wrap his mind around spooky action, despite having just won a Nobel for it lol, as he was originally spurred by his desire to disprove quantum mechanics and prove Einstein correct
though I think that its pretty cool that the universe and nature turned out this way, the QM way

 

[PeterDonis]

he was originally spurred by his desire to disprove quantum mechanics and prove Einstein correct

I'm not sure what you mean by this. Einstein never said QM was incorrect. He only said it was incomplete. If Einstein could have read Bell's papers, he would have agreed at once that QM predicts that the Bell inequalities are violated, and he would have expected actual experiments to bear that out.

 

[tade]

I'm not sure what you mean by this. Einstein never said QM was incorrect. He only said it was incomplete. If Einstein could have read Bell's papers, he would have agreed at once that QM predicts that the Bell inequalities are violated, and he would have expected actual experiments to bear that out.

oh that's interesting, whether or not Einstein would have been hoping that the experimental results would conform to his "pair of gloves" analogous model, certainly that's what Clauser was hoping for

 

[PeterDonis]

certainly that's what Clauser was hoping for

Meaning, Clauser expected the experiments not to match the predictions of QM? Do you have a reference for that? I'm not aware of any physicist who expected that.

 

[tade]

Meaning, Clauser expected the experiments not to match the predictions of QM? Do you have a reference for that? I'm not aware of any physicist who expected that.

oh, as in, I'm not sure what he was expecting, though he was hoping that the results wouldn't match the predictions of QM

 

[PeterDonis]

he was hoping that the results wouldn't match the predictions of QM

"Hoping" in the sense of being saddened when the results did match the predictions of QM, I take it.

 

[tade]

"Hoping" in the sense of being saddened when the results did match the predictions of QM, I take it.

yeah exactly, as the screenshot above says, "I was again very saddened that I had not overthrown quantum mechanics"

 

[vanhees71]

I think what Clauser means is that he finds it hard to comprehend that spooky action at a distance is real

But within relativistic microcausal QFT by construction there are no spooky actions at a distance, only long-ranged correlations between parts of an entangled quantum system.

 

[vanhees71]

Then you have just defined "spooky action at a distance" as "entanglement". Still doesn't give any useful information that we didn't already have.

Yes, and entanglement is a property of the state, i.e., of the preparation procedure on a quantum system and as such it does not describe interactions but correlations and that's why, within relativistic microcausal QFT, there's no violation of causality, because the correlations are not due to some "spooky action at a distance" (i.e., a faster-than-light influence among two space-like separated "measurement events").