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

[tade]

So I'm curious about an idea of using a specially modified polariser to send signals with quantum entanglement, curious to know what's more there is to learn about the idea, for better or worse, its possibilities or impossibilities.

Ok so like in the Bell tests, measuring the spin of an elementary particle, and it has being likened to a coin flip, a 50/50 on either side, heads or tails.

But say we have a specially modified or fabricated polariser which can nudge the measurement results from 50/50 either way to 52/48. As the special polariser has been materially fabricated and synthesized with an anisotropic bias at the molecular level.

And then we are conducting Bell tests, with Alice and Bob monitoring the results at their respective ends. And then if Alice were to swap her polariser from a normal standard one to the special anisotropic one, Bob might be able to tell that she has done so, hence it being a form of signalling.

And I think that a 52/48 shift might be pretty interesting, it can be kinda small or really large depending on the angle you're viewing it from (and using "angle" in a metaphorical sense and not in literal physical one lol)

So I'm just curious and wondering about this idea, and also interested to know if it has been explored before.

 

[DrChinese]

So I'm curious about an idea of using a specially modified polariser to send signals with quantum entanglement, curious to know what's more there is to learn about the idea, for better or worse, its possibilities or impossibilities.

Ok so like in the Bell tests, measuring the spin of an elementary particle, and it has being likened to a coin flip, a 50/50 on either side, heads or tails.

But say we have a specially modified or fabricated polariser which can nudge the measurement results from 50/50 either way to 52/48. As the special polariser has been materially fabricated and synthesized with an anisotropic bias at the molecular level.

And then we are conducting Bell tests, with Alice and Bob monitoring the results at their respective ends. And then if Alice were to swap her polariser from a normal standard one to the special anisotropic one, Bob might be able to tell that she has done so, hence it being a form of signalling.

And I think that a 52/48 shift might be pretty interesting, it can be kinda small or really large depending on the angle you're viewing it from (and using "angle" in a metaphorical sense and not in literal physical one lol)

All Bob ever sees is a mixture of random outcomes, regardless of what Alice does. Kinda hard to send a message with that.

And yes, you can fashion a setup where there is a bias as you imagine. But that doesn't make any difference. The correlations come only when you compare Alice AND Bob's results. Which requires standard signaling (limited by light speed of course).

 

[tade]

All Bob ever sees is a mixture of random outcomes, regardless of what Alice does. Kinda hard to send a message with that.

And yes, you can fashion a setup where there is a bias as you imagine. But that doesn't make any difference. The correlations come only when you compare Alice AND Bob's results. Which requires standard signaling (limited by light speed of course).

oh interesting, though, I think that I'm not looking at any correlations, but just Bob observing some changes only from his own end, changes brought about by Alice swapping the polarisers.

 

[PeterDonis]

Bob observing some changes only from his own end, changes brought about by Alice swapping the polarisers

Bob cannot observe any change at his end based on Alice making changes at her end. He just sees random outcomes regardless. The only way for Bob to know that Alice has made a change at her end is for Alice to send a signal (at no greater than the speed of light) to tell him.

 

[tade]

Bob cannot observe any change at his end based on Alice making changes at her end. He just sees random outcomes regardless. The only way for Bob to know that Alice has made a change at her end is for Alice to send a signal (at no greater than the speed of light) to tell him.

so why doesn't swapping the polarisers lead to a change in the random outcomes

 

[DrChinese]

oh interesting, though, I think that I'm not looking at any correlations, but just Bob observing some changes only from his own end, changes brought about by Alice swapping the polarisers.

Alice sees:
HTTHHTTHHTT

Bob sees:
HTTHHTTHHTT

If Alice changes her polarizer (or doesn't even have one at all!), it doesn't affect Bob's results (as far as can be anyone knows) because they are RANDOM. All Bob ever sees is a random mix of H and T (or 0/1 or U/D or whatever). While you may not care about the correlations, that is what entanglement gives you: a precise estimate of Alice/Bob correlations.

 

[tade]

Alice sees:
HTTHHTTHHTT

Bob sees:
HTTHHTTHHTT

If Alice changes her polarizer (or doesn't even have one at all!), it doesn't affect Bob's results (as far as can be anyone knows) because they are RANDOM. All Bob ever sees is a random mix of H and T (or 0/1 or U/D or whatever). While you may not care about the correlations, that is what entanglement gives you: a precise estimate of Alice/Bob correlations.

though I was thinking that Bob's end might change from 50/50 to 52/48

 

[PeterDonis]

why doesn't swapping the polarisers lead to a change in the random outcomes

Because they are random, and random outcomes can't be controlled.

though I was thinking that Bob's end might change from 50/50 to 52/48

No. Bob's ratio of random outcomes only depends on the setting of Bob's polarizer, not on the setting of Alice's.

 

[tade]

No. Bob's ratio of random outcomes only depends on the setting of Bob's polarizer, not on the setting of Alice's.

hmm how about say Bob's photons being in entangled correlations with what Alice is doing, her polariser swapping

 

[PeterDonis]

how about say Bob's photon being in an entangled correlation with what Alice does

If you want to talk about that (very different) scenario, you will need to figure out how to entangle Bob's photon with "what Alice does" and then describe such a scenario so it can be analyzed.

 

[tade]

If you want to talk about that (very different) scenario, you will need to figure out how to entangle Bob's photon with "what Alice does" and then describe such a scenario so it can be analyzed.

so I imagine that the photons are entangled as per usual, though when it comes to the measurements, Alice has the available option to midway-through swap to the special polariser type, which as mentioned earlier, I'm wondering about the possibilities of the scheme, perhaps such as the possibility of creating such a polariser, and of it not breaking the entanglement

 

[PeterDonis]

so I imagine that the photons are entangled as per usual, though when it comes to the measurements, Alice has the available option to midway-through swap to the special polariser type, which as mentioned earlier, I'm wondering about the possibilities of the scheme, perhaps such as the possibility of creating such a polariser, and of it not breaking the entanglement

This just looks like the same scenario you posed in the OP, which has already been answered.

 

[tade]

This just looks like the same scenario you posed in the OP, which has already been answered.

hmm, seems like we're stuck in a comments loop

 

[tade]

This just looks like the same scenario you posed in the OP, which has already been answered.

hmm, seems like we're stuck in a comments loop

as i have replied to every single reply so far

 

[PeterDonis]

hmm, seems like we're stuck in a comments loop

as i have replied to every single reply so far

That doesn't mean you have added any substance.

 

[tade]

That doesn't mean you have added any substance.

ok, let's take a look at it again then

well, I'm thinking that maybe you might want to not effectively loop back to #8, of you saying "No.", and me asking "hmm how about.." and then it loops back again, and not sure if it really addresses the question, a "vicious circle"

though not meaning to say that someone is at fault, i think that its just the way that it has ended up.

 

[PeterDonis]

me asking "hmm how about.."

Read my response to that in #12 again. If you thought you were proposing some different scenario, you were wrong, and you need to go back and think again.

 

[tade]

ok, let's take a look at it again then

well, I'm thinking that maybe you might want to not effectively loop back to #8, of you saying "No.", and me asking "hmm how about.." and then it loops back again, and not sure if it really addresses the question, a "vicious circle"

though not meaning to say that someone is at fault, i think that its just the way that it has ended up.

hey @DrChinese maybe let's start from #7 and try it out

 

[PeterDonis]

hey @DrChinese maybe let's start from #7 and try it out

The response from @DrChinese in post #6 already answers post #7. So does my response in post #8.

 

[tade]

The response from @DrChinese in post #6 already answers post #7. So does my response in post #8.

oh, from #7 to #6, an instant loop-back

though I'm not sure if it does address, but no worries, i think we can just elaborate and add-on points without much issue

 

[tade]

Read my response to that in #12 again. If you thought you were proposing some different scenario, you were wrong, and you need to go back and think again.

hmm, well, in that case, how about you elaborate on the latter part of #8 referencing on my hopefully elaborations or clarifications of #9 and #11

 

[PeterDonis]

how about you elaborate on the latter part of #8 referencing on my hopefully elaborations or clarifications of #9 and #11

I don't think you quite understand: there are no "elaborations or clarifications" in your posts #9 or #11. Nothing you say in those posts changes my response in post #8 at all. You might want to go re-read that response again and again until its meaning sinks in. I don't think you have fully realized that meaning. It is very general.

i think we can just elaborate and add-on points without much issue

So far you have had zero success in doing this.

 

[tade]

I don't think you quite understand: there are no "elaborations or clarifications" in your posts #9 or #11. Nothing you say in those posts changes my response in post #8 at all. You might want to go re-read that response again and again until its meaning sinks in. I don't think you have fully realized that meaning. It is very general.So far you have had zero success in doing this.

hey cool it with the shade man lol

and just to clarify, by "at all", do you mean keeping #8 just exactly as it is, and no like, adding some elaboration?

 

[PeterDonis]

by "at all", do you mean keeping #8 just exactly as it is, and no like, adding some elaboration?

Yes. The last sentence of post #8 is perfectly straightforward and explicit and needs no elaboration.

 

[tade]

Yes. The last sentence of post #8 is perfectly straightforward and explicit and needs no elaboration.

oh i see, though i feel that despite being straightforward and explicit it could probably benefit more from some elaboration

 

[PeterDonis]

though i feel that despite being straightforward and explicit it could probably benefit more from some elaboration

What kind of elaboration would you like?

 

[tade]

What kind of elaboration would you like?

hmm, well, in that case, how about you elaborate on the latter part of #8 referencing on my hopefully elaborations or clarifications of #9 and #11

oh I'm referring to this

 

[PeterDonis]

i'm referring to this

I've already responded to that. Rather than keep pointing to prior posts that have not gotten across whatever point you think you are trying to make, try making a new post that tries a different way of getting across whatever point you think you are trying to make.

 

[tade]

I've already responded to that. Rather than keep pointing to prior posts that have not gotten across whatever point you think you are trying to make

Yeah no worries I know you already have, I'm just answering your question

 

[tade]

try making a new post that tries a different way of getting across whatever point you think you are trying to make.

ok hmm let's see, I'm finding it kinda hard because of how you're so unequivocal, #22, #24, and not sure what I could do, as I've never encountered anything like this before.

but let me try...

 

[tade]

try making a new post that tries a different way of getting across whatever point you think you are trying to make.

so you said that Bob's ratio of random outcomes only depends on the setting of Bob's polarizer, not on the setting of Alice's, and I was wondering about the entanglement between Alice and Bob, so I was thinking that maybe you could explain about the entanglement together with why Bob's ratio doesn't depend on Alice's settings, and/or about how the inclusion of the special 52/48 polariser may or may not affect things

 

[tade]

@vanhees71 hi, hope I'm not bothering, cos i noticed that you've seen the thread, and i think you're also interested in the field, and so just wondering would like to ask what you think about the OP, thanks

 

[vanhees71]

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.

 

[Nugatory]

But say we have a specially modified or fabricated polariser which can nudge the measurement results from 50/50 either way to 52/48.

Some of the difficulty here is that this nudging violates a basic principle of quantum mechanics, the Born rule. Thus the question is somewhat analogous to asking “say we have a device that violates conservation of energy. Can we use it to build a perpetual motion machine?” and the answer will be the same as you’re hearing in this thread: “Assume what you want, but we still can’t build a perpetual motion machine”.

 

[tade]

Some of the difficulty here is that this nudging violates a basic principle of quantum mechanics, the Born rule. Thus the question is somewhat analogous to asking “say we have a device that violates conservation of energy. Can we use it to build a perpetual motion machine?” and the answer will be the same as you’re hearing in this thread: “Assume what you want, but we still can’t build a perpetual motion machine”.

oh i see, maybe can you elaborate more about the basis behind the Born rule and its exclusion of nudging, thanks

 

[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.

thanks, an interesting informative post
so is it that there's no 'spooky action at a distance' occurring?

 

[vanhees71]

Not according to relativistic local QFT.

 

[tade]

Not according to relativistic local QFT.

and that's pretty interesting, because I think that one of this year's Nobel winners, John Clauser, said that he was initially distressed by the results of his own groundbreaking experiments, because he was more of an Einsteinian in his personal metaphysical notions. In fact I think even now, he's still not too happy about having experimentally proven the incredible properties of quantum mechanics.
Though maybe the way that you describe it could be a way for Clauser to be at peace with both his experimental results and his metaphysical notions.

 

[vanhees71]

Well, I think also Bell was not too happy about the outcome of these experiments. For me it's hard to understand that a first-rank QFT expert like Bell was a proponent of Bohmian QM (which in my opinion only works in non-relativistic QM, and in non-relativistic physics there's anyway no problem with actions at a distance but it's rather the standard Newtonian way to describe interactions as in the Newtonian theory of gravity). Microcausality is at the very foundation of local relativistic QFT precisely because it's a viable realization of the causality principle in relativistic QT, and it's till today the only realization. Whether there are non-local Einstein-causal relativistic theories, I don't know.

 

[Nugatory]

oh i see, maybe can you elaborate more about the basis behind the Born rule and its exclusion of nudging, thanks

I did link to the Wikipedia article, but that is somewhat heavy going. If you are really interested in this stuff but not up for a serious college-level intro to quantum mechanics you might give Giancarlo Ghirardi’s book “Sneaking a look at God’s cards” a try.

 

[DrChinese]

so you said that Bob's ratio of random outcomes only depends on the setting of Bob's polarizer, not on the setting of Alice's, and I was wondering about the entanglement between Alice and Bob, so I was thinking that maybe you could explain about the entanglement together with why Bob's ratio doesn't depend on Alice's settings, and/or about how the inclusion of the special 52/48 polariser may or may not affect things

A couple of points, some of which are reiterations of earlier ones:

a. 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. Further, it is not unusual for the outcomes of actual Bell tests to evidence a slight variance from 50-50. This has no practical effect on the experimental conclusion (which is normally to exclude local realistic theories).

b. All Alice ever sees is a stream of random bits (H/T, U/D, 0/1 or whatever you label it). If Alice has a typical polarizer, the outcomes will average 50-50. If she has a less accurate polarizer, the percentage could be skewed (say 48-52 for purposes of discussion). The stream will still be random. If Alice changes from one polarizer to another, she can send a signal to herself (since one polarizer stream will be 50-50, the other will be 48-52, and she can eventually detect the difference).

c. All Bob ever sees is a stream of random bits (H/T, U/D, 0/1 or again whatever you label it). Here's the important point: nothing Alice does changes this fact! Even IF Alice could somehow change the stream of Bob, then Bob's stream would still be completely random. So Bob has no way to detect or otherwise sense any change by Alice.

d. In the most extreme scenario, Alice removes her polarizer completely and she see a stream that is 0-100. This too changes nothing for Bob's statistics, which never change.

e. The only thing that changes when Alice acts is the CORRELATION between Alice's results and Bob's results. These vary according to the predictions of quantum mechanics, and have values between 0 and 100% correlation. This is what is studied in Bell tests. If you study Bell (or Clauser or Aspect or Zeilinger) you will see that the quantum prediction is incompatible with all hidden variable theories - unless such theories allow for faster-than-light action. There is no known example of faster-than-light action that allows for signaling,

I should point out that PeterDonis and Nugatory are quantum forum moderators, and they are actually being very nice to you (and patient!). While you may feel your questions are not being addressed... the issue is that you aren't actually varying your questions. In every one, there are entangled pairs and in all cases the results are described by my points a-e above. And there is no possibility of FTL signaling because Bob always sees the same thing - a random stream. And a random stream, by definition, lacks any information by itself.

 

[tade]

I did link to the Wikipedia article, but that is somewhat heavy going. If you are really interested in this stuff but not up for a serious college-level intro to quantum mechanics you might give Giancarlo Ghirardi’s book “Sneaking a look at God’s cards” a try.

yeah, thanks, so I'm thinking of like a streamlined explanation of the exclusion of 'nudging' and such

 

[tade]

The only thing that changes when Alice acts is the CORRELATION between Alice's results and Bob's results. These vary according to the predictions of quantum mechanics, and have values between 0 and 100% correlation.

thanks, are these correlations the correlations of Alice and Bob randomly selecting spin directions to measure in?
and also kinda confused because I think Nugatory's suggesting that the issue is that the nudging is unphysical

 

[tade]

Well, I think also Bell was not too happy about the outcome of these experiments. For me it's hard to understand that a first-rank QFT expert like Bell was a proponent of Bohmian QM (which in my opinion only works in non-relativistic QM, and in non-relativistic physics there's anyway no problem with actions at a distance but it's rather the standard Newtonian way to describe interactions as in the Newtonian theory of gravity). Microcausality is at the very foundation of local relativistic QFT precisely because it's a viable realization of the causality principle in relativistic QT, and it's till today the only realization. Whether there are non-local Einstein-causal relativistic theories, I don't know.

and oh yeah, i was wondering how does microcausality/localism fit into Bell's theorem(s) about quantum mechanics

 

[PeterDonis]

you said that Bob's ratio of random outcomes only depends on the setting of Bob's polarizer, not on the setting of Alice's

Yes.

I was wondering about the entanglement between Alice and Bob

The entanglement is between Alice's and Bob's photons, not between Alice and Bob. The statement you quoted above is true when the photons are entangled.

I was thinking that maybe you could explain about the entanglement together with why Bob's ratio doesn't depend on Alice's settings

Because that's how entanglement in QM works. If Alice's and Bob's photons are entangled, that means their polarization measurement results are correlated, but the only way to see the correlations is to compare Alice's and Bob's results, which means Alice and Bob need to communicate those results to each other. @DrChinese already explained this in post #2.

and/or about how the inclusion of the special 52/48 polariser may or may not affect things

A 52/48 polarizer is still a polarizer. If it's Alice's polarizer, which is what you said before, you should be able to deduce what effect it will have on Bob's ratio of random outcomes from the statement I've already given.

 

[PeterDonis]

are these correlations the correlations of Alice and Bob randomly selecting spin directions to measure in?

Read the very sentence you quoted from @DrChinese. What does it say the correlations are correlations of? It's right there in the quote.

 

[tade]

The entanglement is between Alice's and Bob's photons, not between Alice and Bob.

Well I would hope not, otherwise someone might get slapped in the face at the Oscars

the only way to see the correlations is to compare Alice's and Bob's results

so I was thinking that Bob might notice a shift from 50/50 to 52/48 without him having done anything special, though I guess you've already ruled that out, and is there an underlying explanation/mechanism for why it doesn't occur

 

[tade]

Read the very sentence you quoted from @DrChinese. What does it say the correlations are correlations of? It's right there in the quote.

hmm, sorry I'm still really not sure

 

[Nugatory]

so I was thinking that Bob might notice a shift from 50/50 to 52/48 without him having done anything special, though I guess you've already ruled that out, and is there an underlying explanation/mechanism for why it doesn't occur

it is indeed ruled out. As for the underlying reason….

The state (you may have heard it called the wavefunction) of the entangled two-particle system can be written as $|\psi\rangle=\frac{1}{\sqrt{2}}|HV\rangle+ \frac{1}{\sqrt{2}}|VH\rangle$. The $|\rangle$ thingies are called “kets” and they are abstract mathematical objects that add like vectors. The ket $|HV\rangle$ represents the state “Alice’s particle will pass through a horizontal polarizer and Bob’s particle will pass through a vertical polarizer”, and vice versa for the ket ket $|VH\rangle$. The overall state is the ket $|\psi$ and the sum indicates that it is a superposition that will collapse to either $|HV\rangle$ or $|VH\rangle$ when the system interacts with a polarizer in any way. (Using abstract mathematical objects to represent the physics is not as weird as it seems - numbers are abstract mathematical objects but we’re comfortable using them to represent physical quantities like speeds. It’s just that here we have more complicated physics so we need more complicated mathematical objects to represent it).

So that’s the essence of entanglement. If Alice’s measurement is H then the wave function has collapsed to $|HV\rangle$; when and if Bob measures his particle he will get V. If Alice’s measurement is V the wave function has collapsed to $|VH\rangle$ and Bob will get H. But what is the probability of either outcome?

This is where the Born rule comes in. It says that the probability of the wavefunction collapsing to a given state is the square of the coefficient of that state in the superposition. Here both coefficients are $\frac{1}{\sqrt{2}}$, and when we square that we get $\frac{1}{2}$ - both possibilities occur with 50% probability, it is completely random which one we get for any particular entangled pair, and until you introduce your hypothetical modified polarizer both just see an endless series of random H and V results. It’s only when they get together after the fact and compare notes that they see that whenever one of them measured H the other measured V so they must have been working with entangled pairs.

And now we’ve gotten to where we can introduce your hypothetical modified polarizer. There are two possibilities:
1) The modified polarizer violates the Born rule: even though the coefficients and their squares are equal the probability of collapse to either state are not equal. But the Born rule is a fundamental axiom of quantum mechanics, like energy conservation is in classical physics. A device that violates it is impossible the same way that perpetual motion machines violating conservation of energy are impossible. So this line of thought takes us to the logically hopeless situation of trying to apply the laws of physics while assuming that they don’t apply.
2) The modified polarizer doesn’t violate the Born rule, but after the collapse it sometimes changes Alice’s H in the $|HV\rangle$ to a V leaving the system in the state $|VV\rangle$. That will give her the 52/48 ratio you’ve been looking for. However, that doesn’t change Bob’s V/H ratio because he’s still getting the same V and H results. (This, BTW, is why the first measurement on an entangled system breaks the entanglement - once that initial superposition collapses the two sides evolve independently).

 

[PeterDonis]

I was thinking that Bob might notice a shift from 50/50 to 52/48 without him having done anything special, though I guess you've already ruled that out

Definitely.

sorry I'm still really not sure

Here's what you quoted from @DrChinese:

The only thing that changes when Alice acts is the CORRELATION between Alice's results and Bob's results. These vary according to the predictions of quantum mechanics, and have values between 0 and 100% correlation.

Read the first sentence. Then read it again. And again. And keep doing so until you realize what the answer to your question is. The word "correlation" is right there (it's even capitalized) followed by description of what the correlation is a correlation of.