Photons from separated sources can be entangled - after they were detected

[Frame Dragger]

Now I have found THE RIGHT solution of the problem,
completely different from my previous one.
It is so simple and obvious that, I am convinced,
everybody will accept it.
The solution consists of several conceptual steps.

0. Forget everything that I said in my previous posts
of this thread!

1. The standard delayed-choice experiment involves
two entangled particles. It cannot be used for FTL
transfer of information because the interference
is encoded in the COINCIDENCES between the entangled particles.
To observe the coincidences, one needs a CLASSICAL COMMUNICATION
between entangled systems, and classical communication
cannot be FTL.

2. Point 1. above is a special case of the general
property of QM: Without classical communication,
entanglement cannot be used to transmit information.
NOT EVEN SLOWER THAN LIGHT FORWARD IN TIME.

3. As we all know from everyday life,
by classical communication, information can be
transfered ONLY FORWARD IN TIME. This is related
to the second law of thermodynamics.

Now let us apply these facts to two variants of the
DrChinese setting.

4. Assume that Charlie does his choice BEFORE Alice and
Bob make their measurements. Can Alice and Bob observe
any consequences of this choice? Yes, but only if
Charlie sends a classical information to Alice and Bob.

5. Now consider a different situation.
Now assume that Charlie does his choice AFTER Alice and
Bob make their measurements. Can Alice and Bob observe
any consequences of this choice? They could if
Charlie could send a classical information to Alice and Bob.
However, Charlie cannot send classical information to the
past. Therefore, Alice and Bob cannot observe
any consequences of the Charlie's choice.

Q.E.D.

To me it seems that your assumption (one from dBB) lies in Point #3. Dr. Chinese is essentially making the case that such is not the case, or at least, that it is not relevant in a DCQE setting. Isn't this a re-expression of your original objections?

 

[DrChinese]

Can you elaborate on the point of just how that quantum state could reasonably be restored? I'm not disagreeing... I just don't understand.

Sure, here is a diagram which shows what I am referring to, and a reference to where it originated:

http://www.pas.rochester.edu/~AdvLab/Eberly_Bell_Inequalities_AJP.pdf

"We employ an arrangement of polarization analyzer loops to derive several simple Bell inequalities and then discuss the violation of one of them in light of quantum and classical interpretations of the data recorded."

 

[Frame Dragger]

Sure, here is a diagram which shows what I am referring to, and a reference to where it originated:

http://www.pas.rochester.edu/~AdvLab/Eberly_Bell_Inequalities_AJP.pdf

"We employ an arrangement of polarization analyzer loops to derive several simple Bell inequalities and then discuss the violation of one of them in light of quantum and classical interpretations of the data recorded."

Ok. That looks straightforward, if, as you say... challenging. Hmmmm... I wonder if you could get a grant for this? (EDIT: 'this' being the original A-D experiment, not the one in the article here)

 

[DrChinese]

To me it seems that your assumption (one from dBB) lies in Point #3. Dr. Chinese is essentially making the case that such is not the case, or at least, that it is not relevant in a DCQE setting. Isn't this a re-expression of your original objections?

Yes, here is where the language gets so tricky we run the risk of spinning in circles. So if I say something someone doesn't like, it may just be in the language.

The paradox occurs IF you assume the photons A & B were independent to begin with. If they weren't, then it is no weirder (really) that A & D are no longer independent photons after Charlie makes the BSM early. Now, if that isn't a true paradox (they never are), then it shouldn't be weird that Charlie's decision can be made after the fact. Because although A & D no longer exist, they weren't independent anyway. They are - in actuality - part of a chain that consists of A-B-C-D and that chain requires Alice, Bob AND Charlie. In my view, the causal chain is time symmetric and traces a zig-zag path in spacetime.

It is easier to see this when you add quantum repeaters to the chain; and realize that there is no theoretical limit to the zig-zags. So A & Z are entangled after pairs A & B, C & D, ... , Y & Z are entangled and then BSMs are performed on adjoining pair members B & C, D & E, ... X & Y. That should NOT be able to happen in any realistic scenario, or even in a non-local scenario, as all the pairs don't even need to exist at anyone point in time (the only requirement is that adjoining pairs have spacetime overlap).

[Side comment: I mean, where do the pilot waves go after the photons cease to exist? If they are out there, and influence things, why are they otherwise unobservable? In the BM perspective, those pilot waves determine the polarization of A & Z as well as assuring they are correlated when entangled.]

Here is gross speculation on my part: IF there is time symmetry, AND we need classical channels to make sense of quantum non-local signaling (which is the SIGNAL locality requirement), THEN the same thing is probably true of information moving from the Future to the Past. You still need classical channels to interpret it as such. I have no idea why we seem to only send information to the future, and not vice versa. Because it would sure help me if I knew who was going to win this week's sports games...

 

[SpectraCat]

OK, now you are doubling the bet! :grin:

You see, it makes NO difference where in the BSM the extra 10m extensions are added as long as the delay occurs. Now, why do I make this claim? Because we already know that once a beam is split, it can be recombined to restore the quantum state of the original beam. (That is not easy to do, but it is feasible.) So the measurement cannot be the point at which the beamsplitter is encountered.

Ultimately, the measurement occurs at the point at which results can no longer be erased and the process of information gain is not reversible (at the quantum level). That is so even though it was the beamsplitter that was the key element in the measurement process. That might not be true for an electron, but it is true for a photon - it must be detected/detectable.

So line up them beers, I'm coming to collect! (Although I don't know where I need to head yet...)

Hmmm not so fast. I don't necessarily agree that the entangled state after the BSM can be recombined to "restore the quantum state of the original beam", because photons B & C do not come from the same source. If it were possible to do such a recombination, then it would be possible to create a scenario where we could know with 100% certainty the result of a measurement on entangled pairs.

Consider the following. Take two un-entangled photons with known polarizations and combine them in a BSM to create an entangled pair. As I understand it, one of the properties of an entangled pair is that the results of a measurement on the pair are correlated, but random. So AFAICS, there should be no way to "reconstitute" the original quantum state of two unentangled photons with *known* polarizations .. and by "known" here I mean it would be possible to perfectly predict the results before making the measurement.

 

[DrChinese]

Hmmm not so fast. I don't necessarily agree that the entangled state after the BSM can be recombined to "restore the quantum state of the original beam", because photons B & C do not come from the same source. If it were possible to do such a recombination, then it would be possible to create a scenario where we could know with 100% certainty the result of a measurement on entangled pairs.

Consider the following. Take two un-entangled photons with known polarizations and combine them in a BSM to create an entangled pair. As I understand it, one of the properties of an entangled pair is that the results of a measurement on the pair are correlated, but random. So AFAICS, there should be no way to "reconstitute" the original quantum state of two unentangled photons with *known* polarizations .. and by "known" here I mean it would be possible to perfectly predict the results before making the measurement.

We are mixing beers and shots, since you don't get entanglement from the BSM if there was no entanglement to begin with. 2 knowns in gives 2 knowns out.

The question is: does it matter whether the 10m length is added before the beamsplitter or after? The experimental group added it after, and considered the matter closed (at least I didn't think they left any doubt on the matter). And I say that the beamsplitter itself creates beams which, in principle, can be recombined to restore any state which existed prior to the beamsplitter. And I did provide a reference on that as well, specifically referencing entangled systems. So I think the detector is the point where the measurement is said to occur for timing purposes.

Now: I am 2-0 ahead on references. So I think I get to collect my beers (there are 2 at stake now), unless you have a bit more than this. But it is a bit early to start my drinking yet in this time zone... and by the way, I charge interest. But I am not a snob, so anything you will drink with me should suffice.

 

[DrChinese]

0. Forget everything that I said in my previous posts
of this thread!

I will take this as a concession of 1 delicious beer. If you come out this way and don't call me, I will be mad!

 

[peteratcam]

I am trying to understand what everybody is discussing here but there are no equations in this thread at all, and lots of acronyms.
So please help me with this:
Can I think of photons as a two state system? (I prefer to think of spin-1/2s)
Can I think of polarising filters as stern-gerlach aparatuses?
What is a BSM?

I would like to follow through the experimental protocol on paper using schrodinger equation unitary evolution, but I need to know what the degrees of freedom are and what is being measured at each stage, thanks.

 

[Frame Dragger]

I am trying to understand what everybody is discussing here but there are no equations in this thread at all, and lots of acronyms.
So please help me with this:
Can I think of photons as a two state system? (I prefer to think of spin-1/2s)
Can I think of polarising filters as stern-gerlach aparatuses?
What is a BSM?

I would like to follow through the experimental protocol on paper using schrodinger equation unitary evolution, but I need to know what the degrees of freedom are and what is being measured at each stage, thanks.

I can help you with precisely one of those... and the easy one. Go figure. BSM = Bell State Measurement. (edit: to be clear, as in Bell Inequalities)

 

[SpectraCat]

We are mixing beers and shots, since you don't get entanglement from the BSM if there was no entanglement to begin with. 2 knowns in gives 2 knowns out.

Ok, let's consider your statement above (which was exactly *my* point by the way, although I may not have communicated it effectively yet).

I think we both agree that once one member of an entangled pair is measured, the state of the other member is also known, irrespective of distance in space and time between the measuring events. All that matters is that the pair is entangled when the first measurement is made.

Now consider your original example. In that case, at the beginning we have two independent entangled pairs of photons (A/B) and (C/D). Once Alice and Bob make their measurements on photons A and D, which in your variation happens before[/B} B and C enter Charlie's BSM, then B and C are no longer entangled with anything ... they are independent photons with known polarizations. So, by your argument above, not even B & C are entangled by the BSM, since Alice and Bob have already destroyed their respective states entangled states. So that means that A and D aren't entangled either.

So, as I said, the first reference you posted does not cover the situation we are concerned with here, because in all variations discussed in that paper, the entangled pairs (0/1) and (2/3) still exist when photons 1 and 2 enter the BSM.

The question is: does it matter whether the 10m length is added before the beamsplitter or after? The experimental group added it after, and considered the matter closed (at least I didn't think they left any doubt on the matter). And I say that the beamsplitter itself creates beams which, in principle, can be recombined to restore any state which existed prior to the beamsplitter. And I did provide a reference on that as well, specifically referencing entangled systems. So I think the detector is the point where the measurement is said to occur for timing purposes.

That second reference you posted again does not deal with the case relevant to our example. It deals with the case where a single beam corresponding to one member of an entangled pair is split in a birefringent crystal, and then recombined in another matched crystal. That is different from our case, where two photons from different entangled pairs are mixed in a beam-splitter to produce a quantum teleportation event. Perhaps one can extend the example you cited to our case, but it is not at all obvious and should be explicitly proven or experimentally demonstrated. I did a quick literature search, but could find no such examples.

Now: I am 2-0 ahead on references. So I think I get to collect my beers (there are 2 at stake now), unless you have a bit more than this. But it is a bit early to start my drinking yet in this time zone... and by the way, I charge interest. But I am not a snob, so anything you will drink with me should suffice.

Well, you have posted 2 more references that I have, but they haven't really been on target. I'd say we're still even. I am in PA by the way.

 

[SpectraCat]

I am trying to understand what everybody is discussing here but there are no equations in this thread at all, and lots of acronyms.
So please help me with this:
Can I think of photons as a two state system? (I prefer to think of spin-1/2s)

Yes. (me too)

Can I think of polarising filters as stern-gerlach aparatuses?

Yes.

What is a BSM?

It is a "Bell state measurement", or a device to conduct the same. In the example we are considering, it is a beam-splitter. Not sure what the SG analog of that would be.

[QUOTE
I would like to follow through the experimental protocol on paper using schrodinger equation unitary evolution, but I need to know what the degrees of freedom are and what is being measured at each stage, thanks.[/QUOTE]

I would love to see that. I will try to break it down in those terms myself when I have more time.

 

[Frame Dragger]

Ok, let's consider your statement above (which was exactly *my* point by the way, although I may not have communicated it effectively yet).

I think we both agree that once one member of an entangled pair is measured, the state of the other member is also known, irrespective of distance in space and time between the measuring events. All that matters is that the pair is entangled when the first measurement is made.

Now consider your original example. In that case, at the beginning we have two independent entangled pairs of photons (A/B) and (C/D). Once Alice and Bob make their measurements on photons A and D, which in your variation happens before[/B} B and C enter Charlie's BSM, then B and C are no longer entangled with anything ... they are independent photons with known polarizations. So, by your argument above, not even B & C are entangled by the BSM, since Alice and Bob have already destroyed their respective states entangled states. So that means that A and D aren't entangled either.

So, as I said, the first reference you posted does not cover the situation we are concerned with here, because in all variations discussed in that paper, the entangled pairs (0/1) and (2/3) still exist when photons 1 and 2 enter the BSM.



That second reference you posted again does not deal with the case relevant to our example. It deals with the case where a single beam corresponding to one member of an entangled pair is split in a birefringent crystal, and then recombined in another matched crystal. That is different from our case, where two photons from different entangled pairs are mixed in a beam-splitter to produce a quantum teleportation event. Perhaps one can extend the example you cited to our case, but it is not at all obvious and should be explicitly proven or experimentally demonstrated. I did a quick literature search, but could find no such examples.



Well, you have posted 2 more references that I have, but they haven't really been on target. I'd say we're still even. I am in PA by the way.

Sounds to me like a fine case to explore the feasiblity of running this or a related experiment.

 

[Demystifier]

To me it seems that your assumption (one from dBB) lies in Point #3. Dr. Chinese is essentially making the case that such is not the case, or at least, that it is not relevant in a DCQE setting. Isn't this a re-expression of your original objections?

No, point 3 has nothing to do with dBB. Point 3 is about classical communication. dBB is not classical. And this is not a re-expression of my original objections.

 

[Demystifier]

[Side comment: I mean, where do the pilot waves go after the photons cease to exist? If they are out there, and influence things, why are they otherwise unobservable? In the BM perspective, those pilot waves determine the polarization of A & Z as well as assuring they are correlated when entangled.]

To understand the answer to this question, you don't really need to understand pilot wave theory. All you really need to understand is something rather uncontroversial: the theory of decoherence.

In short, the waves are still there and still have an influence. However you cannot practically predict their influence, and in a statistical sense (within the ensemble of many experiments) their influences cancel out in average.

 

[Demystifier]

I will take this as a concession of 1 delicious beer. If you come out this way and don't call me, I will be mad!

I would very appreciate if you could also comment my points 1 - 5. Do you disagree with some of them?

 

[DrChinese]

I think we both agree that once one member of an entangled pair is measured, the state of the other member is also known, irrespective of distance in space and time between the measuring events. All that matters is that the pair is entangled when the first measurement is made.

Now consider your original example. In that case, at the beginning we have two independent entangled pairs of photons (A/B) and (C/D). Once Alice and Bob make their measurements on photons A and D, which in your variation happens before[/B} B and C enter Charlie's BSM, then B and C are no longer entangled with anything ... they are independent photons with known polarizations. So, by your argument above, not even B & C are entangled by the BSM, since Alice and Bob have already destroyed their respective states entangled states. So that means that A and D aren't entangled either.

...

I am in PA by the way.

The interesting thing about the collapse issue is that you really cannot explicitly determine that the first measurement caused the collapse. It could have been the other way around, but we assign causality to coincide with a single direction in time. (I personally use the term "as if" often because it is a simple and easy rule to remember, i.e. it is "as if" the first measurement causes the collapse.) As far as I know: there is no evidence whatsoever for idea that entanglement ends for Alice when Bob is first measured as opposed to vice versa (i.e. that it is in fact Alice that causes the collapse). Certainly, the DCQE experiments don't show anything like that. I think that point would be one which is pretty important. This nuance is what brings out the significance of the term "contextual". You consider the entire relevant context, whatever that happens to be.

Again, to quote Zeilinger et al: "Therefore, this result indicate that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres: this paradox does not arise if the correctness of quantum mechanics is firmly believed." They specifically refer to this as a delayed choice experiment.

 

[DrChinese]

I would very appreciate if you could also comment my points 1 - 5. Do you disagree with some of them?

No, I agree that it takes classical communication to make sense of the bits of information lying around at different places. But don't think I don't look for something anyway! I love to dream up FTL setups just to shoot them down. That is how this thread started!

 

[DrChinese]

To understand the answer to this question, you don't really need to understand pilot wave theory. All you really need to understand is something rather uncontroversial: the theory of decoherence.

In short, the waves are still there and still have an influence. However you cannot practically predict their influence, and in a statistical sense (within the ensemble of many experiments) their influences cancel out in average.

Except, of course, when there is entanglement swapping as here. And in these cases, the rules do not appear to follow what you might expect from either a realistic or a non-local rule set. In other words, the pilot influences seem to sort of make sense for regular Bell tests. But the entanglement swapping protocol seems way to complicated for a mechanism to explain. Now, that is just a qualitative comment on my part and I cannot prove it. But I am sure many others have come to the same conclusion.

 

[DrChinese]

I am trying to understand what everybody is discussing here but there are no equations in this thread at all, and lots of acronyms.
So please help me with this:
Can I think of photons as a two state system? (I prefer to think of spin-1/2s)
Can I think of polarising filters as stern-gerlach aparatuses?
What is a BSM?

I would like to follow through the experimental protocol on paper using schrodinger equation unitary evolution, but I need to know what the degrees of freedom are and what is being measured at each stage, thanks.

I think that is fairly equivalent. The issue is that there is no good analog of photon pair (PDC) production so you can get the entanglement swapping.

EDIT: and there I go with another acronym!

 

[Frame Dragger]

DrChinese: Are you SURE you don't secretly want to make kissy-poos with the Transational Interpreation? The atemporal aspects would seem right up your alley. ;)

EDIT: I would also really love to see peteratcam formulate this on paper.

 

[DrChinese]

DrChinese: Are you SURE you don't secretly want to make kissy-poos with the Transational Interpreation? The atemporal aspects would seem right up your alley. ;)

Aw shucks, I love 'em all!

 

[SpectraCat]

The interesting thing about the collapse issue is that you really cannot explicitly determine that the first measurement caused the collapse. It could have been the other way around, but we assign causality to coincide with a single direction in time. (I personally use the term "as if" often because it is a simple and easy rule to remember, i.e. it is "as if" the first measurement causes the collapse.) As far as I know: there is no evidence whatsoever for idea that entanglement ends for Alice when Bob is first measured as opposed to vice versa (i.e. that it is in fact Alice that causes the collapse). Certainly, the DCQE experiments don't show anything like that. I think that point would be one which is pretty important. This nuance is what brings out the significance of the term "contextual". You consider the entire relevant context, whatever that happens to be.

Actually, I have studied this issue pretty intensely over the last few weeks, and while my readings have been far from comprehensive, I have not found a single example of an experiment that supports your claim that causality is not restricted to the foward direction in time. In all the examples I have seen, there has never been an example where data recorded in the past has been changed by an event that occurred later in time. I have seen lots of claims and smoke and mirrors, but upon a careful reading, the "choice" event always temporally precedes or coincides with the measurements that are dependent on the result of the choice. DCQE (delayed choice quantum eraser) experiments, at least the ones I have seen reported in peer-reviewed journals, always fall into this category. I would very much like to see a DCQE experiment that actually shows a Bell's inequality violation for data that was detected before the DC event occurred.

So anyway, I am not buying the whole future affects the past thing until I see a more convincing experiment. The one that we laid out and debated early in this thread would certainly qualify. I hope someone does it soon.

Again, to quote Zeilinger et al: "Therefore, this result indicate that the time ordering of the detection events has no influence on the results and strengthens the argument of A. Peres: this paradox does not arise if the correctness of quantum mechanics is firmly believed." They specifically refer to this as a delayed choice experiment.

Yeah .. that sounds like doctrine to me, or maybe dogma, but I don't agree that it is a scientific conclusion that follows from the results of that particular paper. I *do* believe in the correctness of QM, that is not at issue here. I have a problem seeing the paradoxes that people refer to in statements like the one from your quote above, because I haven't yet seen any evidence, or even a convincing gedanken experiment, that shows that "the time ordering of the detection events has no influence on the results". So maybe I just don't understand the underlying physics well enough yet, but I kinda doubt it, and it is certainly not for lack of trying ...

 

[Frame Dragger]

Actually, I have studied this issue pretty intensely over the last few weeks, and while my readings have been far from comprehensive, I have not found a single example of an experiment that supports your claim that causality is not restricted to the foward direction in time. In all the examples I have seen, there has never been an example where data recorded in the past has been changed by an event that occurred later in time. I have seen lots of claims and smoke and mirrors, but upon a careful reading, the "choice" event always temporally precedes or coincides with the measurements that are dependent on the result of the choice. DCQE (delayed choice quantum eraser) experiments, at least the ones I have seen reported in peer-reviewed journals, always fall into this category. I would very much like to see a DCQE experiment that actually shows a Bell's inequality violation for data that was detected before the DC event occurred.

So anyway, I am not buying the whole future affects the past thing until I see a more convincing experiment. The one that we laid out and debated early in this thread would certainly qualify. I hope someone does it soon.




Yeah .. that sounds like doctrine to me, or maybe dogma, but I don't agree that it is a scientific conclusion that follows from the results of that particular paper. I *do* believe in the correctness of QM, that is not at issue here. I have a problem seeing the paradoxes that people refer to in statements like the one from your quote above, because I haven't yet seen any evidence, or even a convincing gedanken experiment, that shows that "the time ordering of the detection events has no influence on the results". So maybe I just don't understand the underlying physics well enough yet, but I kinda doubt it, and it is certainly not for lack of trying ...

It sounds to me like your understanding is just fine, but you choose to reject all of the Interpretations of QM. You're concerned with the utility of QM, and the reality of something requires some evidence to satisfy you. You accept theories based on their predictive qualities, but you don't believe they are accurate or complete descriptions of nature. I don't think you'll get much argument here, at least, not from me. Interpretations are just lenses through which to consider the apparently paradoxical nature in some elements of QM.

Determinism, or Uncertainty... I don't find either comforting, and I am willing to believe that nature as a whole is mostly unlike we percieve it on a daily basis. That said, like you, I want to see some evidence before I start questioning my worldview(s) and a universal history of a single arrow of time.

 

[Demystifier]

No, I agree that it takes classical communication to make sense of the bits of information lying around at different places. But don't think I don't look for something anyway! I love to dream up FTL setups just to shoot them down. That is how this thread started!

Now you confused me. At the moment, do you or do you not think that your setup can be used to send information to the past?

 

[zonde]

Sure, here is a diagram which shows what I am referring to, and a reference to where it originated:

http://www.pas.rochester.edu/~AdvLab/Eberly_Bell_Inequalities_AJP.pdf

"We employ an arrangement of polarization analyzer loops to derive several simple Bell inequalities and then discuss the violation of one of them in light of quantum and classical interpretations of the data recorded."

This is definitely not relevant to BSM!
Simply count the inputs.
BSM - 2 inputs, 2 outputs
This example - 1 input, 2 outputs that are then recombined into 1 output again.

Yet another difference is that in this example beam is split using PBSes but BSM is done using polarization independent BS.

 

[Matterwave]

Now you confused me. At the moment, do you or do you not think that your setup can be used to send information to the past?

If I've understood Dr. Chinese's argument correctly (which, I'm not sure I did =P), he believes this experiment can send information to the past only in the sense that entanglement can send information from one particle to another (making sure the pairs are anti-correlated).

No real information can be sent without a classical means of communication. I think, just as in the normal Bell tests, one can't tell if his particle's entangled pair was detected or not because either way, it looks random to him (until he compares notes with his partner).

 

[peteratcam]

There seems to be disagreement on what a BSM is in this thread. In the context of this discussion, should I understand a 'Bell State Measurement' to be a measurement which distinguished between the 4 'maximally entangled' states. eg, a measurement of the operator
\sigma^x_1\sigma^x_2 + 2\sigma^z_1\sigma^z_2
which it can be checked has 4 distinct eigenvalues, with the maximally entangled states as eigenvectors.

Thanks.

 

[Demystifier]

If I've understood Dr. Chinese's argument correctly (which, I'm not sure I did =P), he believes this experiment can send information to the past only in the sense that entanglement can send information from one particle to another (making sure the pairs are anti-correlated).

No real information can be sent without a classical means of communication. I think, just as in the normal Bell tests, one can't tell if his particle's entangled pair was detected or not because either way, it looks random to him (until he compares notes with his partner).

OK, that means that we can agree now that information cannot be sent to the past in any practical sense and that it does not depend on the interpretation.

However, we still have an unanswered question: What happens at the fundamental microscopic level? Can information be sent to the past at THIS level? Unfortunately, this question cannot be answered without referring to any particular interpretation. Therefore, I will answer this question from the point of view of MWI/BI, which are sufficiently similar to provide a common answer for both interpretations. According to these interpretations, nature is DETERMINISTIC at the microscopic level. Therefore, there is no free will (except as an illusion). Therefore, Charlie does not have a free choice to do what he wants. Instead, the Charlie's action is predetermined by conditions present at the time at which Alice and Bob made their measurements (or even earlier). Therefore there is no reason to interpret the correlations as sending microscopic information to the past. Thus, MWI/BI deals with it quite well and there is no particular preference for introducing transactional interpretation. Q.E.D.

 

[Zarqon]

Not sure if I missed something since I just quickly read through the whole thread, but I noted DrChinese mentioning quantum repeaters several times, and I just wish to make a point about this that might clear things up.

In the case of quantum repeaters where you extend the chain of entangled photons to many pairs, it is imperative that the photons are stored in a quantum memory in the intermediate steps, during the time the entanglement swapping measurements (BSMs) are done, otherwise there will not be any coherence left to keep extending the chain. I got the impression that you are talking about photon A and D being "measured" to find entanglement, and surely this measurement corresponds to decoherence of the quantum states, and the photon pairs would not be valid for use in further quantum repeater chains.

I think this might solve the confusion, because I think DrChinese's setup from the original post will only work if the photons A and D are stored in quantum memories, then they can become entangled at any point as soon as Charlie decides to make his swapping. If at anytime before the swapping operation, you would measure the polarization state of A and D (regardless of whether you measure the photon directly or the equivalent state stored in the quantum memory), you would never be able to entangle them.

Also, the change to the quantum state stored in the quantum memory after the BSM, does not violate any causality, since you have to first receive classical information from Charlie on the outcome of the BSM, before you can know what basis to measure the quantum state in, i.e. in what basis the entanglement can be detected.

I hope I understood the problem correctly.

 

[DrChinese]

Now you confused me. At the moment, do you or do you not think that your setup can be used to send information to the past?

I absolutely do not believe that a) information can be sent to the past, nor do I believe that b) information can be sent faster than light (FTL). I hope that is clear

To me, a) and b) are the same restriction. It seems whenever you get close to finding a "loophole" in the no-signaling rule, QM pops up with some funny detail and the "information" you want to send is simply encoded as a random series of bits that requires another key to decode. (That is what quantum communication is all about anyway, isn't it?)

 

[SpectraCat]

Not sure if I missed something since I just quickly read through the whole thread, but I noted DrChinese mentioning quantum repeaters several times, and I just wish to make a point about this that might clear things up.

In the case of quantum repeaters where you extend the chain of entangled photons to many pairs, it is imperative that the photons are stored in a quantum memory in the intermediate steps, during the time the entanglement swapping measurements (BSMs) are done, otherwise there will not be any coherence left to keep extending the chain. I got the impression that you are talking about photon A and D being "measured" to find entanglement, and surely this measurement corresponds to decoherence of the quantum states, and the photon pairs would not be valid for use in further quantum repeater chains.

I think this might solve the confusion, because I think DrChinese's setup from the original post will only work if the photons A and D are stored in quantum memories, then they can become entangled at any point as soon as Charlie decides to make his swapping. If at anytime before the swapping operation, you would measure the polarization state of A and D (regardless of whether you measure the photon directly or the equivalent state stored in the quantum memory), you would never be able to entangle them.

Also, the change to the quantum state stored in the quantum memory after the BSM, does not violate any causality, since you have to first receive classical information from Charlie on the outcome of the BSM, before you can know what basis to measure the quantum state in, i.e. in what basis the entanglement can be detected.

I hope I understood the problem correctly.

I think you did understand it correctly, and your analysis agrees with that of myself and DeMystifier, although I for one am not completely familiar with the concept of quantum memory. Is that equivalent to "persistence of an entangled state"? Stated another way, can we consider two entangled photons propagating along their respective beamlines in an unperturbed state to be "in quantum memory"?

 

[Frame Dragger]

I absolutely do not believe that a) information can be sent to the past, nor do I believe that b) information can be sent faster than light (FTL). I hope that is clear

To me, a) and b) are the same restriction. It seems whenever you get close to finding a "loophole" in the no-signaling rule, QM pops up with some funny detail and the "information" you want to send is simply encoded as a random series of bits that requires another key to decode. (That is what quantum communication is all about anyway, isn't it?)

Yes, to your last question. The main attraction is being tamper-evident in ANY circumstance. That would be a radical leap in crytoplogy and the handling of information, and depending on the cost there is no telling whether this would create an elite with perfectly secure coms, or give ultimate security to the masses?! I don't even know which is a better idea!

 

[SpectraCat]

OK, that means that we can agree now that information cannot be sent to the past in any practical sense and that it does not depend on the interpretation.

However, we still have an unanswered question: What happens at the fundamental microscopic level? Can information be sent to the past at THIS level? Unfortunately, this question cannot be answered without referring to any particular interpretation. Therefore, I will answer this question from the point of view of MWI/BI, which are sufficiently similar to provide a common answer for both interpretations. According to these interpretations, nature is DETERMINISTIC at the microscopic level. Therefore, there is no free will (except as an illusion). Therefore, Charlie does not have a free choice to do what he wants. Instead, the Charlie's action is predetermined by conditions present at the time at which Alice and Bob made their measurements (or even earlier). Therefore there is no reason to interpret the correlations as sending microscopic information to the past. Thus, MWI/BI deals with it quite well and there is no particular preference for introducing transactional interpretation. Q.E.D.

Wait, what? It now sounds like you have reversed yourself and now expect there to be a Bell's inequality violation for Alice and Bob's results, based on Charlie's future choice? Isn't that different from your earlier arguments about the decoherence of the initial entangled pairs (A/B) and (C/D) caused by Alice and Bob's measurements precluding the possibility that there A & D could be entangled "after the fact", as originally proposed by Dr. Chinese?

 

[DrChinese]

Not sure if I missed something since I just quickly read through the whole thread, but I noted DrChinese mentioning quantum repeaters several times, and I just wish to make a point about this that might clear things up.

In the case of quantum repeaters where you extend the chain of entangled photons to many pairs, it is imperative that the photons are stored in a quantum memory in the intermediate steps, during the time the entanglement swapping measurements (BSMs) are done, otherwise there will not be any coherence left to keep extending the chain. I got the impression that you are talking about photon A and D being "measured" to find entanglement, and surely this measurement corresponds to decoherence of the quantum states, and the photon pairs would not be valid for use in further quantum repeater chains.

I think this might solve the confusion, because I think DrChinese's setup from the original post will only work if the photons A and D are stored in quantum memories, then they can become entangled at any point as soon as Charlie decides to make his swapping. If at anytime before the swapping operation, you would measure the polarization state of A and D (regardless of whether you measure the photon directly or the equivalent state stored in the quantum memory), you would never be able to entangle them.

I disagree, there is no mention of "holding" A & D in Zeilinger's published version of the experiment with delayed choice - nor would there need to be for the effect to appear. The entire point of all delayed choice experiments is that it appears "as if" the past was changed by a future choice. However, the histories are always "consistent" when all the facts are brought together. So it is not clear that any particular portion of the setup "caused" the outcome. That is why QM can be labeled "indeterministic" as much as the fact that there is no apparent cause for the particular outcome. (Caveat: As always, such labels are interpretation dependent.)

See the attached figures 1 and 2 from the reference itself.

The issue of quantum repeaters is a little different. There is no requirement that the information be held in a quantum memory for the entanglement swapping to work per se, but that may not lead to a working repeater. I believe, as best as can be determined, the issue has to do with the practicality of synchronizing so many photon pairs over large distances. If you cannot have pairs readily available when you need them, the repeater won't do anything useful. Also, the repeater must repeat with a known Bell state and this leads to additional complexity. There are also fidelity issues of a variety of types as you might imagine.

 

[Frame Dragger]

I disagree, there is no mention of "holding" A & D in Zeilinger's published version of the experiment with delayed choice - nor would there need to be for the effect to appear. The entire point of all delayed choice experiments is that it appears "as if" the past was changed by a future choice. However, the histories are always "consistent" when all the facts are brought together. So it is not clear that any particular portion of the setup "caused" the outcome. That is why QM can be labeled "indeterministic" as much as the fact that there is no apparent cause for the particular outcome. (Caveat: As always, such labels are interpretation dependent.)

See the attached figures 1 and 2 from the reference itself.

The issue of quantum repeaters is a little different. There is no requirement that the information be held in a quantum memory for the entanglement swapping to work per se, but that may not lead to a working repeater. I believe, as best as can be determined, the issue has to do with the practicality of synchronizing so many photon pairs over large distances. If you cannot have pairs readily available when you need them, the repeater won't do anything useful. Also, the repeater must repeat with a known Bell state and this leads to additional complexity. There are also fidelity issues of a variety of types as you might imagine.

This strikes me as an experiment that would require vast resources to build, calibrate, and perform with fidelity. Is it even possible with current technology to build this such that it yields a result worth the building expense?

 

[DrChinese]

This strikes me as an experiment that would require vast resources to build, calibrate, and perform with fidelity. Is it even possible with current technology to build this such that it yields a result worth the building expense?

Sure, there are quite a number of labs doing this research - and they are pushing the boundaries all the time. The experiment I cite was from 2002 (practically ancient now), and labs have been churning out groundbreaking experiments since. Some of the key reseachers are (with my sincere apologies to the many many! many! others who are also active):

Thomas Jennewein, Gregor Weihs, Jian-Wei Pan, and Anton Zeilinger at the University of Austria

Nicolas Sangouard, Christoph Simon, Hugues de Riedmatten, and Nicolas Gisin at the University of Geneva

Harald Weinfurter at the Max Planck Institute for Quantum Optics

Artur Scherer, Gina Howard, Barry C. Sanders, and Wolfgang Tittel at the Institute for Quantum Information Science, University of Calgary

I mention the above specifically because when you see their names as authors, you will know they are working as part of some of these major lab efforts.

Now, if you are referring to the repeaters specifically: here are 2 recent and very comprehensive theoretical works which summarize a lot of the complexities involved. Each is over 50 pages:

http://arxiv.org/abs/0906.2699

http://arxiv.org/abs/0807.3358

 

[Frame Dragger]

Sure, there are quite a number of labs doing this research - and they are pushing the boundaries all the time. The experiment I cite was from 2002 (practically ancient now), and labs have been churning out groundbreaking experiments since. Some of the key reseachers are (with my sincere apologies to the many many! many! others who are also active):

Thomas Jennewein, Gregor Weihs, Jian-Wei Pan, and Anton Zeilinger at the University of Austria

Nicolas Sangouard, Christoph Simon, Hugues de Riedmatten, and Nicolas Gisin at the University of Geneva

Harald Weinfurter at the Max Planck Institute for Quantum Optics

Artur Scherer, Gina Howard, Barry C. Sanders, and Wolfgang Tittel at the Institute for Quantum Information Science, University of Calgary

I mention the above specifically because when you see their names as authors, you will know they are working as part of some of these major lab efforts.

Hmmmm... I wonder if they've considered this scenario? It always seems arrogant to assume they haven't, but then, what if you're on to something interesting? An email to the appropriate group might not be a bad idea.

 

[DrChinese]

If I've understood Dr. Chinese's argument correctly (which, I'm not sure I did =P), he believes this experiment can send information to the past only in the sense that entanglement can send information from one particle to another (making sure the pairs are anti-correlated).

No real information can be sent without a classical means of communication. I think, just as in the normal Bell tests, one can't tell if his particle's entangled pair was detected or not because either way, it looks random to him (until he compares notes with his partner).

Yes, that is exactly as I am trying to portray.

In a time symmetric interpretation (TS), you would be respecting c and could explain this result. In a Bohmian interpretation (BM), likewise. But what is interesting to me is that the standard QM interpretation (the SQM formalism) explains and predicts the entanglement swapping features, while neither TS or BM do. Now I realize my good friends with a BM or TS inclination will vigorously disagree, as they will point out complete equivalence to SQM. But in my opinion, the hoops required to explain entanglement swapping as described in my post 84 become increasing complex for mechanistic interpretations.

This is a point I have been trying to make for some time: that the Bell Theorem distinguished local realistic theories from other candidates (with LR losing). But more recent advances are serving to further narrow the crop of candidate theories (interpretations). Candidate interpretations that do not take these experimental challenges seriously are going to be left behind very quickly*. The researchers who are at the forefront of the advances in quantum communication/non-locality/contextuality - as far as I have read (which is hardly the final word!) - do not subscribe to any of the interpretations debated so vigorously on this board. They tend to stay very close to SQM at all times. This experiment is a perfect example.

* If they haven't already: because they are becoming even LESS useful for research purposes than they have been for the past 80 years.

 

[peteratcam]

The researchers who are at the forefront of the advances in quantum communication/non-locality/contextuality - as far as I have read (which is hardly the final word!) - do not subscribe to any of the interpretations debated so vigorously on this board. They tend to stay very close to SQM at all times. This experiment is a perfect example.

Indeed. I was at a seminar recently given by Anton Zeilinger where, in response to a question, he described Bohmian mechanics as a desperate attempt to maintain pre-quantum realism!
For completeness, I should say he gestured quotation marks for the word 'desperate'.

 

[DrChinese]

Indeed. I was at a seminar recently given by Anton Zeilinger where, in response to a question, he described Bohmian mechanics as a desperate attempt to maintain pre-quantum realism!
For completeness, I should say he gestured quotation marks for the word 'desperate'.

Thanks for sharing that story! Got any more good ones?

(All of the written articles are generally devoid of humor or opinion, so I am always trying to read between the lines to get an idea of the personality behind the names.)

 

[Frame Dragger]

The discussion of Interpretations is fun, because it's metaphysics. We could argue as to the value of the view you take, but ultimately being free of any agenda (Instrumentalism) is where the progress comes from. The rest is meant to explain the seemingly inexplicable to we poor fur-less apes.

 

[SpectraCat]

The discussion of Interpretations is fun, because it's metaphysics. We could argue as to the value of the view you take, but ultimately being free of any agenda (Instrumentalism) is where the progress comes from. The rest is meant to explain the seemingly inexplicable to we poor fur-less apes.

Not always metaphysical .. LHV interpretations were once seriously considered on equal footing with SQM. Now they are considered to be wrong. So, arguing about interpretations is fun for its own sake, but I suspect it's even more fun when you get to blow a hole in one of them and sink it!

 

[Frame Dragger]

Not always metaphysical .. LHV interpretations were once seriously considered on equal footing with SQM. Now they are considered to be wrong. So, arguing about interpretations is fun for its own sake, but I suspect it's even more fun when you get to blow a hole in one of them and sink it!

It's so fun that we have the LHC to whack these theories around with. Just think, if you'd been born a few hundred, or god forbid a few THOUSAND years ago you'd be missing all of this!

 

[Demystifier]

I absolutely do not believe that a) information can be sent to the past, nor do I believe that b) information can be sent faster than light (FTL). I hope that is clear

To me, a) and b) are the same restriction.

They are not exactly the same. If you could invert the thermodynamic arrow of time, then you could send information to the past without FTL.

 

[Demystifier]

Wait, what? It now sounds like you have reversed yourself and now expect there to be a Bell's inequality violation for Alice and Bob's results, based on Charlie's future choice? Isn't that different from your earlier arguments about the decoherence of the initial entangled pairs (A/B) and (C/D) caused by Alice and Bob's measurements precluding the possibility that there A & D could be entangled "after the fact", as originally proposed by Dr. Chinese?

You should distinguish macroscopic and microscopic levels of description. My previous explanations referred to macroscopic phenomena, while the one you are citing above refers to microscopic phenomena. In particular, free will and decoherence make sense only at a macroscopic level.

 

[DrChinese]

Originally Posted by DrChinese

I absolutely do not believe that a) information can be sent to the past, nor do I believe that b) information can be sent faster than light (FTL). I hope that is clear

To me, a) and b) are the same restriction.

Demystifier:
They are not exactly the same. If you could invert the thermodynamic arrow of time, then you could send information to the past without FTL.

Sorry, I did not express myself well. I meant to imply that both of these restrictions (which as you say are not identical) arise from the same source. Whatever that is... as it seems that the same barrier exists whether you are trying to send information to the past or whether you are trying to send information FTL. You still need classical channels to make sense of the otherwise random bits you are holding. And that's all you can ever send through the various forms of entanglement/delayed choice/etc: Random bits.

 

[SpectraCat]

You should distinguish macroscopic and microscopic levels of description. My previous explanations referred to macroscopic phenomena, while the one you are citing above refers to microscopic phenomena. In particular, free will and decoherence make sense only at a macroscopic level.

So I am confused, do you expect a Bell's inequality violation for photons A & D when Charlie makes his BSM on B & C *after* the detection of A & D, or don't you? And in this case, we are talking about 4-way coincidence measurements (with appropriate consideration of travel delays) on A,B,C and D, right?

Also, I am a little unclear on the distinction you are drawing between microscopic and macroscopic phenomena ... are you saying Bell's inequality violations are microscopic?

 

[Frame Dragger]

So I am confused, do you expect a Bell's inequality violation for photons A & D when Charlie makes his BSM on B & C *after* the detection of A & D, or don't you? And in this case, we are talking about 4-way coincidence measurements (with appropriate consideration of travel delays) on A,B,C and D, right?

Also, I am a little unclear on the distinction you are drawing between microscopic and macroscopic phenomena ... are you saying Bell's inequality violations are microscopic?

I have been under the impression that the line between 'micro' and 'macro' in QM is (forgive me) fuzzy. Isn't the notion of where and when macroscopic reality emerges from quantum behaviour one of the bigger unsolved questions of any interpretation of QM?

 

[Demystifier]

I have been under the impression that the line between 'micro' and 'macro' in QM is (forgive me) fuzzy. Isn't the notion of where and when macroscopic reality emerges from quantum behaviour one of the bigger unsolved questions of any interpretation of QM?

I think that decoherence defines the boundary quite well. The boundary is not sharp (there are also mesoscopic systems), but even the "unsharpness" can be well defined in terms of decoherence.

 

[Demystifier]

So I am confused, do you expect a Bell's inequality violation for photons A & D when Charlie makes his BSM on B & C *after* the detection of A & D, or don't you?

I do expect Bell's inequality violation in this case. (Note that this was not my opinion in the beginning. I can make a mistake too.) However, Alice and Bob cannot observe them. Only Charlie can.

And in this case, we are talking about 4-way coincidence measurements (with appropriate consideration of travel delays) on A,B,C and D, right?

Right! That's why Alice and Bob cannot observe it.

Also, I am a little unclear on the distinction you are drawing between microscopic and macroscopic phenomena ... are you saying Bell's inequality violations are microscopic?

No, we can observe them so they are macroscopic. Microscopic stuff is something that we cannot directly observe (e.g., Bohmian trajectories, objective wave functions, objective collapse, absence of any objective microscopic reality, ...) so at the moment we can only speculate about it.