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Frame Dragger said: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!
Frame Dragger said: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. ;)
DrChinese said: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.
SpectraCat said: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 ...
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?DrChinese said: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!
This is definitely not relevant to BSM!DrChinese said: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."
Demystifier said: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?
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.Matterwave said: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).
Demystifier said: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?
Zarqon said: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 said: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?)
Demystifier said: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 said: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.
DrChinese said: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 said: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 said: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.
Matterwave said: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).
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!DrChinese said: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.
peteratcam said: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'.
Frame Dragger said: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 said: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!
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.DrChinese said: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.
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.SpectraCat said: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?
Demystifier said: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.
Demystifier said: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.
SpectraCat said: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 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.Frame Dragger said: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 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.SpectraCat said: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?
Right! That's why Alice and Bob cannot observe it.SpectraCat said: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?
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.SpectraCat said: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?
Demystifier said: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.
SpectraCat said:Incidentally, I still disagree (assuming I understand the situation correctly, which is something I am not completely sure about) ... I am working on a post to explain my (mis?) understanding in more detail
SpectraCat said:... Finally, it is worth noting that in the paper I cited, the authors did not make the same claim that Dr. Chinese made in his post here. They claim that the space-time separation of the detection events doesn't matter, which is consistent with what they tested in their experiment.
SpectraCat said:...This is all fine, but it only works if the first equation I wrote above is valid when B & C become entangled. This is not true if the measurements on A and D have already occurred. Immediately after those measurements have occurred, the total state of the system is known, that is, it has been resolved into some element of the set of separable states:
[tex]\left\{\left[\left|H\right\rangle_{A}\otimes\left|V\right\rangle_{B}\otimes\left|H\right\rangle_{C}\otimes\left|V\right\rangle_{D}\right],\:\:
\left[\left|V\right\rangle_{A}\otimes\left|H\right\rangle_{B}\otimes\left|H\right\rangle_{C}\otimes\left|V\right\rangle_{D}\right],\:\:
\left[\left|H\right\rangle_{A}\otimes\left|V\right\rangle_{B}\otimes\left|V\right\rangle_{C}\otimes\left|H\right\rangle_{D}\right],\:\:
\left[\left|V\right\rangle_{A}\otimes\left|H\right\rangle_{B}\otimes\left|V\right\rangle_{C}\otimes\left|H\right\rangle_{D}\right]\right\}[/tex]
(Note: I used the tensor product notation above to emphasize the separability, but it is just the 4 combinations: HVHV, VHHV, HVVH, VHVH)
So there is now no way to get from just one of these states to the case where there is entanglement between A & D. Note that B & C aren't entangled in this case either ... (otherwise it would be possible to generate entangled pairs from linearly polarized photons simply using beamsplitters).