
042612, 01:01 PM #21
So you're saying they would have to have information from Victor in order to make sense of their data? This doesn't ring true to me, not because I'm doubting your education, but for logical reasons:
Scenario 1: Alice and Bob compare data. They then ask Victor if that data should be correlated and he says yes. "Ahh! It IS correlated!!" they exclaim.
Scenario 2: Alice and Bob compare the same set of data. This time, Victor says that their photons should not be correlated because he took no action. "Ahh! The data is perfectly random just as we should expect!!" they exclaim.
I hope you can understand my doubt here. You're asking me to question the article linked to, which is fine, but I want to know where the confusion lies. You know me well enough to know that I can't just "let it go"...Last edited by RJBeery; 042612 at 01:55 PM.

042612, 05:19 PM #22
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I've kind of explained this two or three times now: the article is misleading where it seems to imply Victor can "correlate" Alice's or Bob's data. He doesn't. What he actually does is perform a certain entangling measurement with four different possible outcomes (well, ideally it would be four, but the implementation in the actual experiment could only distinguish two of them), and he can use his own measurement results to identify correlated subsets of Alice's and Bob's results.
I'll illustrate with an example that simplifies things a bit. Suppose Alice and Bob only measured for horizontal polarisation (outcome 0) or vertical polarisation (outcome 1). They measure 8 sets of particles and get the following results:
Code:Event  Alice  Bob  1  1  0 2  0  0 3  1  0 4  0  1 5  1  1 6  1  1 7  0  0 8  0  1
Things are a little more complicated in the actual experiment cited in the OP (in particular measurements in more than just the horizontal/vertical polarisation basis were performed), but the principle is the same: the data is globally uncorrelated, but you can always identify correlated subsets a bit like the sets A and B I described above. The point of the experiment is that Victor has a way of identifying (but not controlling or creating) such correlated subsets without having or needing access to Alice's or Bob's raw data.
EDIT: The reason for the retrocausalsounding language (and I've used this kind of language myself in some cases) is because once Victor communicates a correlated subset to Alice and Bob and they throw all their other results away, everything is "as if" they had been sharing a certain set of entangled states right from the beginning. In particular they can then do cryptography or deviceindependent random number generation or whatever else they might have liked to do with measurements made on entangled states. Typically they can certify that their protocol is working correctly and securely even if they don't trust Victor.Last edited by przyk; 042612 at 05:46 PM.

042612, 07:48 PM #23
I get it, well done; now let me make it through the Arxiv paper to verify that my understanding of what you're saying and my understanding of what the paper says can be reconciled.

042612, 08:20 PM #24

042912, 04:01 AM #25
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Not exactly, here are AB correlation for the first and the second choice of Victor from the paper:
If AB gets R>R>, it is more likely that V has chosen the left option, if AB gets R>L> that V much more probably has chosen the right one  there is nonzero mutual information and so we could transmit information through such channel.
The simplest way to use nonzero mutual information is repetition code, like sending 3 times the same bit and read as those occurring at least twice . Extension of such (extremely weak!) method is just observing statistics  e.g. Victor fixes some choice and AB estimates correlations using succeeding measurements  improving certainty of Victor's choice.
It's better to see in simpler and much more effective (single SPDC instead of two) setting I've posted before  just set polarizer in erasing or not angle and observe as statistics: light intensity changes on the second arm.

042912, 07:40 AM #26
The caption of your image:
Correlation function between photons 1 and 4 for the three mutually unbiased bases (Hright fence/V right fence,Rright fence/Lright fence,+right fence/−right fence). a,b, Victor subjects photons 2 and 3 to either a BSM (a) or an SSM (b). These results are obtained from coincidence counts of photons 1… http://www.nature.com/nphys/journal/...nphys2294.html
This unavoidably prevents superluminal communication since, even if a random or purposeful decision appears to be affecting events that have already transpired (as in the delayed choice quantum eraser), the signal from the past cannot be seen/decoded until the coincidence circuit has correlated both the past and future behavior. Thus the "signal" in the past is only visible after it is "sent" from the future, precluding quantum entanglement from being exploited for the purposes of fasterthanlight communication or data time travel. http://en.wikipedia.org/wiki/Coincid..._%28physics%29

042912, 08:37 AM #27
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Yes, exactly  thanks of entanglement swapping, by performing Bellstate measurement (BSM  left), or separablestate measurement (SSM  right) on photons 2 and 3, he controls entanglement/correlations of photons 1 and 4 (AB).
There is no using this for FTL communication because of the necessity for coincidence counting.
What is nonintuitive here is that causality works backward, but why it cannot?
Just oppositely  fundamental physics is time/CPT symmetric. From GRT to QFT we use lagrangian mechanics, which has e.g. three equivalent evolution formulations:
a) situation and derivative in the past determines future through EL eq.,
b) situation in past and future determine situation between  through action optimization,
c) situation and derivative in the future determines past through EL eq.
Natural for our intuition is a), but we have to remember that b) and c) are equivalent. There is no reason to emphasize one time direction for causality.
If someone is anxious about the "conflict" of fundamental time/CPT symmetry with our 2nd lawbased intuition, it should be educative to look at very simple model: Kac ring  on a ring there are black and white balls which mutually shift one position each step. There are also some marked positions and when a ball goes through it, this ball switches color.
Using natural statistical assumption ("Stoßzahlansatz"): that if there is p such markings (proportionally), p of both black and white balls will change the color this step, we can easily prove that it should leads to equal number of black and white balls  maximizing the entropy ...
... from the other side, after two complete rotations all balls have to return to the initial color  from 'all balls white' fully ordered state, it would return back to it ... so the entropy would first increase to maximum and then symmetrically decrease back to minimum.
Here is a good paper with simulations about it:http://www.maths.usyd.edu.au/u/gottw...s/kacring.pdf
The lesson is that when on time/CPT symmetric fundamental physics we "prove" e.g. Boltzmann H theorem that entropy always grows ... we could take time symmetry transformation of this system and use the same "proof" to get that entropy always grows in backward direction  contradiction.
The problem with such "profs" is that they always contain some very subtle uniformity assumption  generally called Stoßzahlansatz. If underlying physics is time/CPT symmetric, we just cannot be sure that entropy will always grow  like for Kac ring and maybe our universe ...

042912, 09:25 AM #28
For the same reason that coincidence counting precludes superluminal signaling, so does it preclude reverse causation/signaling. Alice and Bob cannot know anything of the actions taken by Victor until after Victor has acted. And it is trivial that Victor can know information about his own past line cone.

042912, 09:35 AM #29
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042912, 09:55 AM #30

042912, 10:13 AM #31
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I cannot prove the last one and moreover, using Kac ring example, I've explained that no one can.
But the situation with causality is analogous: time/CPT symmetry forbids emphasizing one direction on fundamental level.
However, one direction can be emphasized on solution level: concrete realization we live in  e.g. with well spatially localized (low entropy) big bang relatively near and so creating entropy gradient  our 2nd law.
But it is statistical property (of solution we live in) only  it says nothing about "fundamental directionality of causality".

042912, 11:52 AM #32keith1Guest
Does any of the implications of this experiment's results leave any room for tampering with those results, outside the time limits of 0 and 520 ns, by Alice, Bob, Victor, or any possible privy and undisclosed fourth party participant?
Last edited by keith1; 042912 at 12:01 PM.

042912, 05:15 PM #33
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No. The graph on the left isn't just for the cases where Victor chose to do a Bell state measurement. It's also conditioned on Victor getting the result. The graph on the right is for the cases where Victor chooses the separable measurement and gets either or . Alice and Bob can't predict which measurement Victor will choose to perform, but if Victor tells them which measurement he performed, they can sometimes predict his result.

043012, 03:38 PM #34Originally Posted by RJBeeryOriginally Posted by Przyk

043012, 04:40 PM #35
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What "traditional definition" are you working with? MWI is deterministic in the sense that if you know the initial quantum state of the entire universe (or an isolated subsystem), and you know everything there is to know about the interactions and evolution taking place, then the Schrödinger equation predicts a unique future state at any future time. But individual observers generally won't have access to complete information about that state for various reasons.
In order for the subsets of Alice and Bob's data to be identified by Victor's future measurements, those "branches" would by necessity be required to lead to Victor making those measurements.
If you drop the "free will" assumption and you imagine modeling Alice, Bob, and Victor as physical systems, then there's additional branching depending on the number of decisions they could make. For example, as you say, in the experiment Victor was a quantum random number generator with two possible outcomes, and in a more complete MWI description, you'd consider that the measurements Victor performs are correlated with this outcome, which doubles the number of branches. In the experiment, Alice and Bob each chose between 3 different measurements they could perform, so if you also think of Alice and Bob as quantum random number generators, that's a total of 16 x 2 x 3 x 3 = 288 branches. That's 288 branches per iteration (generation of four photons) and assuming Alice, Bob, and Victor are no more complicated than two or threeoutcome quantum random number generators.
With all that said, if these responses don't make too much sense to you, I wouldn't worry about it too much. I've never really considered determinism a selling point of MWI anyway.

043012, 10:06 PM #36
Yes, exactly. And when you speak of the number of branches created by Alice and Bob you're presuming an equal weight between them. My point is that the weights cannot be equal; they must be skewed depending upon the future and yet to be determined results of the quantum random number generator, or else there would be no correlation between the data. This does not qualify for the traditional definition of determinism.
Originally Posted by przyk

050112, 05:56 AM #37
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 2,966
Where did I say that? The weights of the branches happen to be equal after Alice and Bob have made their measurements, but they're generally not equal after Victor has made his measurements.
Actually, depending on the measurements, some of the 16 branches I counted are actually of weight 0, so you should really read that figure of 16 as a maximum.
My point is that the weights cannot be equal; they must be skewed depending upon the future and yet to be determined results of the quantum random number generator, or else there would be no correlation between the data.
This does not qualify for the traditional definition of determinism.
Perhaps you see my point?

050112, 09:36 PM #38

050212, 08:37 AM #39
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Victor doesn't "correlate" Alice and Bob's data as such. He can only identify correlated subsets of their data that would be there anyway.
What would the data look like if he correlated all photons received?

050212, 11:14 AM #40
It's one thing to claim that ArsTechnica did a poor job summarizing this experiment, but this is directly from the paper:
Originally Posted by ArXiv paper
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