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Explaining DCQE - via coherence in layman terms

by San K
Tags: coherence, dcqe, explaining, layman, terms
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Joncon
#19
Jun11-11, 04:08 PM
P: 57
Quote Quote by unusualname View Post
The sub-samples of course correlate afterwards, but so what? How are the sub-samples created? Even if you have a polarization beam splitter so that ALL photons are measured, it is still a PROBABILSITISIC law that determines which detector they go to.

How do the s-photons know which p-photons will go where if there is a delay after the s-photons are measured?

SpectraCat/Cthuga are WRONG if they think a simple subsampling argument explains this. You HAVE to invoke the nonlocal and/or non-separable nature of QM to explain it.
unusualname, how does probability explain the interference patterns in this version of the experiment? http://arxiv.org/abs/quant-ph/9903047

The thing I don't understand is that the D1 and D2 detectors both show interference fringes and anti-fringes when the sub-samples are examined. What I don't get is that the idler photons encounter a beam splitter before going to either of the detectors. As I understand it, the chance of passing through this BS or reflecting off it is 50/50. So I would expect no interference patterns in these sub-samples.

To put it another way - how do idler photons, of signal photons which contribute to an interference pattern, always end up at the same detector?
Joncon
#20
Jun11-11, 04:14 PM
P: 57
Quote Quote by San K View Post
Joncon, what do you think would happen if we placed QWPs (not polarizer) in the path of the p-photon (and none in the path of s-photons) after s-photon has stuck detector Ds (by going through the double slit without any QWPs)?
I don't see how this can change anything. You still have no information about the s-photon.
unusualname
#21
Jun11-11, 04:19 PM
P: 661
Quote Quote by San K View Post
the s-photons don't know which way p-photons will go. however the probabilities of the p-photons path are frozen at the time s-photons strikes the detector. the entanglement is broken (when s-photon strikes the detector) and the p-photon's behavior/path is "somewhat/probabilistically" determinable.
however the probabilities of the p-photons path are frozen at the time s-photons strikes the detector

but they aren't, that's what Bell tests show us, that's why QM is so nonintuitive. In fact you could do a bell test inbetween the p-photon arm delay to prove this. But much more sophisticated tests of the non-realism of QM have been done, google GHZ states, Hardy, Zeillinger.

EDIt I actually agree with you, but in much more subtle/amazing way.
San K
#22
Jun11-11, 04:23 PM
P: 915
Quote Quote by Joncon View Post
unusualname, how does probability explain the interference patterns in this version of the experiment? http://arxiv.org/abs/quant-ph/9903047

The thing I don't understand is that the D1 and D2 detectors both show interference fringes and anti-fringes when the sub-samples are examined. What I don't get is that the idler photons encounter a beam splitter before going to either of the detectors. As I understand it, the chance of passing through this BS or reflecting off it is 50/50. So I would expect no interference patterns in these sub-samples.

To put it another way - how do idler photons, of signal photons which contribute to an interference pattern, always end up at the same detector?
the don't end up on the same detector, idler goes to Do and the signal ones (that contribute to an int pattern) go to either D1 or D2

not sure what you are asking......
unusualname
#23
Jun11-11, 04:28 PM
P: 661
Quote Quote by Joncon View Post
unusualname, how does probability explain the interference patterns in this version of the experiment? http://arxiv.org/abs/quant-ph/9903047

The thing I don't understand is that the D1 and D2 detectors both show interference fringes and anti-fringes when the sub-samples are examined. What I don't get is that the idler photons encounter a beam splitter before going to either of the detectors. As I understand it, the chance of passing through this BS or reflecting off it is 50/50. So I would expect no interference patterns in these sub-samples.

To put it another way - how do idler photons, of signal photons which contribute to an interference pattern, always end up at the same detector?
Hi Joncon,

I don't want to analyse every type of experiment, but San K accidentially gave the correct answer above, if the experiment is static, then the probabilities are fixed once one side of the entangled pair is measured. Of course this requires non-locality/non-separablity if one arm of the entangled pair is longer than the other.
Joncon
#24
Jun11-11, 04:36 PM
P: 57
Quote Quote by San K View Post
the don't end up on the same detector, idler goes to Do and the signal ones (that contribute to an int pattern) go to either D1 or D2

not sure what you are asking......
No, the signal photons which make up the pattern go to D0. The idlers, which are used to determine the path, go to D1 or D2.

From the document: -
"The signal photon (photon 1, either from A or B) passes a lens LS to meet detector D0"

"The idler photon (photon 2) is sent to an interferometer with equalpath optical arms."
San K
#25
Jun11-11, 04:41 PM
P: 915
Quote Quote by unusualname View Post
however the probabilities of the p-photons path are frozen at the time s-photons strikes the detector

but they aren't, that's what Bell tests show us, that's why QM is so nonintuitive. In fact you could do a bell test inbetween the p-photon arm delay to prove this. But much more sophisticated tests of the non-realism of QM have been done, google GHZ states, Hardy, Zeillinger.

EDIt I actually agree with you, but in much more subtle/amazing way.
Bell tests talk about stuff during entanglement, not after entanglement is broken.

the entanglement (or whatever we later discover the phenomena to be) is broken when one of the pair is measured, ....and whichever is measured first/earlier.....


so bell test don’t come into the picture once entanglement is broken.....

in what (amazing) way do you agree with my post unusual name?


the probabilities are frozen once entanglement is broken .....and this can be verified by the experiment itself....you can (probabilistically) predict the p-photon's (or whichever of the pair is to be detected later) path....because you know what pattern to expect....

just found an actual example.....a commonly used experiment/instrument.....where the probabilities are predicted/known/calculable

http://en.wikipedia.org/wiki/Mach-Ze...interferometer
unusualname
#26
Jun11-11, 04:45 PM
P: 661
Quote Quote by San K View Post
Bell tests talk about stuff during entanglement, not after entanglement is broken.

the entanglement (or whatever we later discover the phenomena to be) is broken when one of the pair is measure, ....whichever is measured first/earlier...

so bell test don’t come into the picture once entanglement is broken.....

in what (amazing) way do you agree with my post unusual name?

No. If you had apparatus in place in the p-photon arm at the time the s-photons are measured you could show the p-photons had RANDOM polarization before they are measured (before they pass through a polarizer)

Sorry if this is confusing, but it's correct.
San K
#27
Jun11-11, 04:51 PM
P: 915
Quote Quote by Joncon View Post
No, the signal photons which make up the pattern go to D0. The idlers, which are used to determine the path, go to D1 or D2.

From the document: -
"The signal photon (photon 1, either from A or B) passes a lens LS to meet detector D0"

"The idler photon (photon 2) is sent to an interferometer with equalpath optical arms."
ok fine.....i swapped the idlers and signals by mistake....but still they are going to different detectors....not sure what you are asking....

are you asking why they are matching (results/patterns)?.......well they are matching because they are entangled....
San K
#28
Jun11-11, 04:53 PM
P: 915
Quote Quote by unusualname View Post
No. If you had apparatus in place in the p-photon arm at the time the s-photons are measured you could show the p-photons had RANDOM polarization before they are measured (before they pass through a polarizer)

Sorry if this is confusing, but it's correct.
you can call that random if you wish however:

you can predict that with X probability p-photons will be this
you can predict that with Y probability p-photons will be that
you can predict that with Z probability p-photons will be this-that
etc

this is provable by the fact that we can predict (probabilistically) what path p-photon will take one it emerges from the polarizer....(if we had information about s-photon, of course)......
unusualname
#29
Jun11-11, 04:56 PM
P: 661
Quote Quote by San K View Post
you can call that random if you wish however:

you can predict that with X probablitiy p-photons will be this
you can predict that with Y probablitiy p-photons will be that
you can predict that with Z probablitiy p-photons will be this-that
etc
Yes, so, if I put the eraser on pluto how does the s-photon know which p-photon to match to?
Joncon
#30
Jun11-11, 04:58 PM
P: 57
Quote Quote by San K View Post
ok fine.....i swapped the idlers and signals by mistake....but still they are going to different detectors....not sure what you are asking....

are you asking why they are matching (results/patterns)?.......well they are matching because they are entangled....
Fair enough, I accept entanglement. But that suggests to me that when the p-photon meets the final BS, the chances of it passing through or reflecting are not 50/50, but are influenced somehow by where the s-photon landed.
San K
#31
Jun11-11, 05:42 PM
P: 915
Quote Quote by Joncon View Post
Fair enough, I accept entanglement. But that suggests to me that when the p-photon meets the final BS, the chances of it passing through or reflecting are not 50/50, but are influenced somehow by where the s-photon landed.
yes, when s-photon landed, the entanglement was broken since s-photon position has been fixed/locked.....and the p-photon also becomes probabilistically determinable.....that’s how I think....some on the forum agree, some disagree and some have another explanation, etc
unusualname
#32
Jun11-11, 06:00 PM
P: 661
Quote Quote by San K View Post
yes, when s-photon landed, the entanglement was broken since s-photon position has been fixed/locked.....and the p-photon also becomes probabilistically determinable.....thatís how I think....some on the forum agree, some disagree and some have another explanation, etc
Well what happens if I just (by mechanical means) put the eraser in place (by microseconds) after EACH s-photon is detected, or if I remove the eraser (by microseconds) before the entangled p-photon can reach it EACH time.

Or I put the eraser so remotely that all s-photons are measured before a single p-photon can reach the eraser, and then I put the eraser in place just before they reach it?

Will the (timing offset if necessary) coincidence counts show interference?

You see, not so simple is it?
San K
#33
Jun11-11, 06:29 PM
P: 915
Quote Quote by unusualname View Post
Well what happens if I just (by mechanical means) put the eraser in place (by microseconds) after EACH s-photon is detected, or if I remove the eraser (by microseconds) before the entangled p-photon can reach it EACH time.

Or I put the eraser so remotely that all s-photons are measured before a single p-photon can reach the eraser, and then I put the eraser in place just before they reach it?

Will the (timing offset if necessary) coincidence counts show interference?

You see, not so simple is it?
the eraser is simply a filter/sieve, so is the co-incidence counter and all of the above can be explained by this/that
unusualname
#34
Jun11-11, 06:50 PM
P: 661
Quote Quote by San K View Post
the eraser is simply a filter/sieve, so is the co-incidence counter and all of the above can be explained by this/that
NO IT CAN'T!!!!!!!!


Until photons are MEASURED/DETECTECTED they have UNKNOWN quantum properties (like polarisation). This is a well establsished experimental fact, it is highly nonintuitive and unsettling but it is the way the world is.

Sorry San K, but this experiment is kinda old school compared to what's been shown with multi-entangled states recently. Reality just ain't really real the way you think. You can allow a non-local interpretation to retain some idea of reality, which is what I thought you were suggesting.
Cthugha
#35
Jun11-11, 07:42 PM
Sci Advisor
P: 1,627
Quote Quote by unusualname View Post
NO IT CAN'T!!!!!!!!
Sure it can. It is of course necessarily a non-local filter, as you would need to get the information about which state photon B has to go to once A is detected across instantly, but definitely sufficient to explain all results. For the first experiments on DCQE (before 2004) the results were, however, not tested for violations of Bell or GHZ inequalities and could therefore indeed have been explained completely classically in terms of a common history in the SPDC process. Or in other words: hidden variables have not been ruled out back in the early days of DCQE. These tests started in 2004. By the way I still do not get your resistance against the subsampling picture. You do realize that also the subsampling explanation requires a non-classical description and non-locality, do you?

Quote Quote by unusualname View Post
Until photons are MEASURED/DETECTECTED they have UNKNOWN quantum properties (like polarisation). This is a well establsished experimental fact, it is highly nonintuitive and unsettling but it is the way the world is.
I do not care much about interpretations, but a lot of people caring about them on these forums will tend to disagree with that statement. It might be that nature is non-realistic and there are no properties before measurement. However, at current it is not experimentally possible to distinguish between non-realistic and non-local descriptions of qm as both can lead to the same results. It is a well established experimental fact that Bell and other inequalities are violated for entangled photons. Non-realism is one possible and appealing conclusion, but not a fact that can be tested experimentally at current.
San K
#36
Jun11-11, 11:34 PM
P: 915
Quote Quote by unusualname View Post
NO IT CAN'T!!!!!!!!


Until photons are MEASURED/DETECTECTED they have UNKNOWN quantum properties (like polarisation). This is a well establsished experimental fact, it is highly nonintuitive and unsettling but it is the way the world is.

Sorry San K, but this experiment is kinda old school compared to what's been shown with multi-entangled states recently. Reality just ain't really real the way you think. You can allow a non-local interpretation to retain some idea of reality, which is what I thought you were suggesting.
Sub-sampling can explain the results of DCQE, sub-sampling does include non-local assumptions/interpretation.

There is no need to assume future can change the past or causality is violated by QM/DCQE or other hypothesis.

Scientists/experimenters, such as Yoon, do discuss sub-sampling (and the associated patterns) in their papers to explain appearance and disappearance of interference patterns.


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