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Explaining DCQE  via coherence in layman terms 
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#19
Jun1111, 04:08 PM

P: 57

The thing I don't understand is that the D1 and D2 detectors both show interference fringes and antifringes when the subsamples 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 subsamples. 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? 


#20
Jun1111, 04:14 PM

P: 57




#21
Jun1111, 04:19 PM

P: 661

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 pphoton arm delay to prove this. But much more sophisticated tests of the nonrealism of QM have been done, google GHZ states, Hardy, Zeillinger. EDIt I actually agree with you, but in much more subtle/amazing way. 


#22
Jun1111, 04:23 PM

P: 915

not sure what you are asking...... 


#23
Jun1111, 04:28 PM

P: 661

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 nonlocality/nonseparablity if one arm of the entangled pair is longer than the other. 


#24
Jun1111, 04:36 PM

P: 57

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." 


#25
Jun1111, 04:41 PM

P: 915

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 pphoton'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/MachZe...interferometer 


#26
Jun1111, 04:45 PM

P: 661

No. If you had apparatus in place in the pphoton arm at the time the sphotons are measured you could show the pphotons had RANDOM polarization before they are measured (before they pass through a polarizer) Sorry if this is confusing, but it's correct. 


#27
Jun1111, 04:51 PM

P: 915

are you asking why they are matching (results/patterns)?.......well they are matching because they are entangled.... 


#28
Jun1111, 04:53 PM

P: 915

you can predict that with X probability pphotons will be this you can predict that with Y probability pphotons will be that you can predict that with Z probability pphotons will be thisthat etc this is provable by the fact that we can predict (probabilistically) what path pphoton will take one it emerges from the polarizer....(if we had information about sphoton, of course)...... 


#29
Jun1111, 04:56 PM

P: 661




#30
Jun1111, 04:58 PM

P: 57




#31
Jun1111, 05:42 PM

P: 915




#32
Jun1111, 06:00 PM

P: 661

Or I put the eraser so remotely that all sphotons are measured before a single pphoton 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? 


#33
Jun1111, 06:29 PM

P: 915




#34
Jun1111, 06:50 PM

P: 661

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 multientangled states recently. Reality just ain't really real the way you think. You can allow a nonlocal interpretation to retain some idea of reality, which is what I thought you were suggesting. 


#35
Jun1111, 07:42 PM

Sci Advisor
P: 1,657




#36
Jun1111, 11:34 PM

P: 915

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 subsampling (and the associated patterns) in their papers to explain appearance and disappearance of interference patterns. 


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