explaining DCQE - via coherence in layman terms

San K

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.

i.e. you can tell what is the probability of p-photon taking a particular path, (out of the choices available)

but this again cannot be used to transmit (volitional) information faster than the speed of light...

when we do co-incidence count, we are simply picking only those photons (from both s and p) marks/positions that match the pattern. the co-incidence counter acts like a filter.

the peaks of one interference pattern coincide with the troughs of the other....causing a no-interference pattern.

and Yoon in his paper also explains it this way.....the sub-sampling way.....

Joncon

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

I don't see how this can change anything. You still have no information about the s-photon.

unusualname

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

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

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

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

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

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

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

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

Yes, so, if I put the eraser on pluto how does the s-photon know which p-photon to match to?

Joncon

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

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

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

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

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.