Brian Green's Beam Splitter Experiments

[moving finger]

But anyway, I was also struggling with the matter of the delayed-choice quantum eraser after having read http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm . In particular, if the setup described on that page would be changed by firing, say, a thousand photons from the laser of which the signal photons would all be detected before any of the idler photons arrived at the other detector. Suppose the 'far' detector used for detecting the idler photons can be any of two detectors, namely one which does detect which-path information and one which doesn't.

Which other one or two detectors are you talking about here? There are 4 in the paper... D1, D2, D3 and D4. It's not clear which ones you intend to keep and which ones throw away.

Best Regards

 

[moving finger]

I need some help please.

In the commentary paper http://www.bottomlayer.com/bottom/ki...scully-web.htm it says :

Without more, we would expect the pattern developing at detector D0 to be an interference pattern. QM predicts that without which-path information, photons arriving from either A or B should interfere and distribute themselves one-by-one according to the statistical distribution of interfering waves.

In other words, it actually says that in absence of which path information, the pattern of signal photons at D0 should be an interference pattern. But we can get that pattern simply by removing mirrors BSB and BSA (remove these mirrors, and all idlers go to D1 or D2 via BS, hence no which path information).

But the original paper at http://xxx.lanl.gov/PS_cache/quant-ph/pdf/9903/9903047.pdf also made it clear that the raw data at D0 does not contain any interference pattern, because the interference patterns from D1 and D2 are 180 degrees out of phase - thus we will only see the pattern when we deconvolve the D0 signal data by correlating it with the idler coincidence data. This 180 degree phase shift is not referred to at all in the commentary paper at http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm.

If we remove BSB and BSA, do we see interference at D0 even without coincidence correlations with D1 and D2? If not, why not?

Am I missing something here?

Best Regards

 

[Brinx]

Moving finger, I proposed to place only one detector in the path of the idler photons instead of the more complex setup used in the paper - namely either a detector which does not read which-path information (such as a lens with a screen behind it, similar to the detector for the signal photons in the paper setup), or a detector which does read which-path information (such as the telescope detector Cesiumfrog suggested).

I should make a sketch to clear things up. I'll do so later on.

 

[moving finger]

Moving finger, I proposed to place only one detector in the path of the idler photons instead of the more complex setup used in the paper - namely either a detector which does not read which-path information (such as a lens with a screen behind it, similar to the detector for the signal photons in the paper setup), or a detector which does read which-path information (such as the telescope detector Cesiumfrog suggested).

I should make a sketch to clear things up. I'll do so later on.

OK, my problem (see my last post above) is that the original paper points out that there is a 180 degree phase shift between the D1 and D2 interference data, which means (I believe) that the raw D0 data (ie the data you want to use) will show no interference at all - I don't understand this and need someone to explain what is going on!

Best Regards

 

[gptejms]

OK, my problem (see my last post above) is that the original paper points out that there is a 180 degree phase shift between the D1 and D2 interference data, which means (I believe) that the raw D0 data (ie the data you want to use) will show no interference at all - I don't understand this and need someone to explain what is going on!

Best Regards

I've just had a quick look at the paper.The paper discusses this point(see page 3 of the paper).The culprit seems to be the beam splitter BS-- reflected and transmitted waves have a phase difference.

 

[moving finger]

I've just had a quick look at the paper.The paper discusses this point(see page 3 of the paper).The culprit seems to be the beam splitter BS-- reflected and transmitted waves have a phase difference.

Yes, I know the original paper discusses this point :

http://xxx.lanl.gov/PS_cache/quant-ph/pdf/9903/9903047.pdf

But the explanation paper does not :

http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

My questions are :

The explanation paper says that :

QM predicts that without which-path information, photons arriving from either A or B should interfere and distribute themselves one-by-one according to the statistical distribution of interfering waves.

But this is obviously not true, since the pattern at D0 is a combination of the two interference patterns which are 180 degrees phase-shifted, so the "raw data" hitting D0 (in absence of any coincidence correlations with the other detectors) shows NO interference pattern.

If the phase shift is due to the beamsplitter BS, what happens if we remove BS - does the raw data at D0 suddenly show an interference pattern?

If the beamsplitter at BS causes a 180 degree phase shift between transmitted & reflected photons, then 50% of all photons reaching D1 and 50% of all photons reaching D2 should be phase-shifted relative to the other 50% - because half of all photons reach each detector are transmitted and half are reflected! Thus the explanation for the phase shift between D1 and D2 data cannot be due to phase shift diferences between reflection and transmission.

Best Regards

 

[gptejms]

But this is obviously not true, since the pattern at D0 is a combination of the two interference patterns which are 180 degrees phase-shifted, so the "raw data" hitting D0 (in absence of any coincidence correlations with the other detectors) shows NO interference pattern.

The joint detections D0,D1 or D0,D2 show intereference patterns--that is the only claim in either of the papers.Besides,I don't see how the phase shift is 180 degrees--seems to be 90 degrees to me(one term contains cos,the other sin).

If the beamsplitter at BS causes a 180 degree phase shift between transmitted & reflected photons, then 50% of all photons reaching D1 and 50% of all photons reaching D2 should be phase-shifted relative to the other 50% - because half of all photons reach each detector are transmitted and half are reflected! Thus the explanation for the phase shift between D1 and D2 data cannot be due to phase shift diferences between reflection and transmission.

Best Regards

I don't really know how a beam splitter works,but the situation may not be the same on either side of the beam splitter.Imagine the following situation:-reflection coeff. R,transmission coeff. 1-R on one side; reflection coeff. -R,transmission coeff. 1+R on the other side.If you work with this, you get the signs as in the paper.

 

[gptejms]

But this is obviously not true, since the pattern at D0 is a combination of the two interference patterns which are 180 degrees phase-shifted, so the "raw data" hitting D0 (in absence of any coincidence correlations with the other detectors) shows NO interference pattern.

The joint detections D0,D1 or D0,D2 show intereference patterns--that is the only claim in either of the papers.Besides,I don't see how the phase shift is 180 degrees--seems to be 90 degrees to me(one term contains cos,the other sin).

If the beamsplitter at BS causes a 180 degree phase shift between transmitted & reflected photons, then 50% of all photons reaching D1 and 50% of all photons reaching D2 should be phase-shifted relative to the other 50% - because half of all photons reach each detector are transmitted and half are reflected! Thus the explanation for the phase shift between D1 and D2 data cannot be due to phase shift diferences between reflection and transmission.

Best Regards

I don't really know how a beam splitter works,but the situation may not be the same on both sides of the beam splitter.Imagine the following situation:-reflection coeff. R,transmission coeff. 1-R on one side; reflection coeff. -R,transmission coeff. 1+R on the other side.If you work with this, you get the signs as in the paper.

 

[moving finger]

The joint detections D0,D1 or D0,D2 show intereference patterns--that is the only claim in either of the papers.Besides,I don't see how the phase shift is 180 degrees--seems to be 90 degrees to me(one term contains cos,the other sin).

The original paper (page 3) clearly says Pi radians phase-shift - that's 180 degrees. A 180 degree phase shift would mean complete loss of fringes (destructive interference) when the data are superimposed - which explains why there is no structure in the D0 data alone. A 90 degree phase shift would not lead to complete loss of interference fringes when you superimpose the data.

I don't really know how a beam splitter works,but the situation may not be the same on both sides of the beam splitter.Imagine the following situation:-reflection coeff. R,transmission coeff. 1-R on one side; reflection coeff. -R,transmission coeff. 1+R on the other side.If you work with this, you get the signs as in the paper.

OK, thanks for that

Best Regards

 

[gptejms]

The original paper (page 3) clearly says Pi radians phase-shift - that's 180 degrees. A 180 degree phase shift would mean complete loss of fringes (destructive interference) when the data are superimposed - which explains why there is no structure in the D0 data alone. A 90 degree phase shift would not lead to complete loss of interference fringes when you superimpose the data.


OK, thanks for that

Best Regards

Right you are--the phase difference is 180 degrees because of the terms being cos^2 and sin^2(not cos &. sin),but D0 data still has a sinc^2.Anyway,the claim in the paper(s) is about D0,D1 or D0,D2--so what's the problem?

 

[moving finger]

so what's the problem?

you answered it - the BS (ie semi-transparent mirror) is spatially asymmetric, and this results in the fact that idler photons going to D1 are phase-shifted with respect to those going to D2 - which in turn explains why there is no structure in the raw D0 data.

Is there such a thing as a symmetric beamsplitter - one that does NOT result in a 180 degree phase shift? If there were, and we were to use it, this would imply there should be structure in the D0 data? But this would also open the door to paradoxes - so maybe a symmetric beamsplitter (one that does not cause a phase shift) is physically impossible.

Best Regards