Undergrad FTL communication via delayed choice measurement

[PeterDonis]

All of the DCQE experiments and variants are based on the fact that you use incoherent light for illuminating the double slit

I'm not sure the light is incoherent. In the Kim experiment described in the Wikipedia article @Alex Torres linked to (and also in the Kim paper referenced in that article), the light is said to be produced by an argon laser. That would be coherent light, not incoherent light.

 

[Alex Torres]

No, it won't. Look at the diagram in the Wikipedia article you linked to. If you take out all the detectors except d0, is what is left just a standard double slit experiment, with d0 in the same position as the detector screen in that setup? I don't think so

Agree, but then... (given we already solved the issue about the incoherent source of light) we have to go back and ask, what could possibly be in Kim's mind for inserting a double slit in the setup, if he wasn't expecting to get an interference pattern somewhere in the setup?

 

[Nugatory]

...would you please give us a link to a source backing up that statement?...just made a little research...and found out they used a laser, a coherent source of light...

The pump laser is a source of coherent photons, but these are not the photons we're talking about here. The experiment is using the photons produced by parametric downconversion in the BBO crystal after the double slit and these are not coherent.

 

[Alex Torres]

The pump laser is a source of coherent photons, but these are not the photons we're talking about here. The experiment is using the photons produced by parametric downconversion in the BBO crystal after the double slit and these are not coherent

Yep.. ... that could solve the whole issue, but if that's the case it would rise concerns about another issue...how did he get an interference pattern by cherry picking only from d1 at d0, or cherry picking only from d2 at d0?

 

[Cthugha]

...would you please give us a link to a source backing up that statement?...just made a little research...and found out they used a laser, a coherent source of light...

Just to clarify: the kind of coherence we talk about here is spatial coherence (that is what a double slit measures) and not any higher order coherence that is typically associated with lasers. This is basically a measure of how point-like the light source is. Accordingly, the degree of coherence depends on the distance between the slit and the light source and the size of the active area of the light source. One can generally show that spatial coherence and entanglement in the spatial/angular degree of freedom are mutually exclusive, see Phys. Rev. A 63, 063803 (2001): https://journals.aps.org/pra/abstract/10.1103/PhysRevA.63.063803 .

An explicit measurement of the visibility of the interference patterns of light emitted from BBO crystals at different distances to the slit has been done in Zeilinger's group. Unfortunately the thesis containing the results is only available in German, but maybe some people here are able to understand it: https://www.univie.ac.at/qfp/publications/thesis/bddiss.pdf
The calculations of the necessary distances are around page 45.

 

[Alex Torres]

Given that ..."During an experiment, detector D0 is scanned along its x-axis, its motions controlled by a step motor."... End quote

Can detectors d1-4 have the same mechanical characteristics?

 

[Nugatory]

Given that ..."During an experiment, detector D0 is scanned along its x-axis, its motions controlled by a step motor."... End quote

Can detectors d1-4 have the same mechanical characteristics?

Yes, but why? There's only one position where they can detect anything.

 

[Alex Torres]

Yes, but why? There's only one position where they can detect anything.

The issue of the BBO generating entangled pairs without capacity to reflect interference at the screen was solved in the Walborn's DCQE version...a more simplified version...
https://files.acrobat.com/a/preview/0b5148b2-be98-4fdb-9f7c-da1f58490d73

The SPDC occurs first, the signals are sent directly to Alice at d0 without going thru the slits. The idlers are sent to Bob who has a standard DS setup that reflects a definite interference pattern at the screen, thus solving the issue of having overlapped results when measuring interference patterns or clumps.

Here's the the assumption...
Alice and Bob have screen type detectors that moves across the x-axis with step motors controlled by individualy coordinated atomic clocks, the rate at which the laser pumps is also controlled by the clocks ...

The question:...will their results correlate don't matter how far appart Alice and Bob are from each other??

 

[Alex Torres]

Here's a summary of Walborn's DCQE

https://files.acrobat.com/a/preview/abdaa0fc-2e1a-4689-97f8-8d866fe89a63

 

[Cthugha]

The SPDC occurs first, the signals are sent directly to Alice at d0 without going thru the slits. The idlers are sent to Bob who has a standard DS setup that reflects a definite interference pattern at the screen, thus solving the issue of having overlapped results when measuring interference patterns or clumps.

No, it does not. You only get the interference pattern directly in the non-delayed version, where the light arriving at the double slit is linearly polarized. Walborn states that at the beginning of section III. In the polarization entangled BBO version of the experiment he uses afterwards, the incoming light is not polarized linearly anymore and there is no direct interference pattern left. This is easier to see in Walborn's popular physics article from American Scientist, where he explains the experiment in a more layman-friendly way:

http://www.mat.ufmg.br/~tcunha/2003-07WalbornF.pdf

 

[Alex Torres]

Walborn states that at the beginning of section III. In the polarization entangled BBO version of the experiment he uses afterwards, the incoming light is not polarized linearly anymore and there is no direct interference pattern left

Ok got it...

Just gave that link by mistake, the following gives a better layman's description, see under the caption "Double slit interference", he performs a test for interference in his setup before touching in anyway the polarization of the entangled photons.

https://files.acrobat.com/a/preview/abdaa0fc-2e1a-4689-97f8-8d866fe89a63

Think he just wanted to be sure the SPDC process was not affecting the interference pattern in anyway.

But,...let us stop right there,...

Assume detector Dp is replaced by another detector which is exactly the same as detector Ds.

The question is... will Dp(assuming was replaced as described above ) show the same interference pattern as it happens to be in Ds??

 

[Alex Torres]

...Still, we might be able to do some FTL signaling with a very simple setup and some basic statistical concepts ...

Using the realization of Asher Peres thought experiment dubbed "steering into the past"...we give Alice and Bob on Earth an atomic clock and another one to Victor on Pluto...
...both clocks are coordinated with settings telling the exact minute an experiment starts and ends so all three, Bob Alice and Victor will know the beginning and the end of any single experiment run...

A single experiment run can consist of the following: ...a single batch of 100 pairs of entangled photons that both Alice and Bob receive, with Victor projecting all them into an entangled state (quantum correlation)...with each measurement from Victor being delayed by the time/distance between Earth and Pluto...

Under these conditions each pair of photons received by Alice and Bob will correlate in a way that if Alice's quantum state is 1 Bob will be 100% certain that his is 0 and vice versa...so by the end of the experiment run 100 pairs of entangled photons received by Alice and Bob with 100% correlation for each pair means Victor projected the whole batch in an entangled state...this whole batch counts as a 1...then it means Victor signaled a 1...

...for signaling a 0... Victor will let Alice and Bob receive the whole batch of entangled pairs of photons with classical correlations within a single experiment run...that means, there will always be less than 100% certainty that all pairs received by Alice and Bob under that single batch will correlate... with that probability result Alice and Bob on Earth will be certain Victor did not projected an entangled state for that batch..

 

[Deepblu]

Under these conditions each pair of photons received by Alice and Bob will correlate in a way that if Alice's quantum state is 1 Bob will be 100% certain that his is 0 and vice versa...so by the end of the experiment run 100 pairs of entangled photons received by Alice and Bob with 100% correlation for each pair means Victor projected the whole batch in an entangled state...this whole batch counts as a 1...then it means Victor signaled a 1...

...for signaling a 0... Victor will let Alice and Bob receive the whole batch of entangled pairs of photons with classical correlations within a single experiment run...that means, there will always be less than 100% certainty that all pairs received by Alice and Bob under that single batch will correlate... with that probability result Alice and Bob on Earth will be certain Victor did not projected an entangled state for that batch..

Alice and Bob will have to wait for all the photons to arrive from Pluto first then do the measurements then compare their results, so they have to wait long time much longer than speed of light to tell what vector did on his end.

When Victor wants to change signalling from 1 to 0, Alice and Bob will have to wait at least 5.5 hours (the time needed for light to travel from Pluto to Earth) first before receiving all the photons.

Why do you insist that FTL communication is possible?! You can't send information by observation .. that's it .. a very simple truth!

 

[DrClaude]

Why do you insist that FTL communication is possible?! You can't send information by observation .. that's it .. a very simple truth!

This is a good place to end this thread.