Undergrad FTL communication via delayed choice measurement

[Alex Torres]

Then, since whatever happens or doesn't happen to the idlers is completely irrelevant to the overall pattern of signal photons at d0, we

Then, a definite interference pattern at d0 can be distinguished from an overall pattern at d0 resulting from different outputs being overlapped. Am i correct?

 

[Nugatory]

Then, a definite interference pattern at d0 can be distinguished from an overall pattern at d0 resulting from different outputs being overlapped. Am i correct?

Quick answer: No.

Longer answer:
In one run of the experiment we accumulate a large number of position measurements, basically observations of the form "detector d0 detected a signal photon when it was at position x". We look at these and find a pattern: some areas get more detections than others. It's always the same pattern every time we run the experiment no matter what happens with the idlers, and that's all the information that we ever get from d0. There will never ever be but that one pattern at d0 (unless we change something else like the frequency of the pump laser, or the position and shape of the slits, or the characteristics of the BBO crystal).

Now suppose we take our large collection of position measurements and divide it into four groups. (We could put measurements 1, 5, 9, ... in the first group, 2, 6, 10, ... in the second group, 3, 7, 11, ... in the third group, and 4, 8, 12, ... in the fourth group. Or we could do the assignments randomly: for each measurement we generate a random number between 1 and 4 and put that measurement in that group. Or we could do something else). Then we look at the pattern for each group. Several things should be obvious:
- The total pattern will be the sum of the patterns for each group. It's nonsense to talk about the sum being somehow different from the total pattern (which is why the quick answer above is "no").
- The pattern for any single group doesn't have to look like the total pattern. For example, we could choose to put all the measurements in which x is less than .1 in one group; then we'll find that the pattern for that one group has a huge spike where the pattern for the other three groups has a huge trough, and neither the trough nor the spike appears in the pattern we actually measured. That doesn't mean that we've observed anything different; it's just an artifact of how we've manipulated our data.

There is one particularly interesting way of dividing our collection of position measurements into four groups. We are sitting around admiring the nice pattern we found at d0 when Alice walks up to us and suggests that we try putting this one, this one and this one into group one, then that one and that one into group two, and so forth. We're intrigued, so we ask her how she's choosing, and she explains that she's putting all the ones whose idler triggered d1 into one group, all the ones whose idler triggered d2 into another group, and so on. So we divide our collection of position measurements into four groups as Alice suggests, then we look at the pattern for each group in isolation... and we get something that looks like the patterns in Kim's paper, with interference fringes in groups 3 and 4 and not in groups 1 and 2.

 

[Alex Torres]

In one one run of the experiment we accumulate a large number of position measurements, basically observations of the form "detector d0 detected a signal photon when it was at position x". We look at these and find a pattern: some areas get more detections than others. It's always the same pattern every time we run the experiment no matter what happens with the idlers, and that's all the information that we ever get from d0. There will never ever be but that one pattern at d0 (unless we change something else like the frequency of the pump laser, or the position and shape of the slits, or the characteristics of the BBO crystal).

Then we get a definite interference pattern for each experiment run under the conditions proposed.

But there's still something i don't get from the rest of your answer,... if we run Kim's version exactly as it is, the overall patterns at d0 is always the same, which happens to be a bell shaped graph, see Strilanc's comment above under "what Alice sees" column.

So we can summarize all the above for the sake of simplicity as follows:

Setup A: is exactly as Kim's description.

Setup B: the idlers path is cleared from BS's and detectors, and has a single piece of paper as a unique target, so no measurement is done.

Overall results at d0, (without sorting the hits)

Setup A: a bell shaped graph
Setup B: a definite interference pattern

 

[Alex Torres]

Or disclosing it in a more specific way. ...

The overall results at d0, (without sorting the hits) will show the following intensity across the screen:

Setup A: a single bell shaped pattern

Setup B: a definite interference pattern

 

[Nugatory]

Or disclosing it in a more specific way. ...

The overall results at d0, (without sorting the hits) will show the following intensity across the screen:

Setup A: a single bell shaped pattern

Setup B: a definite interference pattern

No, both setups produce the same single-hump pattern. Strilanc's picture is correct and I didn't do the addition of the patterns properly the first time around. If you look carefully at the R01 and R03 curves, you'll see that one has a peak where the other has a trough, so the peaks and troughs don't appear in the sum.

 

[Alex Torres]

No, both setups produce the same single-hump pattern. Strilanc's picture is correct

But didn't say it wasn't correct, just said it applies as long as we run Setup A: Kim's set up as it is.

If you look carefully at the R01 and R03 curves, you'll see that one has a peak where the other has a trough, so the peaks and troughs don't appear in the sum

Just to make it sure i got your point...are you referring in this quote to the sum of the patterns at d1/2?

 

[Alex Torres]

This is a quote of what's actually described about hits from d1/d2 selected at d0..."One can get an idea of how this works by looking at the graphs of R01, R02, R03, and R04, and observing that the peaks of R01 line up with the troughs of R02"...R01 and R02 are the individual interference patterns we get from hits at d1 and d2, if they overlap, we get the single hump pattern at d0 described in Strilanc's comment under "what Alice sees" column as the erasure graph.

...But, the proposed Setup B assumes no detectors are in place for the idlers path, only d0 for the signal, ...so no detector is measuring position anywhere in the setup and no erasure is done either, ...then, wouldn't it be reasonable to ask: is there anything that could possibly preclude all signal photons from hitting d0 as a wave in a way that a definite interference pattern at d0 emerges?.... just asking

 

[Alex Torres]

Just a second thought...but first, thanks to all you guys for taking a slice out of you time and give such an useful insights toward this proposal... it's not mine by the way...saw it in the comments section about a DCQE video and found it quite interesting and enough convincing to be posted in a forum like this to get some feedbacks ...

...first thing noted...the fact that Kim's version needs 2 detectors to measure which way and another pair of detectors to measure which way erasure...forces Alice to go back to Bob in order to make sense out of "what she sees at the screen" for having the results overlapped. But then, the results are overlapped since 2 detectors are needed to measure which way and another pair to measure after erasure.
So this is kind of a loophole.

Let's say Bob just realized that, and thinks, the single-hump pattern Alice always get at the screen is a definite pattern after all if compared with something else...

Then he thinks in a radical way and decides to add a second experiment setup totally different to the one he has used so far and gives Alice a second screen to collect the hits from this second setup.

He thinks, to get a message across to Alice i need to find out the way to arrange this second setup in oder to obtain a pattern that just looks distinctively different from the single-hump pattern she always get from the first setup... so if the photons can travel all along to Alice as a wave then she might get a distinctive interference pattern at her screen and that will be a totally different pattern from the single hump pattern she has received so far.

So he decides to discard all detectors and BS's and replaces all that with a single piece of paper as the sole target for all the idlers photons he gets.

Here is the question...is he going to succeed to have a definite interference pattern at Alice's second screen?

 

[SatishR]

You might find this article interesting. The point was that it appears impossible to extract information in the manner you are describing, you need information that you can only get at sub/equal-light speeds.

https://arxiv.org/abs/1707.06995

 

[Strilanc]

So he decides to discard all detectors and BS's and replaces all that with a single piece of paper as the sole target for all the idlers photons he gets.

Here is the question...is he going to succeed to have a definite interference pattern at Alice's second screen?

How many times do we have to say NO IT WON'T MAKE AN INTERFERENCE PATTERN before you'll actually hear the answer?

 

[Alex Torres]

The point was that it appears impossible to extract information in the manner you are describing

Yep...also have seen others rising concerns because on the non communication theorem, but then found out it is conditioned to the use of quantum states as a medium to carry classical bit, there's an obvious reason why you can't use a probability distribution to send any coherent message at all...

 

[Alex Torres]

How many times do we have to say NO IT WON'T MAKE AN INTERFERENCE PATTERN before you'll actually hear the answer?

Come on dude, this is a discussion forum, ...just saying "No" isn't enough... All answers so far are related to why you can't get an interference pattern given Kim's version as it is...
...but, if the reason to have a double slit in the setup is to produce an interference pattern that can be projected or obtained somewhere in the setup, haven't seen an explanation so far about why it would be impossible to obtain that pattern given there's no measurement anywhere in the modified setup...

 

[DrChinese]

Come on dude, this is a discussion forum, ...just saying "No" isn't enough... All answers so far are related to why you can't get an interference pattern given Kim's version as it is...
...but, if the reason to have a double slit in the setup is to produce an interference pattern that can be projected or obtained somewhere in the setup, haven't seen an explanation so far about why it would be impossible to obtain that pattern given there's no measurement anywhere in the modified setup...

Your question about "why it would be impossible..." is always going to be tough, as tough as answering why pigs don't fly.

But it might help if you knew that entangled particle pairs - such as used in the Kim et al experiment - do not usually produce interference patterns. See S290, figure 2. And yet the correlated sub-groups within the overall do, as Kim demonstrated.

https://pdfs.semanticscholar.org/3644/6f15507880c629e06391adf9d21aa6d76015.pdf

 

[PeterDonis]

given Kim's version as it is

Please give a specific reference for "Kim's version as it is". I have seen no link to an actual paper in any of your posts.

 

[PeterDonis]

if the reason to have a double slit in the setup is to produce an interference pattern that can be projected or obtained somewhere in the setup

No, the reason to have a double slit in the setup is...to see what happens when you have a double slit in the setup. Whether it produces an interference pattern depends on the entire setup. A double slit can't magically make interference where none is possible.

All of the answers you have gotten in this thread amount to repeatedly pointing out to you that none of the changes you have suggested making in the setup, as compared to "Kim's version as it is", change anything about the original setup that affects whether interference is possible. So if you can't get interference in the original setup (which you can't, and you appear to realize that), you can't get interference in any of your modified setups.

 

[Alex Torres]

Please give a specific reference for "Kim's version as it is". I have seen no link to an actual paper in any of your posts.

You can scroll up to one of Strilanc's comments above where it clearly explains, with nice graphs, why Alice will never see an interference pattern, given Kim's setup, there's also an acceptable description of the whole experiment in wikipedia...

https://en.m.wikipedia.org/wiki/Delayed_choice_quantum_eraser

 

[Alex Torres]

Whether it produces an interference pattern depends on the entire setup. A double

Thought it was demonstrated, the interference pattern at the screen disappears if a measuring device is placed to see which slit the photon went thru.

 

[PeterDonis]

the interference pattern at the screen disappears if a measuring device is placed to see which slit the photon went thru

Yes, that's an example of "whether it produces an interference pattern depends on the entire setup".

 

[Alex Torres]

Yes, that's an example of "whether it produces an interference pattern depends on the entire setup".

... maybe our only option would be to ask Kim. Did he test the setup for interference pattern before inserting any detector in the idlers path? just to be sure there wasn't something physically destroying the interference pattern before the idlers hit the first pair of detectors d3/4?... otherwise, the presence of those detectors would be totally redundant...

 

[Nugatory]

... maybe our only option would be to ask Kim. Did he test the setup for interference pattern before inserting any detector in the idlers path?

You're forgetting the role of the coincidence counter circuitry in this experiment. No signal photon will be recorded and counted towards the pattern unless its idler is detected by one of the D0-D4 detectors. So if you remove any of these detectors, you're just asking Bob to throw out some of the D0 detections, and any changes in the observed pattern are just the result of not counting all the points in the pattern.

 

[Alex Torres]

No signal photon will be recorded and counted towards the pattern unless its idler is detected by one of the D0-D4 detectors

If you leave all detectors in place and decide not to account for the individual patterns from each detector, you still will have an overall pattern at d0, that will be a single-hump pattern

So if you remove any of these detectors, you're just asking Bob to throw out some of the D0 detections, and any changes in the observed pattern are just the result of not counting all the points in the pattern.

But, that's exactly the purpose of removing some or all detectors besides d0, to get a distinctive pattern from the single hump pattern we get for leaving all detectors in place.

 

[Alex Torres]

So if you remove any of these detectors, you're just asking Bob to throw out some of the D0 detections, and any changes in the observed pattern are just the result of not counting all the points in the pattern

Let me correct the answer to this second quote, should read: But, that's exactly the purpose of removing some or all detectors besides d0, to get an overall distinctive pattern from the overall single hump pattern we get for leaving all detectors in place and not counting towards the individual patterns

 

[mfb]

But, that's exactly the purpose of removing some or all detectors besides d0, to get a distinctive pattern from the single hump pattern we get for leaving all detectors in place.

You don't get a different pattern by removing detectors. You just lose some information.

 

[Alex Torres]

You don't get a different pattern by removing detectors. You just lose some information.

The proposal has a basic assumption: ...if all detectors, besides d0, are removed and all the idlers hit a single target where no measurement takes place, the modified setup will turn out to be a Standard DS experiment with no detector in place at the slits and d0 mimicking the screen behind the slits, should get a definite interference pattern.

 

[Cthugha]

if all detectors, besides d0, are removed and all the idlers hit a single target where no measurement takes place, the modified setup will turn out to be a Standard DS experiment with no detector in place at the slits and d0 mimicking the screen behind the slits, should get a definite interference pattern.

Of course you will not get an interference pattern that way. All of the DCQE experiments and variants are based on the fact that you use incoherent light for illuminating the double slit, so you do not get an interference pattern behind the double slit under any circumstances unless you perform a posteriori filtering.

 

[Alex Torres]

Of course you will not get an interference pattern that way. All of the DCQE experiments and variants are based on the fact that you use incoherent light for illuminating the double slit, so you do not get an interference pattern behind the double slit under any circumstances unless you perform a posteriori filtering.

...got it! You just nailed it!...but there's still something i don't get, what could possibly be in Kim's mind when he designed the setup, given he should have known in advance the double slit in his setup had no chance of rendering an interference pattern anywhere in the setup, that he might still get an interference pattern at d1 or d2?

 

[Alex Torres]

Said "d1 or d2" , if for example, you only cherry pick the hits from d1 at d0 and plot them into a graph, you get a definite interference pattern for all the hits you accounted for at d1 only...the same should stand if you cherry pick from d2 only

 

[Nugatory]

...got it! You just nailed it!...but there's still something i don't get, what could possibly be in Kim's mind when he designed the setup, given he should have known in advance the double slit in his setup had no chance of rendering an interference pattern anywhere in the setup, that he might still get an interference pattern at d1 or d2?

That's what a calculation from the laws of quantum predict will happen - the calculation is in his paper. The point of the experiment was to confirm that this prediction is accurate, and to demonstrate that the relative ordering of the signal and idler detections doesn't matter (in Kim's experiment the idlers were detected about eight nanoseconds after the signal photons).

Said "d1 or d2" , if for example, you only cherry pick the hits from d1 at d0 and plot them into a graph, you get a definite interference pattern for all the hits you accounted for at d1 only...the same should stand if you cherry pick from d2 only

Yes, and Kim's paper has a picture of the d0-d2 coincidence pattern right next to the d0-d1 one.

 

[Alex Torres]

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

...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...

 

[PeterDonis]

if all detectors, besides d0, are removed and all the idlers hit a single target where no measurement takes place, the modified setup will turn out to be a Standard DS experiment with no detector in place at the slits and d0 mimicking the screen behind the slits

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.