FTL communication via delayed choice measurement

  • #1

Main Question or Discussion Point

<Moderator's note: 2 threads merged as it is an identical topic.>

Given Kim's version of DCQE, let's say Bob in on earth looking at d0, Alice is on mars holding the prism that deflects the idler coming from (a) to d3 or (b) to d4. After the experiment run ends Bob should get clump at d0 without a feedback from Alice. In the actual experiment Bob receives each idler at d0, 8 nanoseconds before Alice
 
Last edited by a moderator:

Answers and Replies

  • #2
Given Kim's version of DCQE is it possible FTL communication by having Bob on earth looking at d0, and Alice on mars holding the prism that deflects the idler (a) to d4 and the idler(b) to d3 so by the time the experiment is done Bob would have a definite clump pattern at d0 without the need to receive any feedback from Alice. (To avoid noise from Bob to Alice we can use the same channel used by Victor, Alice and Bob in the Delayed choice entanglement swapping)
 
  • #3
Strilanc
Science Advisor
588
210
No, it's not. Bob sees the same thing on d0 regardless of Alice's choice. You need to split the measurements of d0 into two groups in order to see the interference or not, but in order to do the splitting you need Alice's measurement results.
 
  • #4
Yes, know that will happen if you leave all 4 detectors at Alice. In Kim's papers you see a single band for all hits mixed in a single graph at d0, but if you see d3/d4 only there's a bell shaped pattern that even if you overlap for not cherry picking at them, due to symmetry you also get a bell shaped pattern at d0, assuming a setup with d3/d4 only.
 
  • #5
Let me make just one correction. By Alice having only d3/d4 in place untill the experiment is done, Bob don't even has to plot anything into a graph to see a clump at d0. Next day another run of the experiment starts at the same hour, but this time Alice has nothing in place so her twin idler gets lost in the space, after the experiment is done Bob should get fringes, since no measurenent was done.
 
  • #6
Nugatory
Mentor
12,619
5,171
By Alice having only d3/d4 in place untill the experiment is done, Bob don't even has to plot anything into a graph to see a clump at d0. Next day another run of the experiment starts at the same hour, but this time Alice has nothing in place so her twin idler gets lost in the space, after the experiment is done Bob should get fringes, since no measurenent was done.
That's not how it works. The signal photons whose corresponding idlers will interact with D3 or D4 if they are there contribute to the "clump" subset of the total detections at D0; the signal photons whose corresponding idlers would interact with D1 or D2 if they are there contribute to the "pattern" subset of the total detections at D0. Either way, the total pattern at D0 is the same.

(There are also practical problems separating any signal at D0 from background noise if any of D1-D4 are not providing input to the coincidence counter).
 
  • #7
But... what pattern might you get at d0, by not detecting the idler at all and letting it just hit the wall?
 
  • #8
Nugatory
Mentor
12,619
5,171
But... what pattern might you get at d0, by not detecting the idler at all and letting it just hit the wall?
You get the same pattern at D0 no matter what you do with the idler. Read @Strilanc's post in #3 again.
 
  • #9
You need to split the measurements of d0 into two groups in order to see the interference or not, but in order to do the splitting you need Alice's measurement results.
I understand, that completely applies as long as Alice is measuring both a particle and a wave in the same experiment run. My simple question is, what Bob will get at d0 if he just let all the idlers get absorbed at a piece of paper, since you are not making a definite measurement of anything at the idler, the most reasonable answer is that he gets a definite interference pattern at d0....but that's just my guess.... don't know of any experiment done this way...
 
  • #10
34,060
9,934
What Alice does has no impact on the result Bob gets.

This has been said multiple times now.
 
  • #11
Ok, take Alice out of this....Bob is doing the experiment at home, he takes out all detectors and Beam splitters in the idlers path and put a piece of paper instead....my question is, what pattern, if any, will the signals photons form at d0 after all hits are recorded...
 
  • #12
34,060
9,934
The same as with detectors, but now a sketch of the setup would help.
 
  • #13
PeroK
Science Advisor
Homework Helper
Insights Author
Gold Member
12,395
5,160
Given Kim's version of DCQE is it possible FTL communication ...
In order to send a message, you have to change something under your control. In the case of entanglement, all you can do is observe the state. You cannot control the result of your observation (although you can choose what to observe).

It's fundamentally no different in this respect from the right shoe/left shoe scenario. If Alice receives a box with one shoe in it and Bob receives the other shoe, then they can observe what is in their box. That determines, but doesn't change, what's in the other box. Each has no control over what shoe they observe. And, if one of them replaces the shoe with an opposite shoe, that does not change the shoe the other one has.

Fundamentally, you cannot send a message by observation.
 
Last edited:
  • #14
If Alice receives a box with one shoe in it and Bob receives the other shoe, then they can observe what is in their box.
Again, lets take Alice out of this context ....Bob is doing the experiment at home, he takes out all detectors and Beam splitters in the idlers path and put a piece of paper instead....my simple question is, what pattern, if any, will the signals photons form at d0 after all hits are recorded... im not asking here anything about FTL communication either!...
 
  • #15
495
293
You received an answer:

The same as with detectors, but now a sketch of the setup would help.
 
  • #16
In the proposal Bob is deciding himself no to measure position, my guess is that he should get a definite fringes pattern at d0, in Kim's papers it's pretty well explained why every single detector contributes to the single solid band at d0, but doesn't tell if all detectors are removed from the idlers path...
 
  • #17
Nugatory
Mentor
12,619
5,171
Again, lets take Alice out of this context ....Bob is doing the experiment at home, he takes out all detectors and Beam splitters in the idlers path and put a piece of paper instead....my simple question is, what pattern, if any, will the signals photons form at d0 after all hits are recorded.
That experiment is easier said than done, because you need some way of picking the signal photons out from the much larger number of of incident photons that aren't part of signal/idler pairs. However, the downconverted photons do have a different frequency, so it should be possible in principle.

So what you're really asking is: Suppose we were to identify every detection at D0 that would have been included if we had our D1-D4 detectors and coincidence counter set up; what pattern do they form? Phrased that way, the answer should be clear: it's the sum of the four individual patterns.
 
  • #18
if we had our D1-D4 detectors and coincidence counter set up; what pattern do they form? Phrased that way, the answer should be clear: it's the sum of the four individual patterns.
Agree....then if we remove d1-d4 detectors the pattern at d0 will be________________.
 
  • #19
Nugatory
Mentor
12,619
5,171
Agree....then if we remove d1-d4 detectors the pattern at d0 will be________________.
Unchanged from if the detectors were there, so we can use the distributions in the paper to see what the pattern will be. Qualitatively, the rate-position graph is a hill with bumps on it, or you can visualize the pattern by overlapping the R01, R02, R03, and R04 illustrations from the wikipedia article.
 
  • #20
Unchanged from if the detectors were there, so we can
sorry for no being specific, should read: if we remove d1-d4 detectors and replace them with a piece of paper then run the experiment again, when the experiment is done the pattern at d0 will be________________.
 
  • #21
In other words, if you remove all BS's and detectors in the idlers path an just let it make its run toward a single piece of paper, after the experiment is done, the pattern at d0 will be ______________.
 
  • #22
Nugatory
Mentor
12,619
5,171
sorry for no being specific, should read: if we remove d1-d4 detectors and replace them with a piece of paper then run the experiment again, when the experiment is done the pattern at d0 will be________________.
In all all questions of the form "If <some condition involving the idlers> and then we run the experiment again, the pattern formed by the signal photons at d0 will be _________?" we can fill in the blank with the same words: "What you get when you overlap the R01, R02, R03 and R04 illustrations from the wikipedia article; the rate-position graph is a hill with bumps".

(As a practical matter, we need some way of recognizing the signal photons; for every one that arrives at d0 we will get hundreds of thousands or millions of unrelated photons that will totally overwhelm the pattern formed by the signal photons. The idler detectors and the coincidence counter circuity take care of that when they're present so those are conditions under which it's easiest to actually observe the pattern. But once we've observed it, it's our answer no matter what we do with the idlers).
 
  • #23
"If <some condition involving the idlers
the proposal is that no condition is forced upon the idler, we let it fly freely all along until absorbed by a piece of paper, as someone stated before, the pattern at d0 will be the sum of the patterns provided by d1-d4, given they are in place, so it will still be a definite and distinctive pattern if compared with something else.
 
  • #24
Nugatory
Mentor
12,619
5,171
the proposal is that no condition is forced upon the idler, we let it fly freely all along until absorbed by a piece of paper,
That's a condition (as would also be "we pay no attention to the idler" or "we have no idea what is done with the idler" or "the idler falls into a black hole" or ....).
 
  • #25
That's a condition (as would also be "we pay no attention to the idler" or "we have no idea what is done with the idler" or "the idler falls into a black hole" or ....).
yep.. you are right....so let me use another term: you are not measuring the idlers in anyways..still take into account that it just passed thru the double slit...
 

Related Threads on FTL communication via delayed choice measurement

Replies
7
Views
1K
Replies
5
Views
2K
Replies
1
Views
349
Replies
10
Views
473
Replies
1
Views
546
  • Last Post
Replies
11
Views
4K
  • Last Post
Replies
19
Views
927
Replies
27
Views
2K
Replies
1
Views
802
Top