-1
$\begingroup$

I'm using figure 2 of wikipedia's entry

https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser

enter image description here

If the idler beam is undetected, we get a interference pattern. If it's detected, there's no interference pattern. It seems that if we point the idler beam to Vega, we can observe from the presence of patterns to determine whether there are civilizations detecting the photons.

But this is nonsense. In fact in the experiment described in wikipedia, the pattern on the D0 screen is just a lump without any interference pattern. It's actually an 50%/50% superposition of different interference patterns, such that the peaks and lumps cancel out. The interference pattern only appears if we only count the photons whose idler beam triggers D3 or D4.

My question is : Why don't they just directly detect whether the idler beam should be detected or not, instead of using a beamsplitter, so that the pattern on D0 is not a 50%/50% mixture of signals? So that by observing the pattern we can be sure whether the idler beam is detected or ignored?

Phrased differently, I'm saying that in a double slit experiment, if you detect which-way the electron goes, but then erases the information, you should get a interference pattern. Now if you let people from a light year away to decide whether to erase the information, you have a device that detects whether intelligent life exists in a light year away instantly, by just observing the interference pattern.

$\endgroup$
0
$\begingroup$

If the idler beam is undetected, we get a interference pattern.

No, this is incorrect. If the idler beam is undetected, then no interference pattern will appear. The only way to suss out an interference pattern from the $D_0$ measurements is to plot only the counts that appear in coincidence with either of $D_1$ or $D_2$. If you don't measure the latter, you will never recover the interference pattern; but even if you do, you still require the results of the non-which-way measurement to get the pattern out.

It seems you've completely misread the description of the experiment - either read it again more carefully, or find a better source.

$\endgroup$
  • $\begingroup$ I have not misread it. "But this is nonsense. In fact in the experiment described in wikipedia, the pattern on the D0 screen is just a lump without any interference pattern." $\endgroup$ – seilgu Oct 3 '18 at 8:07
  • $\begingroup$ @seilgu Your question contains assertions which are factually incorrect and in direct contradiction to the material you quote; that's what misreading means. You don't seem to be interested in the answer, though - from your last comment, it seems you're only here to rant, and you can rant on your own if that's your intention. $\endgroup$ – Emilio Pisanty Oct 3 '18 at 8:15
  • $\begingroup$ OK, Consider that D1, D2, D3, D4 are in a box a light year away. Now the idler beam has two beams, red a green, which is parallel and separated by a distance. Now if in that box, some people decides to use the distance as measurement, instead of the prisms. So that guy can be sure whether the green or the blue beam got a detection. He then can tell which slit the particle passed through. However here on earth we cannot know in advance whether he measures two beams separately or used the D1~D4 prisms to erase information. $\endgroup$ – seilgu Oct 3 '18 at 8:17
  • $\begingroup$ If you use this experiment as example, there will be no interference pattern on the screen, for both cases. However what happens when you use a double slit experiment which there are interference patterns, which disappears if which-way is measured? $\endgroup$ – seilgu Oct 3 '18 at 8:22
  • $\begingroup$ That's already addressed in this answer (and explained in depth in Wikipedia). Good day! $\endgroup$ – Emilio Pisanty Oct 3 '18 at 8:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.