Author Topic: Delayed Choice Quantum Eraser  (Read 3872 times)

Offline Lynx Cat

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Delayed Choice Quantum Eraser
« on: July 27, 2012, 06:41:45 PM »
I was just thinking about the Delayed Choice Quantum Eraser experiment (if you're not familiar with it, I suggest you do some reading on it, and on its predecessor the double-slit experiment as well if necessary), and went back to wondering about how/whether we could use it to send information back in time. So I've read through it again, considered the objections explained in Wikipedia (I tried reading some more academic papers commenting on it, but quite frankly they went totally over my layman head), and thought about a possible way to get around to it. It seems so simple that someone with enough physics knowledge has to have considered it before, so I'd just like to know what's wrong with my reasoning, if anything.

Using the Wikipedia image as a reference:



Let's say we set it up so that beam splitter BSc (the one that leads to detectors 1 and 2) can be removed or put back into place at will. I imagine it being lowered into the circuit or raised out of it. So, if we lower it, the experiment progresses as described; however, if we raise it, detector 1 will detect only a "red" photon, while detector 2 will detect only a "blue" one. Therefore, which-path information is retained no matter what - the photon went through the "lower" ("blue") path if detected by D3 or D2, and through the "upper"/"red" path if detected by D4 or D1.

OK. Now, let's say that, instead of a single photon, we send a bunch of photons - not simultaneously, so their path can still be detected (or not), but one after the other in a machine-gun-like sequence. Let's call this sequence a "string". Say the position of BSc is set between strings, so you control how the experiment behaves in each string. That's set up so you have enough photons to form an interference pattern (or not), and you have direct control over how they'll behave.

I guess you can probably see where I'm going. The idea is to deliberately (rather than randomly) determine the idler photon's behavior, so when its quantum-entangled pal, the signal photon, shows a particle-like or wave-like behavior "in the past", that reflects an actual, deliberate message sent "from the future". (For example, say "particle"/no interference is 0, and "wave"/interference is 1, then use that to encode information in a binary format such as ASCII.) If you can make the strings and respective processing times sufficiently short, and the path taken by the idler photon sufficiently long, you may have some sweet retrocausality going on there.

"But Lynx!" you might exclaim. "There's no way to get anything useful out of the signal photon before you get feedback from the idler-side, because the interference patterns in the signal's detector cancel each other out, so you can only see the interference patterns if you know which idler photon corresponds to each signal photon, which requires information getting back from the 'sender' back to the 'receiver' the slow way before you can get at the quantum-entangled info! Otherwise all you'll see is a big blur!" (At least that's what I got from the paragraph titled "Problems with using retrocausality".) Hah! That's where the moving beam-splitter comes to the rescue. First, let's look at the results of the "regular" experiment:



The above compares the total result of detector 0 (the signal detector), which is always a big blur, to the results of detectors 1 through 4. That's boring and not very helpful. The experiment then uses a coincidence counter to correlate each signal photon to its idler brother, and record them separately, so you get to view the results of detector 0 corresponding to each of the four other detectors when you sort them according to the counter, as below:



Thus, with a "lowered" BSc, the unsorted result from the signal photons is a big blur corresponding to the sum of results 1 through 4. However, if you "lift" BSc, detector 1 now shows the same result as 4, and 2 shows the same result as 3 as well. So, adding them all up ends up with a total result that's concentrated in the middle, not spread out as before. The two outcomes (lowered vs. raised BSc) are different even without counter information to sort the photons. So, by controlling the beam splitter's position, the "sender" (the person at the idler end) can actually send information back in time to the "receiver" at the signal end.

So? What do you say about that? Is there anything that will make it not work? I've heard a lot about a "no quantum teleportation" theorem that forbids this sort of information transmission, but I've yet to see a layman-understandable explanation of that, or of anything else that shoots down my proposal. Time travel can't be as simple as installing a hinge on one part of that experiment... right?
Did I do, O CROM, did I as I said Id do? Good! I did.

Offline stephako

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Re: Delayed Choice Quantum Eraser
« Reply #1 on: August 01, 2012, 03:04:25 AM »
I heard a talk once by a Professor who was working on these experiments and he explained it rather nice. A have to find the motivation to search for the notes though...
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Offline bn

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Re: Delayed Choice Quantum Eraser
« Reply #2 on: August 02, 2012, 06:22:39 PM »
hmmmmmmmmm...
i'll consider a show on this type of topic.
i'm having trouble finding the right kind of quantum mechanics people to fill my bench for this type of topic.
quantum information as well.

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Offline Lynx Cat

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Re: Delayed Choice Quantum Eraser
« Reply #3 on: August 03, 2012, 06:22:47 AM »
Wow, cool! Didn't know it was going to get this tricky. (Which is great, of course :D) I hope my post wasn't too confusing or difficult to navigate, I tend to get a bit carried away sometimes. I guess I didn't want to wait for people to raise the more obvious objections and then respond to those objections, so I laid it all down at once.
Did I do, O CROM, did I as I said Id do? Good! I did.

Offline bn

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Re: Delayed Choice Quantum Eraser
« Reply #4 on: August 03, 2012, 06:30:01 AM »
well, it's not just that.
i realized two things as i tried to make sense of it.
first, that it's pretty interesting.
second, that most of the comments i made were liable to be incorrect since this is so far outside my field.
which is pretty much the kind of thing i like to put on my show.

except...

my bench isn't big enough.
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Offline Lynx Cat

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Re: Delayed Choice Quantum Eraser
« Reply #5 on: August 03, 2012, 06:48:06 AM »
Did I do, O CROM, did I as I said Id do? Good! I did.