Delayed choice quantum eraser

  • Thread starter Spacezilla
  • Start date
  • #1

Main Question or Discussion Point

http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser" [Broken]: The results from Kim, et al. have shown that, in fact, observing the second photon's path will determine the particle or wavelike behavior of the first photon at the detector, even if the second photon is not observed until after the first photon arrives at the detector. In other words, the delayed choice to observe or not observe the second photon will change the outcome of an event in the past.
Question: What would happen if this was delayed even further? We are talking nanoseconds here, but what if the particle was slowed or sent far away, before being reflected back? What I'm getting at is... What if the delay wasn't nanoseconds, but seconds? What if it was minutes? What if it was enough time to manually change whether the second photon was observed or not?

The results from Kim, et al. have shown that, in fact, observing the second photon's path will determine the particle or wavelike behavior of the first photon at the detector, even if the second photon is not observed until after the first photon arrives at the detector.
In other words, the photon behaves one way if we observe the second photon nanoseconds later and another way if we don't. So we start the experiment and we observe our detector. Depending on whether we see an interference pattern or not, we observe each secondary photon or we don't and that's the point.

I'm been thinking about this for a while and I can't figure out what would happen. I searched Google and found http://uplink.space.com/showflat.php?Board=sciastro&Number=141733 where someone else asks the same question, but unfortunately no one really seems to understand the experiment, so he gets no real answer.

Now, I turn on my photon generator.. and what do I see? Either a "particle" pattern or an interference pattern, one would presume.

Well, according to the results of the already-performed original "Delayed Choice Quantum Eraser" experiment, what I see depends on what the state of the 'which-path/no which-path' switch will be 10 minutes from now when the idler photon hits it?

What if I see an interference pattern, so then when I see that, I make sure the switch is on "which-path" no matter what... which will clearly violate the results of the previous expirement. I can deliberately violate the rules of what I see on the detector, by switching the switch to what WOULD give me the opposite result, using my own free will.

What is the deal here, and what would happen in this case?
This is exactly what I'm asking too. Now, depending on how you choose to interpret the delayed choice quantum eraser experiment, there's no getting around that we're either changing the past or predicting the future. So what happens if we delay it long enough to change whether we'll detect or erase the information about the second photon's after observing the results?

This is not just a theoretical question, this is something that should be possible to test for someone who has access to this kind of equipment, but unfortunately I don't. However, many people have in the past predicted the outcomes of quantum experiments correctly, so I hope someone here can predict the outcome of this experiment as well.

Thank you for your time. :)
 
Last edited by a moderator:

Answers and Replies

  • #2
JesseM
Science Advisor
8,496
12
Referring to the first set of photons as "signal photons" and the second set as "idler photons", the way it works is that the total pattern of signal photons actually never shows interference--even if you measure all the idlers in such a way that the which-path information is erased, you will see interference if you do a "coincidence count" between signal photons and idlers which went to a certain detector, but if you add all the subsets, they add up to a non-interference pattern. So, it's impossible to find an interference pattern until after the idlers have already been measured, ruling out the possibility of any backwards-in-time hijinx. This was discussed on this thread a while ago. And here's my summary from this thread:
Even in the case of the normal delayed choice quantum eraser setup where the which-path information is erased, the total pattern of photons on the screen does not show any interference, it's only when you look at the subset of signal photons matched with idler photons that ended up in a particular detector that you see an interference pattern. For reference, look at the diagram of the setup in fig. 1 of this paper:

http://xxx.lanl.gov/PS_cache/quant-ph/pdf/9903/9903047.pdf

In this figure, pairs of entangled photons are emitted by one of two atoms at different positions, A and B. The signal photons move to the right on the diagram, and are detected at D0--you can think of the two atoms as corresponding to the two slits in the double-slit experiment, while D0 corresponds to the screen. Meanwhile, the idler photons move to the left on the diagram. If the idler is detected at D3, then you know that it came from atom A, and thus that the signal photon came from there also; so when you look at the subset of trials where the idler was detected at D3, you will not see any interference in the distribution of positions where the signal photon was detected at D0, just as you see no interference on the screen in the double-slit experiment when you measure which slit the particle went through. Likewise, if the idler is detected at D4, then you know both it and the signal photon came from atom B, and you won't see any interference in the signal photon's distribution. But if the idler is detected at either D1 or D2, then this is equally consistent with a path where it came from atom A and was reflected by the beam-splitter BSA or a path where it came from atom B and was reflected from beam-splitter BSB, thus you have no information about which atom the signal photon came from and will get interference in the signal photon's distribution, just like in the double-slit experiment when you don't measure which slit the particle came through. Note that if you removed the beam-splitters BSA and BSB you could guarantee that the idler would be detected at D3 or D4 and thus that the path of the signal photon would be known; likewise, if you replaced the beam-splitters BSA and BSB with mirrors, then you could guarantee that the idler would be detected at D1 or D2 and thus that the path of the signal photon would be unknown. By making the distances large enough you could even choose whether to make sure the idlers go to D3&D4 or to go to D1&D2 after you have already observed the position that the signal photon was detected, so in this sense you have the choice whether or not to retroactively "erase" your opportunity to know which atom the signal photon came from, after the signal photon's position has already been detected.

This confused me for a while since it seemed like this would imply your later choice determines whether or not you observe interference in the signal photons earlier, until I got into a discussion about it online and someone showed me the "trick". In the same paper, look at the graphs in Fig. 3 and Fig. 4, Fig. 3 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D1, and Fig. 4 showing the interference pattern in the signal photons in the subset of cases where the idler was detected at D2 (the two cases where the idler's 'which-path' information is lost). They do both show interference, but if you line the graphs up you see that the peaks of one interference pattern line up with the troughs of the other--so the "trick" here is that if you add the two patterns together, you get a non-interference pattern just like if the idlers had ended up at D3 or D4. This means that even if you did replace the beam-splitters BSA and BSB with mirrors, guaranteeing that the idlers would always be detected at D1 or D2 and that their which-path information would always be erased, you still wouldn't see any interference in the total pattern of the signal photons; only after the idlers have been detected at D1 or D2, and you look at the subset of signal photons whose corresponding idlers were detected at one or the other, do you see any kind of interference.
 
Last edited by a moderator:
  • #4
Demystifier
Science Advisor
Insights Author
10,801
3,502
For those who have not read the paper above, here is a plain-English summary:
The delayed-choice experiment looks paradoxical if you think that the photon passes either through both slits or through one slit only. The paradox removes if you assume that something (the wave) passes through both slits, while something different (the particle) passes through one slit only. But then you need to assume that the photon consists of two separate things, which contradicts the standard interpretation of QM. Fortunately, there is an interpretation - the Bohmian interpretation - that provides such a wave-and-particle picture consistently.
 
  • #5
Thank you for your comments, I greatly appreciate it.
 
  • #6
vanesch
Staff Emeritus
Science Advisor
Gold Member
5,028
16
It might be interesting to do a search in this forum on delayed choice quantum eraser. Many threads have discussed this in the past here.

BTW, although Bohmian mechanics can give an explanation for the phenomenon, every self-consistent interpretation of quantum theory can do so.
 
  • #7
Demystifier
Science Advisor
Insights Author
10,801
3,502
BTW, although Bohmian mechanics can give an explanation for the phenomenon, every self-consistent interpretation of quantum theory can do so.
I agree. But why then this phenomenon is regarded as a big deal, even by experts?
 
  • #8
vanesch
Staff Emeritus
Science Advisor
Gold Member
5,028
16
I agree. But why then this phenomenon is regarded as a big deal, even by experts?
I already pointed out a few times that many experimental papers on this issue, which are *experimentally* very rich, are very misleading/poorly written on the interpretation side.

I guess that both Bohmians and MWI-ers don't find it a big deal (although nevertheless quite fun). The "projectionists" are of course a bit puzzled because they don't know anymore when to project :smile:
(although, if you really understand von Neumann, you wouldn't be puzzled either).
 
  • #9
Simplification

I too have seen this alot and have found a one-line version of the question:

What would you see if you looked at the detector in a DCQE before the choice was made?



Now as for the implementation, I see Spacezilla's point also that the implementation would seem at least as straightforward as the original experiment.
I would imagine you would use a Bose-Einstein condensate to considerably slow the signal photons. While you replaced the mirrors with a manual (manually controlled electronic) switch which merged the signal photon path from slit B into the path from slit A.
 
  • #10
Understanding Bohmian

Demystifier

So, under the bohmian interpretation of quantum mechanics, every particle is a particle and a wave always.
And that the wave influences the particle based on where the wave is most intense.

But does that mean manipulating the which-way information affects the wave aspect? Otherwise it would always show an interference pattern (over time).
 
  • #11
Which pattern?

Now take a modification of the experiment where many photons are sent to the interferometer and hit the detector so that a pattern could emerge (and those are the idlers?).

While the signal(?) photons are still travelling for such a time that the last idler has hit the detector before the first choice is made on a signal photon.

Then what might you see on the detector if measured prematurely?
 
  • #12
2,006
5
RP, if you look at only the detector, you just see noise. Gaussian. Doesn't matter whether the "choice" has been made "yet" or "not". Read those other threads.
 
  • #13
Gaussian

So if you look at the detector prematurely you see noise?
Because if you always see noise this isn't much of an experiment.
 
  • #14
Signal vs Idler

Sorry, you had them correct JesseM,
The signal is measured by the detector, and the idler is put through the choice.
 
  • #15
2,006
5
RP, the only way not to see noise is by looking at correlations.. which is obviously impossible until after you receive information about the result of the measurement on the other (idler) set of photons.
 
  • #16
Confusion

Ok, now I think I've been confused all this time.

Here's how I thought the measurement was taken (of the signal photons, D0 in the paper):

The signal photons either exhibited a particle or a interference tendency (although only to be seen over meny repetitions).
D0 was on a stepper motor moving left and right to capture the photons in a certain position.
D0 would be moved to the next position after a good number of photons were sent through from the pump through the whole setup.
Then the total number of hits from each position was mapped together with each other position to form a sort of 2D graph from which either an interference pattern or a particle pattern would be seen.

Please correct me if I'm wrong.
 
  • #17
Demystifier
Science Advisor
Insights Author
10,801
3,502
I already pointed out a few times that many experimental papers on this issue, which are *experimentally* very rich, are very misleading/poorly written on the interpretation side.

I guess that both Bohmians and MWI-ers don't find it a big deal (although nevertheless quite fun). The "projectionists" are of course a bit puzzled because they don't know anymore when to project :smile:
(although, if you really understand von Neumann, you wouldn't be puzzled either).
Good points! :approve:
 
  • #18
2,006
5
Please correct me if I'm wrong.
..or you could just carefully read the original journal article yourself. Or those other threads you've been directed to here.

Think of it this way: if it worked the way you were assuming, it would be possible to tell the future or transmit information faster than light (and if there's one thing that physicists certainly would have noticed by now...).
 
  • #19
Kim et al.

RP, the only way not to see noise is by looking at correlations.. which is obviously impossible until after you receive information about the result of the measurement on the other (idler) set of photons.
I thought that the signal photons were passing through a Young's slit type apparatus - in which case shouldn't they usually show an interference pattern overall ??? - in other words why should the interference pattern only appear with the correlations ??
 
  • #20
2,006
5
I thought that the signal photons were passing through a Young's slit type apparatus - in which case shouldn't they usually show an interference pattern overall ???
not if there's some way (in principle) to figure which slit each went through
 
  • #21
Kim et al.

Many thanks for your reply. However, if we choose not to observe which slit the photons went through - then I suppose the photons are free to produce interference patterns. In which case it is surprising to me that the interference patterns produced by two sets of photons cancel to give a Gaussian. Could you tell me in simple language where the phase shift comes from - and why this is not present in the original Young's slit experiment ??
 
  • #22
2,006
5
One way to interpret it is this: in the original Young's slit, once the signal photon's position is measured, there is no way even in principle to determine which slit the photon came from. But in the DCQE, when the signal photon's position is measured, it is still possible in principle to determine (by making a particular measurement of the idler) which slit the photon went through.
 
  • #23
Kim et al

Many thanks again. It appears from further reading that there are many interpretations of quantum mechanics which physicists have difficulty in deciding which one is the more appropriate.
In Kim et al 1999 the two wave components from an idler photon are well separated and presumably pass through D1 and D2, impinge on the beam splitter BS thereby erasing the 'which slit' information - allowing the interference pattern to be seen from the signal photon.
Does this mean that it might be possible to send one of these wave components a very long way and back again - and then to the beam splitter BS - (processing one photon at a time) - if the same results are obtained this would provide experimental evidence that the wave components of a photon are effectively timeless and probably infinitely extended ?
 
  • #24
131
0
.... would provide experimental evidence that the wave components of a photon are effectively timeless and probably infinitely extended ?
I thought this was already implied because it is a wave "function". Yes/no?
 
  • #25
Kim et al.

I thought this was already implied because it is a wave "function". Yes/no?
Implication from a theoretical equation does not negate the positive benefits of getting empirical evidence to support the relevance of the equation - does it ?
 

Related Threads on Delayed choice quantum eraser

Replies
1
Views
2K
  • Last Post
Replies
1
Views
674
  • Last Post
Replies
1
Views
540
  • Last Post
Replies
6
Views
2K
  • Last Post
Replies
3
Views
2K
  • Last Post
Replies
11
Views
2K
  • Last Post
Replies
2
Views
6K
  • Last Post
3
Replies
66
Views
13K
Replies
2
Views
684
Replies
5
Views
2K
Top