Implications of Quantum Eraser, Delayed Choice, etc.

[DrChinese]

But the coincidence count rates depends on path length difference! Look at figures 4-7, the coincidence count rate is in fact an interference pattern on its own simply based on path-length difference. I'm sure I do not need to remind you that a coincidence count involves BOTH A and B arms of the experiment.

Sure, but the interferometer and its sensitivity are a function of a portion of one arm only. I believe the adjustment of path length is only on the A side and has nothing at all to do with the time window being used for the coincidence matching. In other words, the path length adjustment has no discernible impart on the arrival time at the detectors.

The article does not appear to state, but I believe the usual coincidence window is on the order of nanoseconds. That would imply a distance on the order of 1 meter. But perhaps I am incorrect in my reading of the setup.

 

[StevieTNZ]

This is your unreviewed personal opinion. The article cited above is a published peer-reviewed article. It is obvious here who is out of line.

You do realize the peer-review process has its problems, don't you? If not, do a Google search.

 

[harrylin]

The behavior of the particle at the slits seemingly depends on what the photon encounters after the particle has passed through the slit(s).

"Seemingly" is rather appropriate I think.

A discussion of this is also in Wikipedia:
http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

Regretfully it's too long ago that I looked into this, and not me but my colleague who left understood it very well... Still I'll give here my 2cts:

Does this prove that:
1. Particles can go back in time and do stuff?

Certainly not (that was indeed my main question at the time).

[..]
3. Does this experiment prove that particles are not really particles but just related to the wavy field that can form interference?

Such interference experiments support wave/field theory.

4. Does this prove that particles don't travel in between but only the detection event is important?

I don't think so...

5. What's the explanation why if which-path information is suppressed, intereference form, does this has to do with decoherence? [..]

We think that it has to do with selection (filtering) of information. Depending on the way you manipulate the information, you may obtain a visible interference pattern or not. It's also possible to do this partially.

Regards,
Harald

 

[unusualname]

Unfortunately you are uninformed about this issue. I will welcome you to look at the experiment described in PHYSICAL REVIEW A 71, 052103(2005) [http://people.whitman.edu/~beckmk/QM/qe/qe.pdf] . Specifically look at figure 4 where they show how the number of coincidence counts between arms A and B vary as the pathlength difference between the two arms is changed. Also note that the figure shows changes within 4 microns and it takes less than half a micron difference to drop the coincidence counts from maximum to minimum.

I've been busy but took a quick look at the paper (following the correct link posted by DrChinese) and saw it was not talking about the Walborn experiment under discussion, so your weird idea that quarter wave plates can sufficiently disturb the flight time of photons within the sensitivity of modern coincidence counters is not at all supported by the paper you (tried to) link to.

It seems to show the expected oscillation you would expect from standard QM considerations if you change the interferometer pathlengths. Whether this can also have a classical explanation isn't of much interest to me.

 

[DrChinese]

Referring to a post by billschnieder:

...so your weird idea that quarter wave plates can sufficiently disturb the flight time of photons within the sensitivity of modern coincidence counters is not at all supported by the paper you (tried to) link to.

I'm not accusing billschnieder of any particular position. I have noted that a number of Local Realists have hypothesized that photons are delayed, absorbed, or otherwise transformed by wave plates, beam splitters, or other elements of apparatus in such a way that they are not counted. This is intended to explain, in some manner, the violation of the Bell Inequality (which, if upheld, then rules out Local Realism).

The funny part about this is that such effects have no basis in quantum theory. Which means these are new - and presumably discoverable - physical effects. In fact, there must be dozens (if not hundreds) of such new & previously unknown effects if their idea is correct! That is because there are dozens of entirely different mechanisms for observing violations of Bell Inequalities!

In the modern post-Aspect world of Bell tests, standard quantum theory has been used to devise many different ways to observe entanglement. Different types of particles, different bases of entanglement, different numbers of particles, different sources, etc. as well as new tests such as GHZ, Hardy etc. All tests say the same thing, which is LR is ruled out. So far, the score is something like QM 1000, LR 0.

But the LR crew is mostly still stuck on debunking the traditional type. (The exception is the LR computer simulations of De Raedt et al, which at least have the benefit of applying to more that just one type of entanglement. Of course, these simulations have other serious issues which I will not go into here for fear of getting even further off track.)

 

[San K]

Would it be correct to say that the delayed-choice quantum eraser experiments don't actually change the past, as the particle going through either one or both slits (before we choose to find out which-way info, or 'erase') never existed until we 'erase' or find out which-way info?

the DCQE - does not change the past...i agree/think

however remember one of the photons has stuck the detector before the other has arrived...so the one that is detected has already registered its position