Delayed-choice double slit experiment

[etamorphmagus]

And of course, in the entangled particles experiments - the delayed quantum erasers, you could wait well after one was "detected", according to wiki. Is this so, in the "delayed choice quantum eraser"? I don't understand it fully, but it's something like that right?

 

[Jamma]

I repeat for emphasis: If we change the apparatus at any time during the experiment, the results obtained correspond to the experimental configuration in place at the moment the experiment is closed, i.e. the instant when the particle is detected. We can wait until the last possible moment to make changes. It doesn't matter. Only the final experimental configuration matters.
Best wishes

I'm sorry, I'm either misinterpretting you, or this just can't be right. So if you fired a particle through a slit 1m from the source, and the detector was 10 light years away, you could technically just block off the slit just before the particle "hits" the detector screen, and it will appear back at the wall because in that configuration, particles wouldn't be able to get through? This is obviously ridiculous.

Also, just because the particles travel at less than the SoL, that doesn't mean that you can't come up with some way of doing FTL communication. For example; you could fire the particles through at a constant rate and agree to measure (or not measure) a particle at each time. The sender could put inplace a polariser, or brick wall even, according to what you are saying, and this would effect the particle which was just about to hit the screen. This is clearly FTL communication if we do things on a large enough scale.

 

[DrChinese]

An actual delayed choice experiment with slits and polarizers is Walborn, et al, Phys Rev A 033818 (2002).

Here is the link:

http://arxiv.org/abs/quant-ph/0106078

"We report a quantum eraser experiment which actually uses a Young double-slit to create interference. The experiment can be considered an optical analogy of an experiment proposed by Scully, Englert and Walther. One photon of an entangled pair is incident on a Young double-slit of appropriate dimensions to create an interference pattern in a distant detection region. Quarter-wave plates, oriented so that their fast axes are orthogonal, are placed in front of each slit to serve as which-path markers. The quarter-wave plates mark the polarization of the interfering photon and thus destroy the interference pattern. To recover interference, we measure the polarization of the other entangled photon. In addition, we perform the experiment under delayed erasure circumstances. "


On the other hand, this is not the same thing as saying what happens at the slits matters much AFTER the photon has passed by. Because that is not part of the context, it doesn't matter.

 

[DrChinese]

I'm sorry, I'm either misinterpretting you, or this just can't be right. So if you fired a particle through a slit 1m from the source, and the detector was 10 light years away, you could technically just block off the slit just before the particle "hits" the detector screen, and it will appear back at the wall because in that configuration, particles wouldn't be able to get through?

I think there are different versions of the time element being discussed. There is not a single point in time in which context is relevant. There are many different points in space time to consider. After a photon has passed the double slit, what happens at the slit doesn't change the outcome. But the words before and after are somewhat misleading, as a context 10 light years away CAN be relevant to something "here and now". It depends.

 

[etamorphmagus]

I think there are different versions of the time element being discussed. There is not a single point in time in which context is relevant. There are many different points in space time to consider.

Which points?

After a photon has passed the double slit, what happens at the slit doesn't change the outcome. But the words before and after are somewhat misleading, as a context 10 light years away CAN be relevant to something "here and now". It depends.

To what?

This?

"We report a quantum eraser experiment which actually uses a Young double-slit to create interference. The experiment can be considered an optical analogy of an experiment proposed by Scully, Englert and Walther. One photon of an entangled pair is incident on a Young double-slit of appropriate dimensions to create an interference pattern in a distant detection region. Quarter-wave plates, oriented so that their fast axes are orthogonal, are placed in front of each slit to serve as which-path markers. The quarter-wave plates mark the polarization of the interfering photon and thus destroy the interference pattern. To recover interference, we measure the polarization of the other entangled photon. In addition, we perform the experiment under delayed erasure circumstances. "

 

[zonde]

An actual delayed choice experiment with slits and polarizers is Walborn, et al, Phys Rev A 033818 (2002). A Google search will yield many other articles on actual experiments.

There are no changes in setup that are made on the fly as experiment is performed.

There is no faster than light communication here. The particles, i.e. the things moving through space-time, always travel at speeds less than the speed of light.

Yes there are no FTL in those experiments so I do not understand why you make statements that imply FTL.
If two places, one where we change the setup and the other place where photon is detected, are spatially separated and our change at one place can be observed as certain difference in observed results at other place without delay we could make Bell telephone.

The point is this: Bohr said we could wait until the very last minute to make a change. He was right! Modern delayed choice experiments confirm this.

And he means delay along the trajectory of photon. Not something like you imply with your statements.

I repeat for emphasis: If we change the apparatus at any time during the experiment, the results obtained correspond to the experimental configuration in place at the moment the experiment is closed, i.e. the instant when the particle is detected. We can wait until the last possible moment to make changes. It doesn't matter. Only the final experimental configuration matters.
Best wishes

I have a question - in what reference frame we should determine this "experimental configuration in place at the moment the experiment is closed"?
Because if we have made any changes to setup that are outside past and future light cones of detection event then this "configuration at the moment of detection" will depend from chosen reference frame.

 

[DrChinese]

Which points?

All spacetime points along every possible path (or history). So a lot of points contribute to the context!

 

[etamorphmagus]

All spacetime points along every possible path (or history). So a lot of points contribute to the context!

Sure, but what points are the critical ones? You leave a lot of room for questions with your answers, sorry if this is frustrating for you.

And what do you mean by context?

 

[DrChinese]

Sure, but what points are the critical ones? You leave a lot of room for questions with your answers, sorry if this is frustrating for you.

And what do you mean by context?

All would conceptually be equally relevant in a sense. But you could consider the end point of each possible path as making a "more relevant" contribution. (Although you can't exactly it an end point in the strictest sense.)

It may be easiest to picture the context from the position of the source, and look into the future. What could you "see" if you could look towards the future?