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grav
2009-Nov-07, 07:36 PM
Here (http://www.youtube.com/watch?v=_OWQildwjKQ&feature=related) and here (http://www.youtube.com/watch?v=QBOaXcG3sJ0&feature=related) are a couple of consecutive videos for a lecture about the double slit experiment. It describes an experiment where the particles are sent through the slits one at a time and which slit each passes through is detected. The next day, the results are examined to determine which slits they passed through. When the screen is then looked at, there are only two piles for where the particles struck. However, if the results are not examined but erased instead and then the screen is looked at, the screen shows an interference pattern.

How is this possible? I thought Relativity was strange but it is nothing compared to this. The lecture doesn't tell, but if the results are saved but the screen is looked at first, does it still show an interference pattern even though the results are examined afterward? Wouldn't the results be the same in either case anyway, that about half the particles went through each slit, so the results really wouldn't make a difference anyway, whether it was observed before or after the screen is observed?

I don't believe this description is completely accurate. It can't be. Does anyone have some precise details about how the experiment is performed? Does anyone have a good link or video of the actual experiment being performed?

speedfreek
2009-Nov-07, 07:54 PM
The next day, the results are examined to determine which slits they passed through. When the screen is then looked at, there are only two piles for where the particles struck. However, if the results are not examined but erased instead and then the screen is looked at, the screen shows an interference pattern.

How is this possible?

It is not possible, as described. The experiment is known as the delayed choice quantum eraser and the "lecturer" in those videos is grossly misrepresenting both the experimental set up and the results.


I don't believe this description is completely accurate. It can't be. Does anyone have some precise details about how the experiment is performed? Does anyone have a good link or video of the actual experiment being performed?

http://strangepaths.com/the-quantum-eraser-experiment/2007/03/20/en/

http://arxiv.org/abs/quant-ph/9903047

http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

grav
2009-Nov-07, 09:23 PM
Thanks, speedfreek. Looks like there is more to the quantum erasure experiment than was being described in the lecture. I have read through the first and last links, but I'm still not sure what is being measured. In the diagram in the Wiki link, what are they looking at with the detectors to compare results and what are they doing to change the outcome of the experiment?

grav
2009-Nov-07, 10:13 PM
Let's see if I've got this straight. Detector d0 is at a screen and the detector can be moved along that screen to various positions to detect whether signal photons strike at those positions. Whenever an idler photon strikes detector d3 or d4, where the slit that the entangled photons passed through would be known, the signal photon can be detected anywhere upon the screen but centered upon a single central position with lesser probability of incidence further out along the screen in both directions. Whenever an idler photon strikes d1 or d2, where the slit that the entangled photons passed through would not be known because the paths are brought back together, the signal photon strikes the screen at d0 in an interference pattern, more signal photons being incident to the positions where the crests of the pattern would be and less where it the troughs would be.

Also, depending upon whether the idler photon strikes d1 or d2, more signal photons will strike the screen at the crests for d1 and the troughs of d1 become the crests of d2, so the interference pattern again becomes a single bright area if counted for both detectors since the positions of the signal photons for their idler photons that strike d1 and d2 even out. The quantum information part of that would be that the splitting of the idler photons occur after the signal photons have already struck the screen, so it is as if the signal photons knew beforehand which path the idler photons would take in order to strike d1 and d2 and produce an interference pattern on the screen at d0, or d3 and d4 and produce no pattern on the screen. Is all of this correct?

Ken G
2009-Nov-07, 10:29 PM
Yes, that's all correct, except the "as if the signal photons knew beforehand" is oversold. That's the point that everyone makes great hay over, but it's actually just imprecise use of language. The key thing to remember is that the signal photons never do anything that appears the least bit strange to any observer who is not privy to the idler photon data. If the signal photons really "knew beforehand", wouldn't their behavior be apparent in some way that they possess this "knowledge"? But it isn't, their behavior is completely normal if you don't know what the idler photons do. This is quite fortunate for being able to do quantum mechanics-- in most quantum mechanics experiments, the rest of the universe acts like an "idler photon", to some degree, but we can still do quantum mechanics on the photons we have in our experiments. Note that what the signal photons do in a sense "know beforehand" is the setup of their piece of the experiment-- they know that an idler photon is being split apart on some other path, and they know that some piece of them in a sense goes with that idler photon-- the piece that has to do with correlations between quantum systems. But they don't "know beforehand" what other apparatus that idler photon is going to encounter, and anything that happens that involves that other apparatus is going to have to involve correlations with objects that interact with that other apparatus.

In other words, the "weird" behavior appears only when you correlate the data between the signal photons and the idler photons. Then we get the strangeness that is quantum mechanics-- quantum mechanics is above all a theory of correlations, often correlations that we don't even recognize the existence of in our macroscopic lives.

grav
2009-Nov-07, 11:33 PM
Yes, that's all correct, except the "as if the signal photons knew beforehand" is oversold. That's the point that everyone makes great hay over, but it's actually just imprecise use of language. The key thing to remember is that the signal photons never do anything that appears the least bit strange to any observer who is not privy to the idler photon data. If the signal photons really "knew beforehand", wouldn't their behavior be apparent in some way that they possess this "knowledge"? But it isn't, their behavior is completely normal if you don't know what the idler photons do. This is quite fortunate for being able to do quantum mechanics-- in most quantum mechanics experiments, the rest of the universe acts like an "idler photon", to some degree, but we can still do quantum mechanics on the photons we have in our experiments. Note that what the signal photons do in a sense "know beforehand" is the setup of their piece of the experiment-- they know that an idler photon is being split apart on some other path, and they know that some piece of them in a sense goes with that idler photon-- the piece that has to do with correlations between quantum systems. But they don't "know beforehand" what other apparatus that idler photon is going to encounter, and anything that happens that involves that other apparatus is going to have to involve correlations with objects that interact with that other apparatus.

In other words, the "weird" behavior appears only when you correlate the data between the signal photons and the idler photons. Then we get the strangeness that is quantum mechanics-- quantum mechanics is above all a theory of correlations, often correlations that we don't even recognize the existence of in our macroscopic lives.Okay, great. So from what I can see so far, nothing can be gained by studying d0 first, because we wouldn't be able to tell from which slit a particular photon strike came from or what the probability would be for it to strike at any particular location on the screen in that case. If we study d1 first, however, we can tell the probability of signal strike locations on the screen and will then find them distributed that way when we then look at only those signal photons that are entangled with idler photon strikes at d1. Same thing with d2 but with the opposite crests and troughs. We can also look at the idler photon strikes at d3 and d4 also and find that their associated entangled signal photons are distributed across the screen in a single wide diminishing crest. Is that right?

speedfreek
2009-Nov-07, 11:40 PM
Yes, that's right. :)

Ken G
2009-Nov-07, 11:58 PM
I'm good with it. :)

grav
2009-Nov-08, 12:01 AM
Yes, that's right. :)


I'm good with it. :)
Okay, cool. Thanks again, guys. :) I suppose the "strangeness" of the entangled particles, then, is simply that the idler photons that strike at the detectors whose paths are indeterminate are the only ones whose entangled signal photons produce an interference pattern, even though the paths of the signal photons are independent of the idler photons once they separate. Looking at the diagram for the setup, then, I'm seeing something pretty obvious. I know I'm only one day into this, but I want to see if it is plausible or if it has been tested.

I notice that the paths for the idler photons that strike at d3 and d4 and therefore can be determined from which slit they came from have been reflected off of the splitters, while those that strike at d1 and d2 have passed through the splitters. Since the ones that pass through the splitters are the only ones whose entangled counterpart produce an interference pattern, I'm thinking there must be something special about this. If I'm not mistaken, the polarization of the light can determine whether it passes through certain materials, so I'm thinking that only the idler photons which are polarized a certain way will pass through the splitters and reach d1 or d2. Their counterpart signal photons will also be polarized in a certain way that coincides with the way that the idler photons are polarized, and this determiines whether or not they will produce an interference pattern on the screen. The idler photons that are not polarized in the right way to pass through the splitters will strike d3 and d4 and the entangled signal photons will also not be polarized in the right way to produce an interference pattern. I'm not sure about the details of such a polarization scheme, but we could possibly test the idea by reversing the paths of one of the splitters such that the idler photons that pass through the splitter are the ones that strike d3. Does this sound plausible? Has it already been tested?

grav
2009-Nov-08, 12:49 AM
Okay, let's say we add the interference patterns of d1 and d2 as shown in the middle of the first link (http://strangepaths.com/the-quantum-eraser-experiment/2007/03/20/en/) together. That will then produce the same overall single area that either of the other two detectors shown above them produce, right? Here's my question. Will the intensity of that illuminated area equal the intensity of either of the other two or will it be twice as bright? It would seem it should be twice as bright since it contains half the total number of photons while the other two contain one quarter each, but I'm not completely sure since interference is taking place, but I'm pretty sure it would have twice the intensity or equal to the other two added together.

Ken G
2009-Nov-08, 02:38 AM
The idler photons that are not polarized in the right way to pass through the splitters will strike d3 and d4 and the entangled signal photons will also not be polarized in the right way to produce an interference pattern. I'm not sure about the details of such a polarization scheme, but we could possibly test the idea by reversing the paths of one of the splitters such that the idler photons that pass through the splitter are the ones that strike d3. Does this sound plausible? Has it already been tested?I don't know if it's been tested, but the polarization is not particularly special, even unpolarized particles (particles with zero spin) should show this effect.

Ken G
2009-Nov-08, 02:40 AM
Here's my question. Will the intensity of that illuminated area equal the intensity of either of the other two or will it be twice as bright? You can send photons through one at a time, and the pattern you get must account for all the photons. The screen is not measuring "brightness" in that case, it is measuring photon counts. If you use lots of photons at once, brightness is the same thing as photon counts.

grav
2009-Nov-08, 03:49 AM
You can send photons through one at a time, and the pattern you get must account for all the photons. The screen is not measuring "brightness" in that case, it is measuring photon counts. If you use lots of photons at once, brightness is the same thing as photon counts.Thanks, Ken. Okay, here's another question. This link (http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslid.html) shows diagrams for a comparison of the single slit and double slit experiments. Now, when only one slit is open, the interference is that of the dotted line seen in the double slit diagram. But when both slits are open, the amplitudes for that diagram contain more troughs. If the amplitudes are indicative of the photon number count or a particle count, then if two slits are open, twice as many particles should make it through the slits, making twice the total areas below the lines of the crests and troughs in the double slit diagram. But the double slit diagram looks to cover only about half the area below the wave line instead, indicating half the number count of particles than that for the single slit wave. How come?

grav
2009-Nov-08, 04:32 AM
This interference stuff is getting very interesting. Consider this diagram (http://en.wikipedia.org/wiki/Mach-Zehnder_interferometer) of an M-M type setup. Apparently, detector 2 always gains destructive interference of the beams. Does that mean that if the photons are sent through one at a time, they will never be detected at detector 2, so always at detector 1, thereby determining that even given the arbitrary choice of paths taken by each single photon, the path chosen will always be the same, that of detector 1 where every individual photon will always be detected to have struck?

grav
2009-Nov-08, 05:02 AM
Wow, this (http://en.wikipedia.org/wiki/Elitzur-Vaidman_bomb-testing_problem) just keeps getting more and more interesting.

Ken G
2009-Nov-08, 06:46 AM
Thanks, Ken. Okay, here's another question. This link (http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslid.html) shows diagrams for a comparison of the single slit and double slit experiments. Now, when only one slit is open, the interference is that of the dotted line seen in the double slit diagram. But when both slits are open, the amplitudes for that diagram contain more troughs. If the amplitudes are indicative of the photon number count or a particle count, then if two slits are open, twice as many particles should make it through the slits, making twice the total areas below the lines of the crests and troughs in the double slit diagram. But the double slit diagram looks to cover only about half the area below the wave line instead, indicating half the number count of particles than that for the single slit wave. How come?The figure must not be normalized, I think it's just showing the pattern for comparison.

grav
2009-Nov-08, 07:15 AM
The figure must not be normalized, I think it's just showing the pattern for comparison.So by normalized, do you mean that if the amplitude pattern shown in that way in the diagram gives half the total number count, then the true pattern would be about the same for the location of the crests and the troughs but have four times the amplitude across all fronts in order to give twice the total number count with two slits?

speedfreek
2009-Nov-08, 11:30 AM
You earlier linked to http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslid.html

But did you look at the grating intensity comparisons? :)

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/gratint.html

Ken G
2009-Nov-08, 01:10 PM
So by normalized, do you mean that if the amplitude pattern shown in that way in the diagram gives half the total number count, then the true pattern would be about the same for the location of the crests and the troughs but have four times the amplitude across all fronts in order to give twice the total number count with two slits?Yes, I would expect that's true.

grav
2009-Nov-08, 01:18 PM
You earlier linked to http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/mulslid.html

But did you look at the grating intensity comparisons? :)

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/gratint.htmlAh, yes. It was right there, wasn't it? :) Looks like the intensities for the two slit would indeed be four times greater than those shown for the one slit in the comparison. Thanks.

WayneFrancis
2009-Nov-09, 01:42 AM
Nice Thread, Thanks for Gav for asking the question and Ken G for giving great answers.

Ken G
2009-Nov-09, 06:08 AM
Wow, this (http://en.wikipedia.org/wiki/Elitzur-Vaidman_bomb-testing_problem) just keeps getting more and more interesting.Yes, that's quite cute, I hadn't seen that before. Note that the article uses the "Many worlds" picture to describe one way of imagining what is happening there, but a far more philosophically neutral approach is simply to track the correlations involved in the action of the apparatus. Many worlds is a way to understand correlations by associating with them additional worlds, but one can also simply talk in terms of the existence of correlations, period. I'm always left to wonder, if we have a theory about correlations, why do we need to pretend it is a theory about something other than correlations? I suppose you can call me an ontological minimalist.

grav
2009-Nov-18, 03:15 PM
Yes, that's quite cute, I hadn't seen that before. Note that the article uses the "Many worlds" picture to describe one way of imagining what is happening there, but a far more philosophically neutral approach is simply to track the correlations involved in the action of the apparatus. Many worlds is a way to understand correlations by associating with them additional worlds, but one can also simply talk in terms of the existence of correlations, period. I'm always left to wonder, if we have a theory about correlations, why do we need to pretend it is a theory about something other than correlations? I suppose you can call me an ontological minimalist.Now I'm wondering how that would affect the M-M experiment if according to quantum mechanics, when the apparatus is set up in a particular way, the interference along one path will always be null anyway.

I'm also thinking that what we consider to be interference in the slit experiments might not be interference at all, but more related to the "particle in a box" (http://en.wikipedia.org/wiki/Particle_in_a_box) phenomenon. I have been trying to plot "interference" graphs using a computer simulation, but based upon the particle in a box and trying to match the interference pattern accordingly, although so far haven't succeeded.

Ken G
2009-Nov-18, 03:44 PM
Now I'm wondering how that would affect the M-M experiment if according to quantum mechanics, when the apparatus is set up in a particular way, the interference along one path will always be null anyway.M-M is well predicted by either Maxwell's equations or quantum mechanics-- there's no difference when dealing with bright light levels (lots of quanta). That's called the "correspondence principle." There's also a relativistic version of quantum mechanics if you want to know what one quantum in the M-M experiment does as seen from relativistically moving reference frames.


I'm also thinking that what we consider to be interference in the slit experiments might not be interference at all, but more related to the "particle in a box" (http://en.wikipedia.org/wiki/Particle_in_a_box) phenomenon. I have been trying to plot "interference" graphs using a computer simulation, but based upon the particle in a box and trying to match the interference pattern accordingly, although so far haven't succeeded.It's considered to be interference either way, as there's a direct connection between the amplitude of a classical field and the wave function of a quantum of that field-- again the correspondence principle.

grav
2009-Nov-18, 04:08 PM
M-M is well predicted by either Maxwell's equations or quantum mechanics-- there's no difference when dealing with bright light levels (lots of quanta). That's called the "correspondence principle." There's also a relativistic version of quantum mechanics if you want to know what one quantum in the M-M experiment does as seen from relativistically moving reference frames.Yes, I was thinking the relativistic effects would still need to take place as well, since the distance light travels along paths of equal length according to the stationary observer would still be different with a relative speed, but if a single photon were to be emitted at a time, they would still all take the one path over the other, so its not quite the same as a pure interference acting between the paths with two steadily emitted beams.

It's considered to be interference either way, as there's a direct connection between the amplitude of a classical field and the wave function of a quantum of that field-- again the correspondence principle.The word "interference" brings to mind the phase difference between two steadily emitted beams or across two slits to me, particles from one beam interfering with the paths of the other, but the same thing occurs with particles emitted one at a time. With the particle in a box, the wave function is meaningless except to predict the probability for the position of the single particle. Classically the particle can be anywhere equally but quantum mechanically the particle has certain positions it is more likely to be than others. Something similar could be happening with the distribution of the photons on the screen, more likely to show up some places over others, whether emitted one at a time or steadily.

Ken G
2009-Nov-18, 09:55 PM
Yes, I was thinking the relativistic effects would still need to take place as well, since the distance light travels along paths of equal length according to the stationary observer would still be different with a relative speed, but if a single photon were to be emitted at a time, they would still all take the one path over the other, so its not quite the same as a pure interference acting between the paths with two steadily emitted beams. I don't understand, if all the individual quanta would all take the same path, so would the amplitude of the classical electromagnetic wave. There's nothing quantum mechanical about the type of interference they are using in that setup, the quantum piece is just how it allows you to test something without exposing it to any light (since the quantum is an "either/or" phenomenon, while classical amplitides are a "little here, little there" phenomenon). But if the probability of the quantum is zero, then the classical amplitude is also zero, so that's not unique to quantum mechanics.


The word "interference" brings to mind the phase difference between two steadily emitted beams or across two slits to me, particles from one beam interfering with the paths of the other, but the same thing occurs with particles emitted one at a time. With the particle in a box, the wave function is meaningless except to predict the probability for the position of the single particle. That's just how the correspondence principle works: what is classically a wave amplitude, which can interfere, is quantum mechanically a probability amplitude, which can also interfere in exactly the same way. The sole difference is the event that is being predicted: classically, you are predicting the intensity of a light beam, quantum mechanically, you are predicting the probability of detection of a quantum. Anything that does not fundamentally rely on the difference between an intensity and a quantum does not distinguish classical light from quantum mechanical light.


Classically the particle can be anywhere equally but quantum mechanically the particle has certain positions it is more likely to be than others. Classically there is no particle-- light is an electromagnetic field only.


Something similar could be happening with the distribution of the photons on the screen, more likely to show up some places over others, whether emitted one at a time or steadily.As soon as you are talking about a distribution of a large number of photons, there is no difference between the classical treatment and the quantum treatment, because an intensity is the same thing as a distribution of quanta.