# Hypothesizing on photon mode of travel in double slit or similar experimental setups

 P: 69 we don't know where the photon would strike, so how would moving the detector help? there is something abotu the experiment that i am missing
 Sci Advisor P: 1,626 This is a genuine TWO-PHOTON interference phenomenon. The position where a photon strikes depends on its wavevector, which is random, so the position where it strikes D0 is random too. However, the position where it is detected will give you some information about the wavevector and as you have entangled photons also about the wavevector of the other photon. Therefore although the total photon distribution on the other detector is random the pattern of a subset of photons with well defined wavevector (as determined by the position of D0) is not random, but gives an interference pattern.
 P: 69 thanks cthugha and others when the signal photon is detected at Do ....does not the wave function (entanglement) collapse? if so, then whatever we do (to idler) after detection (of signal) at Do is immaterial?
P: 1,395
 Quote by sanpkl thanks cthugha and others when the signal photon is detected at Do ....does not the wave function (entanglement) collapse? if so, then whatever we do (to idler) after detection (of signal) at Do is immaterial?
Yes, it is true that the entanglement is destroyed by the measurement of the signal photon at D0, however, that leaves the idler photon in a *well-defined* state, which is correlated to the photon detected at D0. It is this correlation which, through coincidence counting, reveals the interference pattern when the quantum eraser is in place.
P: 69
thanks spectracat.

now let's see if i got the next part right....

by delayed choice we also mean that....

even after the signal has been detected at Do, we can still "play" with the idler and get or not get interference pattern......(of course we would have to do photon by photon..)

i.e.

1 we can erase which way info and cause intereference pattern to disappear (after of course validating via coincidence counter..that only...the "matches/pairs" do-d1 etc..)
2. we can bring back which way info and get interference.

so in a sense .....does this mean/say...we can change the position of signal of Do.....that happened in THE PAST...so to speak....

 Quote by SpectraCat Yes, it is true that the entanglement is destroyed by the measurement of the signal photon at D0, however, that leaves the idler photon in a *well-defined* state, which is correlated to the photon detected at D0. It is this correlation which, through coincidence counting, reveals the interference pattern when the quantum eraser is in place.
P: 1,395
 Quote by sanpkl thanks spectracat. now let's see if i got the next part right.... by delayed choice we also mean that.... even after the signal has been detected at Do, we can still "play" with the idler and get or not get interference pattern......(of course we would have to do photon by photon..) i.e. 1 we can erase which way info and cause intereference pattern to disappear (after of course validating via coincidence counter..that only...the "matches/pairs" do-d1 etc..) 2. we can bring back which way info and get interference. so in a sense .....does this mean/say...we can change the position of signal of Do.....that happened in THE PAST...so to speak....
Well, that is the way that DCQE experiments are often sold .. you can decide if you like that interpretation or not. I have a different view, which is that the coincidence measurements reveal different components of the overall signal at D0, which does not show any interference. Since these coincidence measurements are by definition not complete until the second detector has registered, it is unclear to me why that shows anything relevant to temporal ordering. Basically, it shows that the predictions of QM are correct for this system.

As far as I am aware, no one has ever observed that recorded data has changed its values based on some delayed choice mechanism. What they see is that for two *different* data sets, recorded using *different* experimental configurations, the results are different: interference is observed when the QE is in place, which-path data is observed when it is not.

So, you have to be quite careful when saying that QM shows that past events can be changed, because this has never been shown directly to be true. No observed event has ever been shown to change its value. What people mean is that they infer a temporal ordering from perfectly reasonable deductions, such as the travel distance to detector D0 is shorter than for the other detectors, so the photon at D0 *must* have been recorded first. This seems reasonable to me. The next step is where they get weird, because they start saying things like, "the detector at D0 cannot know at the time the signal photon is measured whether we will have the QE inserted or not", to justify their interpretations of the rest of the measurements (i.e. that a past event has been changed.) However it has been shown time and time again that such statements simply do not pertain to QM measurements of this kind.

Anyway I hope this helps ... basically it can all be summed up as, "we can never observe a quantum system in the act of being quantum". I don't know who said it first (certainly not me), but it is worth remembering.
 Sci Advisor P: 1,626 You cannot change the detections at D0 afterwards. But you have a choice whether you will be able to pick a subset of the detections at D0 by means of coincidence counting, which gives an interference pattern. If you get which-way information on the other side, there is no such subset available. So the delayed choice is more or less just a choice of a subset. You do not change the detections or their position at D0 afterwards at all.
P: 69

i am with ya.

i am not a fan of the "past can be changed" hypothesis nor of the "many worlds" hypothesis....

however........i am holding/liking the below hypothesis in my mind....for the near future....

the signal photon at Do gets detected/recorded *only when* idler is...

till then signal photon "sort of hovers"...in a narrow range above Do...thus entanglement is broken only till the last.....

this would explain most of the things in this experiment....i guess...

 Quote by SpectraCat Well, that is the way that DCQE experiments are often sold .. you can decide if you like that interpretation or not. I have a different view, which is that the coincidence measurements reveal different components of the overall signal at D0, which does not show any interference. Since these coincidence measurements are by definition not complete until the second detector has registered, it is unclear to me why that shows anything relevant to temporal ordering. Basically, it shows that the predictions of QM are correct for this system. As far as I am aware, no one has ever observed that recorded data has changed its values based on some delayed choice mechanism. What they see is that for two *different* data sets, recorded using *different* experimental configurations, the results are different: interference is observed when the QE is in place, which-path data is observed when it is not. So, you have to be quite careful when saying that QM shows that past events can be changed, because this has never been shown directly to be true. No observed event has ever been shown to change its value. What people mean is that they infer a temporal ordering from perfectly reasonable deductions, such as the travel distance to detector D0 is shorter than for the other detectors, so the photon at D0 *must* have been recorded first. This seems reasonable to me. The next step is where they get weird, because they start saying things like, "the detector at D0 cannot know at the time the signal photon is measured whether we will have the QE inserted or not", to justify their interpretations of the rest of the measurements (i.e. that a past event has been changed.) However it has been shown time and time again that such statements simply do not pertain to QM measurements of this kind. Anyway I hope this helps ... basically it can all be summed up as, "we can never observe a quantum system in the act of being quantum". I don't know who said it first (certainly not me), but it is worth remembering.
P: 69
nice insight Cthugha. ....still trying to fully understand what you said...

just so i understand (the below) better..

why is there no subset available? (for which way info)

 Quote by Cthugha You cannot change the detections at D0 afterwards. But you have a choice whether you will be able to pick a subset of the detections at D0 by means of coincidence counting, which gives an interference pattern. If you get which-way information on the other side, there is no such subset available. So the delayed choice is more or less just a choice of a subset. You do not change the detections or their position at D0 afterwards at all.
 Sci Advisor P: 1,626 I am not sure I get your problem exactly. Some time before I gave a rough and a bit simplified explanation of DCQE experiments in a different topic. See this link: http://www.physicsforums.com/showpos...60&postcount=8 Maybe that explanation is a bit easier to digest.
P: 69
Cthugha,

I understand it somewhat.....would you like to take a stab at the below cases and provide a short "layman" explanation?

the only explanation i can think off is that..somehow a subset won't be created...

case 1 we change from "which way" to "no which way info" after signal photon has been detected

(and of course before idler photon is detected)

case 2 we change from "no which way info" to "which way info" after signal photon has been detected

(and of course before idler photon is detected)

 Quote by Cthugha I am not sure I get your problem exactly. Some time before I gave a rough and a bit simplified explanation of DCQE experiments in a different topic. See this link: http://www.physicsforums.com/showpos...60&postcount=8 Maybe that explanation is a bit easier to digest.
 Sci Advisor P: 1,626 Ok, let us assume that you have the DCQE setup as used by Kim, Kulik, Shih and Scully which we used earlier in this discussion and assume that we have some kind of mechanism which allows us to choose whether we have which path information (photon goes to D3 or D4) or we do not have which-way information (photon goes to D1 or D2). Now let's have a look at the detections with which-way information. All detections going one way will end up at the same detector. There is no phase dependence of the detections at this detector so you get no subsets. If you erase which-way information, you send the photon to the mirror leading to detectors D1 or D2. This part of the setup is pretty similar to a Mach-Zehnder interferometer. Whether a photon will end up at D1 or D2 will depend on the relative phase difference corresponding to the events "photon comes from slit A and reaches the mirror" and "photon comes from slit B and reaches the mirror". In a common Mach-Zehnder interferometer this phase shift is introduced by putting some sample in one arm of the interferometer. Here it is (randomly) produced by the downconversion process. Therefore this gives you the possibility to define two subsets: photons going to D1 and photons going to D2, which are characterized by different dependencies on the relative phase shift - just like in the Mach-Zehnder interferometer one will behave like $$sin^2(\frac{\Delta\Phi}{2})$$ and one will behave like $$cos^2(\frac{\Delta\Phi}{2})$$. These subsets are also visible on the other side. One certain position of D0 corresponds to some well defined value of this phase difference as the paths from slit A and slit B to this position are different. Therefore you can correlate the detections at D0 with those at D1 or D2 and get the interference pattern. If you now send photons to D1/D2 and put in some other which-way marker (for example by using polarization) all you do is to destroy the interference at the last mirror. This is like trying to use a Mach-Zehnder interferometer where you have different polarizations in both arms, which will also not show any interference.
 P: 1,540 "This is like trying to use a Mach-Zehnder interferometer where you have different polarizations in both arms, which will also not show any interference. " Well said, thanks for the excellent example. I didn't think I was going to fully appreciate this one, but I THINK I do now.
P: 69
Yes, well said Cthugha. Give me a day or two to digest it.

Simple questions

1. is a subset basically the patterns caused by EITHER whichway or "nowhichway"?
the total set being the positions of all the entangled photons that could be captured and this total set would show no pattern.
2. when both are there....no interference is noted?
3. is it possible, in the experiment, to get the location of the signal photon on Do prior to idler getting to the incidence counter?

a) the idler is delay by about 8 ns (of course we can increase/decrease this time difference). However during this 8ns are we able to tell where on the Do x-axis did the signal photon register? or do we have to wait for idler to be matched with signal in the co-incidence detector?

 Quote by Frame Dragger "This is like trying to use a Mach-Zehnder interferometer where you have different polarizations in both arms, which will also not show any interference. " Well said, thanks for the excellent example. I didn't think I was going to fully appreciate this one, but I THINK I do now.
P: 1,626
 Quote by sanpkl 1. is a subset basically the patterns caused by EITHER whichway or "nowhichway"? the total set being the positions of all the entangled photons that could be captured and this total set would show no pattern. 2. when both are there....no interference is noted?
Well, just compare this to the Mach-Zehnder interferometer where you also have two detectors.
If you send light along both paths, you can also get clicks at both detectors, but whether a photon goes to one detctor or the other will depend on the phase difference at the beam splitter. Therefore you get one subset "First detector" telling you that this subset will also have some well defined possible values of the relative phase and you get one subset "second detector" telling you this subset will have some different well defined possible values of the relative phase.
If you send the light only along one arm or the other you will have clicks at both detectors, but whether the light will go one way or the other at the beam splitter, is completely random. So the two subsets "first detector" and "second detector" do not carry any additional information, while these subsets are also subsets in terms of the relative phase in the case of no which-way information present. This later subset is the useful one, which allows for creation of an interference pattern (as it depends on phase).

 Quote by sanpkl 3. is it possible, in the experiment, to get the location of the signal photon on Do prior to idler getting to the incidence counter? a) the idler is delay by about 8 ns (of course we can increase/decrease this time difference). However during this 8ns are we able to tell where on the Do x-axis did the signal photon register? or do we have to wait for idler to be matched with signal in the co-incidence detector?
No, you can get the signal detection positions and times well before the idler is detected. No problem with that.
P: 69
Cthugha,

cthugha wrote ---No, you can get the signal detection positions and times well before the idler is detected. No problem with that.[/QUOTE]

1. since we know what we did with the idler photon (before, as well as after, the signal was detected at Do....) .......
do we really need to match/compare/check with the idler photon in the coincidence counter?

 Quote by Cthugha Well, just compare this to the Mach-Zehnder interferometer where you also have two detectors. If you send light along both paths, you can also get clicks at both detectors, but whether a photon goes to one detctor or the other will depend on the phase difference at the beam splitter. Therefore you get one subset "First detector" telling you that this subset will also have some well defined possible values of the relative phase and you get one subset "second detector" telling you this subset will have some different well defined possible values of the relative phase. If you send the light only along one arm or the other you will have clicks at both detectors, but whether the light will go one way or the other at the beam splitter, is completely random. So the two subsets "first detector" and "second detector" do not carry any additional information, while these subsets are also subsets in terms of the relative phase in the case of no which-way information present. This later subset is the useful one, which allows for creation of an interference pattern (as it depends on phase). No, you can get the signal detection positions and times well before the idler is detected. No problem with that.
P: 1,626
 Quote by sanpkl 1. since we know what we did with the idler photon (before, as well as after, the signal was detected at Do....) ....... do we really need to match/compare/check with the idler photon in the coincidence counter?
Of course we have to. You just know that the photon will go to the beam splitter, but without coincidence counting you do not know, which exit port the photon will take. This is the necessary bit of information you still need.
P: 69
 Quote by Cthugha Of course we have to. You just know that the photon will go to the beam splitter, but without coincidence counting you do not know, which exit port the photon will take. This is the necessary bit of information you still need.
you are talking about the idler, i assume.

do you mean ....

we don't know if the photon will go to d1/d2 or d3/d4 ?

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