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

[SpectraCat]

in the below and other quantum eraser experiments...

the detector is moved by a trackor...however since the position of photon is randomly determined...how would the experimenter know where (which position) to keep the detector?

are a lot of photons missed because the detector is not at the correct position in time?


pls see...http://en.wikipedia.org/wiki/Quantum_eraser_experiment

The point of the experiment is not to detect all of the photons, but rather to establish correlations between the two members of the entangled pair based on whether or not the quantum eraser is in place. These coincidence measurements are required to reveal the interference pattern at the D0 detector. A large number of counts are acquired at each detector position.

 

[sanpkl]

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

 

[Cthugha]

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.

 

[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?

 

[SpectraCat]

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.

 

[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...

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.

 

[SpectraCat]

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.

 

[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.

 

[sanpkl]

nice answer SpectraCat. well presented.

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...

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.

 

[sanpkl]

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)

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.

 

[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:
https://www.physicsforums.com/showpost.php?p=2241460&postcount=8

Maybe that explanation is a bit easier to digest.

 

[sanpkl]

Cthugha,

I read your posting at https://www.physicsforums.com/showpost.php?p=2241460&postcount=8

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)

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:
https://www.physicsforums.com/showpost.php?p=2241460&postcount=8

Maybe that explanation is a bit easier to digest.

 

[Cthugha]

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.

 

[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.

 

[sanpkl]

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?

"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.

 

[Cthugha]

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).

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.

 

[sanpkl]

Cthugha,

first...let me ask about (slightly off tangent) the below:

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?

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.

 

[Cthugha]

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.

 

[sanpkl]

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 ?

 

[Cthugha]

You do not know whether a photon for which there is no which-way information available will end up at D1 or D2. What the other photons do, does not really matter.

 

[sanpkl]

You do not know whether a photon for which there is no which-way information available will end up at D1 or D2. What the other photons do, does not really matter.

and this is because do-d1 and do-d2 have a phase difference and if combined no interference would show?...thus we need to separate d1 and d2...and get the patterns are shown in the paper

anything more you want to add to this?

 

[Cthugha]

No, sounds good to me.

 

[sanpkl]

a typographical error in the below link?
http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

a quote from the above link...

However, what makes this experiment possibly astonishing is that, unlike in the classic double-slit experiment, the choice of whether to preserve or erase the which-path information of the idler need not be made until after the position of the signal photon has already been measured by D0.

the word "after" should be replaced with "before"
or better still
the above sentence needs be rephrased/corrected with something like below

The which-path
or both-path information of a quantum can be erased or
marked by its entangled twin even after the registration
of the quantum. - kim paper

 

[sanpkl]

thanks cthugha and spectracat for enhancing my understanding of quantum mechanics

i think i have figured the fallacy about the past being changed...

when the signal photon is detected (at Do)...all the below happens

- the wave function collapses

- the state of both the entangled photons is frozen (which way or both way and in case of which way...also the slit A or slit B)

- its just that we don't what that state is till the idler photon arrives and is checked with signal via the conincidence counter

- thus both the photon become "determinate" once the signal is detected, even the idler path becomes determinate...however we can only tell once we compare via coincidence counter

- thus the past cannot be changed

 

[sanpkl]

can the (screen of) Do detector be made more bigger so that we don't have to constantly keep moving it by a step motor?

 

[Cthugha]

In principle yes, but you will get another problem if you do so.

If you just increase the size without making the detector itself position sensitive you lose any spatial resolution and can of course trivially never get any interference pattern.

If you instead use a large position sensitive detector (or a line of single detectors, which is more or less the same) you have to do coincidence counting between each of these detectors and the detectors on the other side individually. Therefore just using one detector and moving it from left to right is often the easier solution.

 

[sanpkl]

one cannot measure the position of the signal independently of the idler.

thus the position of the signal is relative to the position of idler.

this solves the delayed choice riddle because the idler chooses a position such that ...the signal will form a pattern (relative to idler) such that is is consistent with what we did last to idler before it hit the detector.

look forward to your thoughts...

 

[sanpkl]

in the coincidence counter:

what parameters/properties are used to pair/match/verify the signal photon with the idler photon?

is it just the time/timing? or is it something else?

are both made to arrive at same time in coincidence counter? or it does not matter?

 

[sanpkl]

in the yooh-ho kim paper ...http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser


the word "registering" of the quantum is used, it does not talk about the position of the signal photon on Do.

1. The position can only be determined after comparing with idler in the co-incidence counter...

"In a two detector system, a coincidence counter alleviates this problem by only recording detection signals that strike both detectors simultaneously (or more accurately, recording only signals that arrive at both detectors and correlate to the same emission time)"

http://en.wikipedia.org/wiki/Coincidence_counting_(physics )


Ques 1: this is with reference to determining the position of hte signal photon on Do.

Since we know the emission time of the signal and idler photon (and its same)...
can we not figure out the "correct" signal photon position simply by using emission time and without comparing with idler?
why do we need to compare with idler (to determine singal position on Do)?

Ques 2: only the detection ("registration of the quantum") of the signal photon is made, but position is not determined until idler "arrives". if we were to determine the position of signal photon (prior to comparing with idler in coincidence counter) would that break the entanglement? would the wave function collapse?


thanks

 

[Cthugha]

the word "registering" of the quantum is used, it does not talk about the position of the signal photon on Do.

Eh? Let me quote the paper:
"Photon 1, propagating to the right, is registered by detector D0, which can be scanned by a step motor along its x axis[...]".
The position of the detector on the x-axis automatically gives you the position of the photon.

1. The position can only be determined after comparing with idler in the co-incidence counter...

If you mean the position of the signal photon this is plain wrong. You know its position as soon as you detect it.

Ques 1: this is with reference to determining the position of hte signal photon on Do.

Since we know the emission time of the signal and idler photon (and its same)...
can we not figure out the "correct" signal photon position simply by using emission time and without comparing with idler?
why do we need to compare with idler (to determine singal position on Do)?

Ques 2: only the detection ("registration of the quantum") of the signal photon is made, but position is not determined until idler "arrives". if we were to determine the position of signal photon (prior to comparing with idler in coincidence counter) would that break the entanglement? would the wave function collapse?

Sorry, but this is complete nonsense. The position of the signal photon is well determined by the detection. This is absolutely NOT what the coincidence counting is for.

 

[sanpkl]

Cthugha,

Are you saying coincidence counter is simply for seperating (correlating) with each detector i.e. Do-d1, D0-d2, Do-d3, Do-d4?

Thus this helps give an interfernce pattern...actually two...that are shifted by (half) a phase?

I think the confusion arose when someone told me that they had discussed with Dr Kim a couple of years ago and Dr Kim said signal position cannot be determined without correlating with idler. I think what might have happened is that this person mis-understood, or memory issue. Dr Kim might have been talking about the interference pattern.

However if this is the case then i have an experimental variation of the below that I wanted to discuss.

Eh? Let me quote the paper:
"Photon 1, propagating to the right, is registered by detector D0, which can be scanned by a step motor along its x axis[...]".
The position of the detector on the x-axis automatically gives you the position of the photon.



If you mean the position of the signal photon this is plain wrong. You know its position as soon as you detect it.



Sorry, but this is complete nonsense. The position of the signal photon is well determined by the detection. This is absolutely NOT what the coincidence counting is for.

 

[Cthugha]

Are you saying coincidence counter is simply for seperating (correlating) with each detector i.e. Do-d1, D0-d2, Do-d3, Do-d4?

Thus this helps give an interfernce pattern...actually two...that are shifted by (half) a phase?

In principle: yes.

 

[sanpkl]

Cthugha,

Thanks for clarifying/validating my understanding.

the kim scully experiment that i refer to is this article...http://arxiv.org/abs/quant-ph/9903047. it is the same we have been discussing.

Now continuing further ...if we carry on the kim scully experiment for say...a million photons ( so that we can get two clear, but separate, interference patterns).

we would have two clear interference patterns (separated by pie phase shift) namely Do-D1 and Do-D2. i.e. Figure 3 and Figure 4 on the kim scully paper.

now we send the million plus one photon...we keep which way info before signal photon is detected and then erase which way before idler strikes (say 8 ns later) on either D1 or D2.

now...(please correct/modify the below where required):

where would be find the million plus one photon? on figure 3/4 or figure 5/6? (Figure 6 is nto given in the paper however we can assume is it same as figure 5.)

assuming it is found on either fig 3 or fig 4 intereference pattern..we assume this... because which way info has been erased prior to idler striking the detector...

we measure the position of signal photon and see if its on the interference of Figure 3 OR Figure 4.

we should be able to tell if signal is on which curve because the million photons before it have created two nice interference patterns. we stopped the experiment after million photons and drew the nice/clear interference patterns before sending the millionth plus one photon.

so now we can predict which detector idler will strike...if signal position is discovered on figure 3...then idler will strike detector D1. if signal position is discovered on figure 4 then...idler will strike D2.

I will stop here for now and look forward to your reply.

 

[SpectraCat]

Cthugha,

Thanks for clarifying/validating my understanding.

the kim scully experiment that i refer to is this article...http://arxiv.org/abs/quant-ph/9903047. it is the same we have been discussing.

Now continuing further ...if we carry on the kim scully experiment for say...a million photons ( so that we can get two clear, but separate, interference patterns).

we would have two clear interference patterns (separated by pie phase shift) namely Do-D1 and Do-D2. i.e. Figure 3 and Figure 4 on the kim scully paper.

now we send the million plus one photon...we keep which way info before signal photon is detected and then erase which way before idler strikes (say 8 ns later) on either D1 or D2.

now...(please correct/modify the below where required):we measure the position of signal photon and see if its on the interference of Figure 3 OR Figure 4.

we should be able to tell if signal is on which curve because the million photons before it have created two nice interference patterns. we stopped the experiment after million photons and drew the nice/clear interference patterns before sending the millionth plus one photon.

so now we can predict which detector idler will strike...if signal position is discovered on figure 3...then idler will strike detector D1. if signal position is discovered on figure 4 then...idler will strike D2.

I will stop here for now and look forward to your reply.

No No No No No No No . You cannot see the interference patterns for the signal photons without the coincidence detection. If you ignore the idler photons, all that would be observed for the signal photons is a big Gaussian-looking blob with no interference fringes whatsoever. In fact, even for the idler photons that pass through the interferometer and are detected at D1 and D2, the interference patterns are only evident in the separated coincidence channels. If you add the D1 and D2 coincidences together, you just get the blob again (this is shown explicitly in the paper).

EDIT: the reason for this last point is obvious if you think about it ... the signal photons come from two spatially distinct sources, and propagate directly to the D0 detector ... therefore there is ALWAYS which-path information available for the signal photons, and so the pattern from the signal photons can never show interference fringes directly. It is only by looking at the separate coincidences from the idler photons that passed through the QE interferometer that we find out that the "Gaussian-blob" actually contains components that have interference fringes. There is no way to obtain that information without looking at the coincidence measurements. It is another example of the maxim, "it is impossible to directly observe a quantum mechanical system being quantum-mechanical".

I think you may be forgetting what you have learned previously, because it seemed that you had absorbed this a few pages back. If I may offer some unsolicited advice: the following is a good rule of thumb to remember when trying to draw conclusions based on concepts/experiments that you are just learning about: "if it sounds impossible, it probably is" .. I have found that when I talk myself into similar positions, it is a good idea to think it through carefully and write down what I know about the system. Then I write down my "seemingly crazy" conclusion ... then I go back and fact check everything against the sources to make sure my understanding is correct. Going through this exercise (sometimes multiple times), usually helps me find my mistake/misunderstanding, and it ALWAYS helps me to gain a deeper understanding of the system.

 

[sanpkl]

Spectra cat , thanks for your reply and desire to help in the understanding. i do remember what i learned earlier. i do understand it conceptually.
not to worry i did absorb it fine.

what i am saying is...we already have an interference pattern from the first million photons that we sent one by one...we got it the way its done in the kim-scully paper.

i am now talkign about a single photon striking D0 (after a million photons have already struck) which already has two well defined interefrence patterns marked/drawn on it via use of the conincidence counter for the first million photons.

all i am saying is we already have two *well defined*, *well demarcated* intereference patterns after a million photons have struck. now i am talking about just one photon. the millionth and one photon, say...

Ques: can we not tell if its on the first interference pattern or the second? is not a single photon position enough to judge if it lies on the first or second internfrence pattern.


Attempted self answer: we cannot tell its on the first or second because its too early too tell? just one photon could lie on either of the interfernce patterns?... we need more photons and then also use the conincidence counter (and correlate wioth idler) to separate them?