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

[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?

 

[SpectraCat]

Spectra cat , i do remember what i learned earlier. i do understand it conceptually.

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?

Attempted self answer: we cannot tell its on the first or second because its too early too tell? one photon not a interfrence maketh? we need more photons and then also use the conincidence counter (and correlate wioth idler) to separate them?

You still don't seem to get the fundamental point here ... we do not "have two well-defined interference patterns" until we look at the *separate* coincidence channels. Your original question was asking about drawing conclusions from looking only at the signal photon measurements to make a *prediction* about the fate of the idler photon *passing through the interferometer*. That is clearly a physical impossibility based on common sense, let alone the laws of QM.

You cannot say anything regarding the idler photon until it has been measured. If it shows up on D1 or D2, then it passed through the interferometer, and which-path info was erased. If it shows up on D3 or D4, then we know which source (A or B) it came from. The interference pattern you are talking about only exist/make sense for *pairs* of photons corresponding to coincidence measurements at detectors D1 and D2 .. there is no "measurement" in this case until both photons have been detected. The number of coincidences preceding a particular measurement, whether it is the first or the million-and-first, is completely irrelevant.

 

[Cthugha]

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.)

As you erase which-way information (assuming that you can do it willingly which is not possible in the Kim paper), the idler photon will end up at D1 or D2. This means when doing coincidence counting you will find it at figure 3 or 4.

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.

Well, if you had no background noise, perfect detectors detecting every photon, perfect fringe visibility (this means a large number of counts at one position in figure 3 and no counts at all at the same position in figure 4 or vice versa) and have already recorded the interference patterns by coincidence counting you could predict at which detector (D1 or D2) the idler will end up if you recorded the corresponding idler at a position with zero counts for one of the two coincidence count interference patterns and send so few photon pairs around that you can be unambiguously sure which idler photon belongs to which signal photon. That this works is trivial. The measurement at D0 defines the relative phase between A and B and this relative phase is exactly what determines the idler photon behavior at the final beam splitter.
But of course it does not work the other way around. If you detect an idler photon at D1 or D2 you cannot say at which position the signal will hit D0.

Your original question was asking about drawing conclusions from looking only at the signal photon measurements to make a *prediction* about the fate of the idler photon *passing through the interferometer*. That is clearly a physical impossibility based on common sense, let alone the laws of QM.

Eh? The possibility to make a prediction on one side based on the measurement on the other side is pretty much what entanglement is about.

 

[SpectraCat]

Ok, maybe I see where your confusion arises. Are you are thinking of the D0 detector as representing some "screen" where we have a 2D-trace representing the interference patterns? That is certainly not correct. If we replaced D0 by a "screen", we would see a 1-D line (or band, since its thickness will not be zero). The integrated pattern of all the photons along that band will show a roughly Gaussian distribution peaked in the center. The question you are asking concerns a signal photon that hits such a "screen" at a particular location x'. Do you really think we can learn anything about its corresponding idler photon simply from observing that location? From the context of your question, it seems like you think that such a "screen" would somehow be divided into "bins", and all the photons landing in a given "bin" are correlated with idler photons going to a particular detector. This is not the case ... signal photons can hit any location on the "screen", irrespective of where their idler photon ends up.

The y-axis in figures 3 and 4 corresponds to coincidence counts, which are proportional to the detection rate (i.e. intensity) of photons hitting D0 at a particular location. So all we can say based on the observed interference patterns is that a signal photon that hits a particular "bin" has a X probability of corresponding to a coincident detection event of the idler at D1, and Y probability of corresponding to a coincident detection at D2.

So, perhaps that helps put my earlier answers into the proper context, and helps you to understand why the interference patterns are observable only through the coincidence measurements.

 

[sanpkl]

thanks Spectra,

sorry if this is getting too much for you. maybe you can make a last attempt...:-) or its ok...you can not respond...i do understand the whole thing...and i know you and cthugha are saying same thing and i agree with it...

yes my post was abotu predicting idler from signal...and i see your logic...and i realize that i my logic is way off...

(though i think you and cthugha are correct, i want to be sure i am 100% convinced, i am 99% now, i do understand you)

however before we get to that...

let's say we have looked at the *separate* coincidence channels for the first million photons.

now we get two well defined patterns...let's say we have done it exactly the way kim-scully et al would have done it.

now i send one photon...we measure its position... can we tell if it falls on the first or the second interference pattern?

if its close to the first pattern then its on first, if its close to the second then its on second.

however i think the reason we cannot tell is because ** both ** the interfernce patterns overlap...in a sense.

thus we cannot tell from a single photon on which interference pattern it lies...

reading cthugha's response.

You still don't seem to get the fundamental point here ... we do not "have two well-defined interference patterns" until we look at the *separate* coincidence channels. Your original question was asking about drawing conclusions from looking only at the signal photon measurements to make a *prediction* about the fate of the idler photon *passing through the interferometer*. That is clearly a physical impossibility based on common sense, let alone the laws of QM.

You cannot say anything regarding the idler photon until it has been measured. If it shows up on D1 or D2, then it passed through the interferometer, and which-path info was erased. If it shows up on D3 or D4, then we know which source (A or B) it came from. The interference pattern you are talking about only exist/make sense for *pairs* of photons corresponding to coincidence measurements at detectors D1 and D2 .. there is no "measurement" in this case until both photons have been detected. The number of coincidences preceding a particular measurement, whether it is the first or the million-and-first, is completely irrelevant.

 

[Cthugha]

So all we can say based on the observed interference patterns is that a signal photon that hits a particular "bin" has a X probability of corresponding to a coincident detection event of the idler at D1, and Y probability of corresponding to a coincident detection at D2.

Indeed, that might be the best wording.

 

[sanpkl]

yes spectra you see my confusion...i am imagining a 2d...or a 2d being constructed...after noting position of Do on x axis...let me read and digest your post ...before i respond.

i just realized that my understanding about the pattern/graphs generation is wrong ...after reading your post below...

ok...i got it...in summary/short:

1. one cannot tell by the position of single signal photon...if it lies on Do-D1 pattern or Do-D2 pattern because of 2

2. the graphs are probability distributions.

3. plus there is a bit more...

now i will post the next thought i had ( after i have read Cthugha's last post) ...thanks Spectra cat and Cthugha...

Ok, maybe I see where your confusion arises. Are you are thinking of the D0 detector as representing some "screen" where we have a 2D-trace representing the interference patterns? That is certainly not correct. If we replaced D0 by a "screen", we would see a 1-D line (or band, since its thickness will not be zero). The integrated pattern of all the photons along that band will show a roughly Gaussian distribution peaked in the center. The question you are asking concerns a signal photon that hits such a "screen" at a particular location x'. Do you really think we can learn anything about its corresponding idler photon simply from observing that location? From the context of your question, it seems like you think that such a "screen" would somehow be divided into "bins", and all the photons landing in a given "bin" are correlated with idler photons going to a particular detector. This is not the case ... signal photons can hit any location on the "screen", irrespective of where their idler photon ends up.

The y-axis in figures 3 and 4 corresponds to coincidence counts, which are proportional to the detection rate (i.e. intensity) of photons hitting D0 at a particular location. So all we can say based on the observed interference patterns is that a signal photon that hits a particular "bin" has a X probability of corresponding to a coincident detection event of the idler at D1, and Y probability of corresponding to a coincident detection at D2.

So, perhaps that helps put my earlier answers into the proper context, and helps you to understand why the interference patterns are observable only through the coincidence measurements.

 

[sanpkl]

Indeed, that might be the best wording.

yes that is what i was looking for.

Well worded! Spectra cat. now reading Cthugha's post

 

[SpectraCat]

As you erase which-way information (assuming that you can do it willingly which is not possible in the Kim paper), the idler photon will end up at D1 or D2. This means when doing coincidence counting you will find it at figure 3 or 4.

Ok, I guess I glossed over that aspect of his question .. I was assuming that all 4 idler detectors were still in play.

Well, if you had no background noise, perfect detectors detecting every photon, perfect fringe visibility (this means a large number of counts at one position in figure 3 and no counts at all at the same position in figure 4 or vice versa) and have already recorded the interference patterns by coincidence counting you could predict at which detector (D1 or D2) the idler will end up if you recorded the corresponding idler at a position with zero counts for one of the two coincidence count interference patterns and send so few photon pairs around that you can be unambiguously sure which idler photon belongs to which signal photon. That this works is trivial. The measurement at D0 defines the relative phase between A and B and this relative phase is exactly what determines the idler photon behavior at the final beam splitter.

Are you sure this is correct? First of all, it would only provide definitive answers for a very small subset of photons that hit D0 at locations corresponding to the minimum/maximum intensities of the fringes. Second of all, even for those subsets, it will only work if the minimum intensities in a given pattern correspond to a detection rate of zero, and it was my understanding that this is not possible. Is it really possible to tailor the phase characteristics of a single photon so that one can predict with certainty which path it will take upon encountering a beam-splitter or a half-silvered mirror? If so, then I will have to reconsider my comments.

But of course it does not work the other way around. If you detect an idler photon at D1 or D2 you cannot say at which position the signal will hit D0.

Why not? It seems to me that there is no difference between the two cases, assuming that we have position-sensitive detection at both locations. If predictive power is allowed from observing the signal photon path, it should also be from observing the idler-photon path.

Eh? The possibility to make a prediction on one side based on the measurement on the other side is pretty much what entanglement is about.

Of course, but the position of detection at D0 does not give us the information we need to predict unambiguously the path which the idler photon will take upon encountering the beam splitter, does it? (see my question above).

 

[sanpkl]

SpectraCat wrote:

<Of course, but the position of detection at D0 does not give us the information we need to predict unambiguously the path which the idler photon will take upon encountering the beam splitter, does it? (see my question above).>

I agree with Spectra Cat.

However, do we all also agree that, this is because:

we don't know (with *absolute certainty*) if the detected position of signal photon "falls/lies" on the interference pattern of figure 3 or 4 (or fig 5 or 6)?

Until we correlate with idler in the co-incidence counter

 

[Cthugha]

Are you sure this is correct? First of all, it would only provide definitive answers for a very small subset of photons that hit D0 at locations corresponding to the minimum/maximum intensities of the fringes. Second of all, even for those subsets, it will only work if the minimum intensities in a given pattern correspond to a detection rate of zero, and it was my understanding that this is not possible. Is it really possible to tailor the phase characteristics of a single photon so that one can predict with certainty which path it will take upon encountering a beam-splitter or a half-silvered mirror? If so, then I will have to reconsider my comments.

Well, this would of course just work under perfect experimental conditions, with no background noise and of course just for extremely few positions of D0. In principle this is possible. Whether this can indeed be realized under realistic lab conditions is a different question. I mean - this is pretty much the same as asking "is it possible to have completely destructive interference in one exit port of a Mach-Zehnder-interferometer". Under some well defined experimental conditions it is possible.

Why not? It seems to me that there is no difference between the two cases, assuming that we have position-sensitive detection at both locations. If predictive power is allowed from observing the signal photon path, it should also be from observing the idler-photon path.

This case would be more difficult. There are just two detectors D1 and D2, but lots of possible positions of D0. So while it is possible to say that D0 detections at some positions x1, x2 and x3 will lead to idler detections at D1, you will only be able to say that a detection at D1 will mean a signal detection at position x1 or x2 or x3 and maybe even some other position. However, you could get some probability distribution.

 

[sanpkl]

Cthugha,

are you saying that:

once we have determined the position of signal photon on D0,
the entanglement is broken
the path of idler is now fixed/sealed/predetermined

 

[Cthugha]

Broken? Well, you break the superposition.

Just compare this to the simplifying case of two spin-entangled particles. You can have particle 1 with spin up and particle two with spin down or the other way round. If you measure for example spin down on one particle, you now know what that you would measure spin up on the other.

In the DCQE experiments the measurement is different. You gain phase information. And once you do so on the side of D0 you get a well defined phase and can therefore predict what will happen on the other side - as phase is the property which determines what happens in a Mach-Zehnder interferometer. However in most cases this will be a probabilistic prediction like "with 70% probability that photon will go to D1".

 

[sanpkl]

Cthugha,

assuming you have not given up on my questions/posts...:-)

so proceeding further on our line of thought/understanding...

you wrote:

<As you erase which-way information (assuming that you can do it willingly which is not possible in the Kim paper), the idler photon will end up at D1 or D2. This means when doing coincidence counting you will find it at figure 3 or 4.>


in the experiment...(either the scully kim one or my minor modification of it)

at the time/moment when signal photon position was measured, we did *have* which way information.

the which-way information is erased (in idler) after the signal photon has been measured.

however quantum mechanics (or your quote above) would say that the position of signal photon would correspond to which way information being erased?

1. how do we explain this?

2. while in the kim scully setup we might not "willingly" be able to erase which way info, there are sereval other delayed choice quantum eraser experiments where we do have the ability to erase or not erase which way information at our will?

for example Wheeler's delayed choice ?

http://news.sciencemag.org/sciencenow/2007/02/16-04.html

As you erase which-way information (assuming that you can do it willingly which is not possible in the Kim paper), the idler photon will end up at D1 or D2. This means when doing coincidence counting you will find it at figure 3 or 4.



Well, if you had no background noise, perfect detectors detecting every photon, perfect fringe visibility (this means a large number of counts at one position in figure 3 and no counts at all at the same position in figure 4 or vice versa) and have already recorded the interference patterns by coincidence counting you could predict at which detector (D1 or D2) the idler will end up if you recorded the corresponding idler at a position with zero counts for one of the two coincidence count interference patterns and send so few photon pairs around that you can be unambiguously sure which idler photon belongs to which signal photon. That this works is trivial. The measurement at D0 defines the relative phase between A and B and this relative phase is exactly what determines the idler photon behavior at the final beam splitter.
But of course it does not work the other way around. If you detect an idler photon at D1 or D2 you cannot say at which position the signal will hit D0.



Eh? The possibility to make a prediction on one side based on the measurement on the other side is pretty much what entanglement is about.

 

[sanpkl]

well said!

Broken? Well, you break the superposition.

Just compare this to the simplifying case of two spin-entangled particles. You can have particle 1 with spin up and particle two with spin down or the other way round. If you measure for example spin down on one particle, you now know what that you would measure spin up on the other.

In the DCQE experiments the measurement is different. You gain phase information. And once you do so on the side of D0 you get a well defined phase and can therefore predict what will happen on the other side - as phase is the property which determines what happens in a Mach-Zehnder interferometer. However in most cases this will be a probabilistic prediction like "with 70% probability that photon will go to D1".

 

[Cthugha]

at the time/moment when signal photon position was measured, we did *have* which way information.

the which-way information is erased (in idler) after the signal photon has been measured.

however quantum mechanics (or your quote above) would say that the position of signal photon would correspond to which way information being erased?

1. how do we explain this?

The position of the signal photon itself does not depend on whether you erase which-way info or not (still assuming that we can somehow do it willingly). However, if you do, the idler will enter the Mach-Zehnder like part of the setup leading to detectors D1 and D2. If you do not erase which-way info the idler photon the idler will never enter that part of the setup and will go to D3 or D4. This part is not phase sensitive so any position detected at D0 can lead either to a detection at D3 or D4.

2. while in the kim scully setup we might not "willingly" be able to erase which way info, there are sereval other delayed choice quantum eraser experiments where we do have the ability to erase or not erase which way information at our will?

For example in the double slit quantum eraser experiment performed by Walborn (http://pra.aps.org/abstract/PRA/v65/i3/e033818]) you can willingly choose whether you keep or erase which way info in terms of the polarization of the light in the two paths.

 

[sanpkl]

how do we explain the fact that the same/fixed position (of signal)... say (x,y) = (5,7)

can lie on
figure 3/4 if we erase which way (for idler)
and
figure 5/6 if we keep which way (for idler)




if photon is driving on highway I-5 the photon is on I-5
if photon is driving on I-10, the photon is on I-10
if the photon is on intersection of I-5 and I-10 then its possibe for the photon to be aligned with either I-5 or I-10


also see below from http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

<<The position of a photon at detector D0 has been registered and scanned. Yet the actual position of the photon arriving at D0 will be at one place if we later learn more information; and the actual position will be at another place if we do not.>>

 

[Cthugha]

how do we explain the fact that the same/fixed position (of signal)... say (x,y) = (5,7)

can lie on
figure 3/4 if we erase which way (for idler)
and
figure 5/6 if we keep which way (for idler)

Why should there be a problem? Figures 3/4 and figures 5/6 are the results of two completely different measurements on the idler.

As soon as the signal is detected the signal part of the experiment is done. You determined a position - or equivalently a relative phase. I think the whole experiment would be much easier to understand if you exchanged the position axis with an axis showing the relative phase. And now the choice is whether to send the idler to a detector setup, which is sensitive to this relative phase (detectors D1/D2 leading to figures 3/4) ot to a detector setup which is not sensitive to the relative phase (leading to figures 5/6).

also see below from http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

<<The position of a photon at detector D0 has been registered and scanned. Yet the actual position of the photon arriving at D0 will be at one place if we later learn more information; and the actual position will be at another place if we do not.>>

Please be careful with that page. Most of the commentary there is nonsense - and I do not quite believe the endnote saying Dr. Kim reviewed the commentary.

 

[sanpkl]

Cthugha, thanks for clarifying about the experiment.
My understanding on the generation of figures 3,4,5,6, is very limited.
Can you please correct/modify the below?

0. Does "joint detection" mean anything beyond correlating in the co-incidence counter?

1. What is the y-axis (labeled coincidence) measuring in figures 3,4,5,6? Searching through the kim paper to see if I can find the answer.

for example on figure 3, one of the signal photon's position is (x,y) = (1.5, 120).

what does 120 mean?

2. Figures 3/4, 5/6 are constructed based on position of signal photon, the idler (other than simply confirming it’s the right twin signal photon) has nothing to do with it
3. You wrote: Figures 3/4 and figures 5/6 are the results of two completely different measurements on the idler.
I thought/assumed: Figures 3/4 and figures 5/6 are the result of measurements on the signal. What did you mean?
4. Phase difference between signal and idler can be measured, as well as determined theoritically, however the signal has already made the pattern/position, so

 

[Cthugha]

0. Does "joint detection" mean anything beyond correlating in the co-incidence counter?

Not really.

1. What is the y-axis (labeled coincidence) measuring in figures 3,4,5,6? Searching through the kim paper to see if I can find the answer.

for example on figure 3, one of the signal photon's position is (x,y) = (1.5, 120).

what does 120 mean?

It is just the number of simultaneous detections of the signal at a given position and the idler at a given detector in a fixed time interval. So all they do is to put the detector at position x and wait for say 60 seconds and count all the simultaneous detections of signal photons at D0 at this position and for example idler photons at D1. The absolute number of simultaneous detections is then shown on the y-axis. Then they move the detector to the next position and wait again for 60 seconds.

So for example for Fig. 3 there have been 120 simultaneous detections if the detector is positioned at position x=1.5.

2. Figures 3/4, 5/6 are constructed based on position of signal photon, the idler (other than simply confirming it’s the right twin signal photon) has nothing to do with it

Well the idler at least tells whether the detection of the signal needs to be added to figure 3, 4, 5 or 6. This is more than nothing.

3. You wrote: Figures 3/4 and figures 5/6 are the results of two completely different measurements on the idler.
I thought/assumed: Figures 3/4 and figures 5/6 are the result of measurements on the signal. What did you mean?

As I said before conincidence counting is always a measurement on signal AND idler. But the measurement on the signal side is always the same. But on the other side the measurement apparatus is either phase sensitive (idler going to D1/D2 in a Mach-Zehnder-like setup) or not (idler going to D3/D4).

4. Phase difference between signal and idler can be measured, as well as determined theoritically, however the signal has already made the pattern/position, so

The phase difference between signal and idler is completely irrelevant. The phase difference between the possibilities of having the signal and idler emitted from position A or from position B (or equivalently the phase difference between the photon going through one slit or the other slit of the double slit and ending up at one certain position at the detector) is the quantity that matters.

 

[sanpkl]

conincidence counting is always a measurement on signal AND idler. But the measurement on the signal side is always the same. But on the other side the measurement apparatus is either phase sensitive (idler going to D1/D2 in a Mach-Zehnder-like setup) or not (idler going to D3/D4).

1. since we know the emission time of the signal and idler photons, (and path length) we can figure out what time signal (and even idler) would strike Do.

thus can we not separate the signal photon, from the noise by simply considering the photon which struck Do at the expected/calculated arrival time?
or
is the "fine tuning" via formula (or clock) is not good enough and thus we need a coincidence counter?

2. still trying to understand how signal photons patterns can be "clump" or "interference" after the fact.

one idea that comes to mind is that:
once the position of signal is determined, the path of idler is fixed between D3/D4 (i.e.. figure 3/4) and D1/D2 (figure 5/6). though we cannot control the choice between 3 and 4.

however in some experiments we can "willingly" change the path of idler between fig 3/4 or fig 5/6...