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

[Cthugha]

however, in actuality, we cannot tell which "bin" the signal photon has hit, till we compare with the idler?

we can only say *if* the signal photon has hit the (Do-D1) bin (i.e lies on figure 3) then the likely hood of idler photon being detected at D1 is say...80%

No, that is not the problem. If you already recorded the interference patterns for a while, you could tell the probability distribution of the idler hitting D1 or D2 by knowing the detection position on D0.

However, you can never tell before whether the signal "hit the D0-D1 bin" or the "D0-D2 bin" (or equivalently whether it lies on Fig. 3 or 4). You can just tell where it hit on D0. To get the D1 or D2-info beyond a probabilistic description you need information from the idler.

However, the remaining answers will really have to wait until tomorrow morning.

 

[sanpkl]

one of the major fallacy with "past is/canbe changed, many worlds" is the misunderstanding that:

the "decision" is happening when idler i detected. this is incorrect.

the truth is that:

the experiment is over when signal photon is detected (earlier detection) and not when idler photon i detected (later).

the "probalilistic firming/decision/choice" happens when the signal photon (or which ever is detected first) is detected.

the wave function (of various superimposition states) collapses when signal photon (or to be more precise when whichever photon is detected earlier) is detected and not when idler is detected.


QED. thanks to Cthugha, EagleLake and Spectra Cat

 

[DrChinese]

one of the major fallacy with "past is/canbe changed, many worlds" is the misunderstanding that:

the "decision" is happening when idler i detected. this is incorrect.

the truth is that:

the experiment is over when signal photon is detected (earlier detection) and not when idler photon i detected (later).

I think the context of the entire experiment is relevant. Not just the "first" detection. How you interpret the results is dependent on that context, and that will not be known until later - when all of the results can be brought together into a single place. And then it will in fact appear "as if" the past was dependent on the future.

You can interpret this in different ways. And there are other delayed choice experiments which evidence the same thing. Consider Zeilinger et al:

http://arxiv.org/abs/quant-ph/0201134

From middle of page 5:

"Such a delayed-choice experiment was performed by including two 10 m optical fiber
delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed
by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and
photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore,
this result indicate that the time ordering of the detection events has no influence on the
results..."

 

[Cthugha]

the experiment is over when signal photon is detected (earlier detection) and not when idler photon i detected (later).

the "probalilistic firming/decision/choice" happens when the signal photon (or which ever is detected first) is detected.

the wave function (of various superimposition states) collapses when signal photon (or to be more precise when whichever photon is detected earlier) is detected and not when idler is detected.

Well, this is a rather hard wording. The experiment is not really over when you detect the signal - however, the setting is fixed. You can still choose whether to make a which-way experiment or an interference experiment on the idler. Either of those is a measurement, too.

You could say the signal side of the experiment is over. You will not change the position the signal was detected after the detection. However, you can still change the kind and amount of information you can gain from knowing the position where the signal was detected.

 

[sanpkl]

Well, this is a rather hard wording. The experiment is not really over when you detect the signal - however, the setting is fixed.

cthugha, thanks for correcting and providing insight into the experiment.

You can still choose whether to make a which-way experiment or an interference experiment on the idler. Either of those is a measurement, too.

ok. now let's say we detected the signal photon position and its at (x, y) = (1.5, 120) and it lies on figure 3.

actually y does not matter..its just a count (that can be converted to probability).

so, let's assume, we are reasonably sure (after pattern formed by gazillion photons prior) that signal photon position lies on figure 3.

now

1. if we choose which way info...are we likely to detect idler at D1?
2. if we do not chose which way info...are we likely to detect idler at?
2b) the signal is on fig 3, suggesting interference and no which way..however if we make which way experiment on idler...then won't signal be saying interference and idler which way?

my guess/answer to 2b) is that ...when we force idler (to say which way) the entanglement with signal is broken..so we can no longer expect them (idler and its twin signal)to say the same thing..

or in other words what behavior of idler (i.e. which detector it will land up at) can we expect (probabilistically) if

Case 1. we choose to make a which-way experiment on idler
Case 2. we choose to make an interference experiment on the idler.

**given that** signal seems to be (with high probability) on the pattern of figure 3...

sorry if this makes you feel i am going back to square one...don't worry...just answer the best you can...if you will...

You could say the signal side of the experiment is over. You will not change the position the signal was detected after the detection. However, you can still change the kind and amount of information you can gain from knowing the position where the signal was detected.

i have a feeling this is really well said...well summarized...till i fully digest and assimilate this.

Please give example of kind and amount of information one can gain (about idler?) from knowing the position of where signal was detected. this will help me understand better.

 

[sanpkl]

I think the context of the entire experiment is relevant. Not just the "first" detection. How you interpret the results is dependent on that context, and that will not be known until later - when all of the results can be brought together into a single place. And then it will in fact appear "as if" the past was dependent on the future.

You can interpret this in different ways. And there are other delayed choice experiments which evidence the same thing. Consider Zeilinger et al:

http://arxiv.org/abs/quant-ph/0201134

From middle of page 5:

"Such a delayed-choice experiment was performed by including two 10 m optical fiber
delays for both outputs of the BSA. In this case photons 1 and 2 hit the detectors delayed
by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon 0 and
photon 3 matches the fidelity in the non-delayed case within experimental errors. Therefore,
this result indicate that the time ordering of the detection events has no influence on the
results..."

interesting...it will take me a few days to understand the paper...thanks for the link and your post

cthugha do you want to take a stab at this and summarize it...?

 

[SpectraCat]

interesting...it will take me a few days to understand the paper...thanks for the link and your post

cthugha do you want to take a stab at this and summarize it...?

To save some time and re-typing efforts, you may also want to have a look at this thread:

https://www.physicsforums.com/showthread.php?t=376225

DrChinese and I have been hashing out two points of view on this work which don't quite agree interpretation-wise ... we have worked through some of the details of the experiment there.

 

[Cthugha]

ok. now let's say we detected the signal photon position and its at (x, y) = (1.5, 120) and it lies on figure 3.

actually y does not matter..its just a count (that can be converted to probability).

so, let's assume, we are reasonably sure (after pattern formed by gazillion photons prior) that signal photon position lies on figure 3.

Ok, so we have a detection at x=1.5 and have recorded the coincidence patterns for a while, right?

1. if we choose which way info...are we likely to detect idler at D1?

No, if you choose to destroy which-way info, you are likely to detect the idler at D1. If you keep which-way info you will see a detection at D3 or D4 with equal probability.

2. if we do not chose which way info...are we likely to detect idler at?

See above.

2b) the signal is on fig 3, suggesting interference and no which way..however if we make which way experiment on idler...then won't signal be saying interference and idler which way?

Well, "signal is on Fig.3" is a sloppy formulation and strictly speaking wrong. You can only be sure that it is likely to end up on fig. 3 if you will erase which-way info on the idler side. Otherwise the corresponding coincidence count cannot end up on Fig. 3 (Fig. 3 are the D0-D1 coincidence counts - if the idler does not go to D1, you are not on Fig. 3). The signal alone does not give you any possibility to choose which-way or interference. In a nutshell the detections on D0 alone are never on a figure. Just the coincidence counts are. However, you can predict where the coincidence counts will end up if you already measured the coincidence count pattern for a while and already decided whether to perform a which-way or interference experiment beforehand.

Please give example of kind and amount of information one can gain (about idler?) from knowing the position of where signal was detected. this will help me understand better.

What I meant was the phase information as seen in the coincidence count interference pattern. This information cannot be gathered by looking at the signal detections alone and it is impossible to get this information by looking at the idler detections alone. It is just present if you have both informations present. However, you can choose to discard this kind of information to get which-way information of the idler instead. However, doing so does not change what happened at the signal side.

interesting...it will take me a few days to understand the paper...thanks for the link and your post

cthugha do you want to take a stab at this and summarize it...?

Well, I often post on topics on delayed choice quantum eraser experiments because it helped me understand coherence, the quantum mechanical meaning of photon bunching and interference of two-photon probability amplitudes in general back in the days of my diploma thesis. Therefore I am quite familiar with that experiment. Regarding the Zeilinger paper you refer to, there are for sure lots of people around here who are able to summarize it much better than I could do.

At first sight it is a different kind of delayed choice experiment. You have two entangled photon pairs and can perform entanglement swapping. Accordingly you can detect two of these photons and can afterwards choose to project the other two photons into a state which should result in also entangling the two photons which are already detected and will find violations of the Bell inequalities if you look for them in the earlier detected photons and do coincidence counting with the other two photons. Although this might seem counterintuitive (from a classical point of view), it is fully consistent with QM. But as I said before: I am sure DrChinese and some others around here will be able to give a much more precise summary of Zeilinger's work.

 

[sanpkl]

Ok, so we have a detection at x=1.5 and have recorded the coincidence patterns for a while, right?

correct.

does this mean the signal photon has a higher probablity of being on figure 3? when its is later correlated with idle

can the probablities of idler striking d1, d2, d3, d4 be, now, calculated
given that: signal was detected at x=1.5?

if you choose to destroy which-way info, you are likely to detect the idler at D1. If you keep which-way info you will see a detection at D3 or D4 with equal probability.

if we leave the "choice" (of "which way" or not) upto the idler photon..(as in the kim scully experiment)...is it likely to be detected at D1 ...instead of d2, d3, d4?

given the context above...i.e. ...signal photon was detected at x = 1.5 mm

 

[Cthugha]

if we leave the "choice" (of "which way" or not) upto the idler photon ...is it likely to be detected at D1 ...instead of d2, d3, d4?

given the context above...i.e. ...signal photon was detected at x = 1.5 mm

maybe its possible to calc the probalitities?

Yes, that is possible. Whether a photon goes to D1/D2 or D3/D4 depends on the splitting ratio of the beamsplitters BSA and BSB (see the setup in the Kim paper). Assuming they are 50/50 beamsplitters, 25% of all photons will go to D3 and another 25% will go to D4. The remaining 50% will go to either D1 or D2. How many of these 50% go to D1 and how many go to D2 can be extracted from figures 3 and 4. Having a look at position x=1.5 again, you see that there are roughly 120 coincidence counts for D1 (fig. 3) and roughly 40 coincidence counts for D2 (fig. 4). so for that portion of the total counts you have a distribution of 75% D1 and 25% D2.

So in total you get:
D1: 75% of 50% =37.5%
D2: 25% of 50% =12.5%
D3: 25%
D4: 25%

 

[sanpkl]

Yes, that is possible. Whether a photon goes to D1/D2 or D3/D4 depends on the splitting ratio of the beamsplitters BSA and BSB (see the setup in the Kim paper). Assuming they are 50/50 beamsplitters, 25% of all photons will go to D3 and another 25% will go to D4. The remaining 50% will go to either D1 or D2. How many of these 50% go to D1 and how many go to D2 can be extracted from figures 3 and 4. Having a look at position x=1.5 again, you see that there are roughly 120 coincidence counts for D1 (fig. 3) and roughly 40 coincidence counts for D2 (fig. 4). so for that portion of the total counts you have a distribution of 75% D1 and 25% D2.

So in total you get:
D1: 75% of 50% =37.5%
D2: 25% of 50% =12.5%
D3: 25%
D4: 25%

Cthugha,

at x= 1.5mm ,

figure 3 = 120, fig 4 = 40

why are fig 5 and fig 6 (not shown) not 80 each?

given that the paper says this is 50/50 beam splitter...

Answer: not all idler photons were able to be collected...fig 3 and 4 are comparable to each other but not comparable with fig 5 and 6


why would figure 6 have a small shift of center relative to figure 5? are the path lengths not same?

 

[Cthugha]

why are fig 5 and fig 6 (not shown) not 80 each?

given that the paper says this is 50/50 beam splitter...

That can have lots of different reasons: beamsplitters, which are not perfect, small absorption of the mirrors, not exactly equal quantum efficiency of the detectors, bad alignment, not ideally chosen time windows for coincidence counting and so on and so on...

why would figure 6 have a small shift of center relative to figure 5? are the path lengths not same?

What makes you think figure 6 would show a shift compared to figure 5?

 

[sanpkl]

Yes, that is possible. Whether a photon goes to D1/D2 or D3/D4 depends on the splitting ratio of the beamsplitters BSA and BSB (see the setup in the Kim paper). Assuming they are 50/50 beamsplitters, 25% of all photons will go to D3 and another 25% will go to D4. The remaining 50% will go to either D1 or D2. How many of these 50% go to D1 and how many go to D2 can be extracted from figures 3 and 4. Having a look at position x=1.5 again, you see that there are roughly 120 coincidence counts for D1 (fig. 3) and roughly 40 coincidence counts for D2 (fig. 4). so for that portion of the total counts you have a distribution of 75% D1 and 25% D2.

So in total you get:
D1: 75% of 50% =37.5%
D2: 25% of 50% =12.5%
D3: 25%
D4: 25%

Continuing with the above experiment...for the millionth plus one signal and idler..where signal photon has been detected at D1..

Case 1: now if we change the beam splitters to 100/0 (where 100 is no which way), what are the probablities?

D1= 0, D2 = 0, D3 = 50, D4 = 50?

Case 2: we add a quantum eraser (the polarizer kind) in the path of idler where we change to no which way. what are the probablities?

D1= 0, D2 = 0, D3 = 50, D4 = 50?

Would adding a quantum eraser (in which we control the choice), say of the polarized kind, in the kim scully experiment work?

 

[Cthugha]

Continuing with the above experiment...for the millionth plus one signal and idler..where signal photon has been detected at D1..

Do you mean D0?

Case 1: now if we change the beam splitters to 100/0 (where 100 is no which way), what are the probablities?

D1= 0, D2 = 0, D3 = 50, D4 = 50?

Do you mean that 100/0 is complete which-way information (No which-way means you get the interference pattern, complete which-way info means no interference pattern)? Then your reasoning is correct.

Case 2: we add a quantum eraser (the polarizer kind) in the path of idler where we change to no which way. what are the probablities?

D1= 0, D2 = 0, D3 = 50, D4 = 50?

Would adding a quantum eraser (in which we control the choice), say of the polarized kind, in the kim scully experiment work?

In principle you can add a polarizer kind quantum eraser. The results will then of course depend on your choice of settings.

 

[sanpkl]

Do you mean D0?




Do you mean that 100/0 is complete which-way information (No which-way means you get the interference pattern, complete which-way info means no interference pattern)? Then your reasoning is correct.



In principle you can add a polarizer kind quantum eraser. The results will then of course depend on your choice of settings.

yes, i meant Do, typing error..sorry

1. thus essentially the signal photon randomly chooses its position on Do and idler's path is determined, in a probabilistic way, by it.?

or better still

the signal and idler are in a balance (momentum balance/conservation)...as a single entity in a sense...as two faces of the same coin...

when the signal strikes Do the information (about signal's "choice/state") is instantaneously transmitted to idler

2. we don't know what the signal photon "decided/choose" ...till we compare with idler?

3. the idea that idler path is influencing the signal photon position on Do (at the moment of impact on Do or even after impact) is wrong?

 

[sanpkl]

Entanglement Hypothesis

Hypothesis (just a hypothesis):

Entanglement of the photons (say signal and idler) happens outside space and time.

Thus some of their "properties" are really not separated by space and time.

Every particle (mass/energy) has waves associated with it. In case of particles larger than a few atoms the wave effect is negligible.

 

[sanpkl]

I think the context of the entire experiment is relevant. Not just the "first" detection. How you interpret the results is dependent on that context, and that will not be known until later - when all of the results can be brought together into a single place. And then it will in fact appear "as if" the past was dependent on the future.

..."

Dr Chinese

i am reading the paper you sent..will get back in 2 days...

yes, good point. agreed. look forward to your comments on the below:

however half of the experiment is done when signal photon strikes D0?

At that moment in time the signal photon has made it's "choice/mark/determination/frozen" of its position.

- the wave-function of the signal photon has collapsed
- this past (signal photon detection position) does not changed ever
- and it will (probabilistically) effect how the idler photon behaves when choosing paths? i.e. the idler gets the information regarding the frozen state of the signal photon (instantaneously)

 

[eaglelake]

cthugha, eagle,

all of the below might have been answered before but i just wanted to go over it again...with a different rephrasing...

with reference to the delayed choice quantum eraser...http://arxiv.org/abs/quant-ph/9903047

1 a) when a signal photon has been detected on Do, has not the pattern of signal photon (Though *unknown* to us, till we compare with idler in coincidence counter) already been fixed/sealed? does that mean same as "experiment is over"?

i understand that a single photon not a pattern make, i am referring to the direction/potential

1. b) once the signal photon is measured is the fate/path of the idler also "somewhat" sealed (with a high probability)?

this would resolve/invalidate the "past can be changed" hypothesis/misunderstanding, i guess

2. once the signal photon is measured, we till don't know which figure 3,4,5,6 would it fall, until we compare with idler?

this would resolve/invalidate the "faster than light information travel" hypothesis/misunderstanding, i guess

3. the position of the signal photon on Do has nothing to do with what is what we are doing to the idler at that point in time (i.e. at the exact time the signal strikes Do). if i remember correctly cthugha said similar.

this would help resolve/invalidate the "past can be changed" hypothesis/misunderstanding, i guess

cthugha wrote
<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">

for me the "70% (or higher than 50%) probablity" explains a lot...

4. i guess that this would also help reduce the need for "many worlds" hypothesis

5. wave function collapses (for both twins-- idler and signal) when either the signal or idler photon is detected?

My response was to the original post which referred to a single photon experiment. Your questions now concern an entangled two-photon experiment. The paper you cite is not an easy read. (At least I did not find it so.) There is a more understandable, non-mathematical, discussion in Walborn et al, arXiv:quant-ph/0503073v1 and also in Roussel and Stefan, arXiv:0706.2596v1. I hope this helps.

Best wishes

 

[sanpkl]

My response was to the original post which referred to a single photon experiment. Your questions now concern an entangled two-photon experiment. The paper you cite is not an easy read. (At least I did not find it so.) There is a more understandable, non-mathematical, discussion in Walborn et al, arXiv:quant-ph/0503073v1 and also in Roussel and Stefan, arXiv:0706.2596v1. I hope this helps.

Best wishes

thanks eagle...

from the walborn cunha paper...

they write..

”Bob, that is amazing! You have control
over the past! While you are at it, can you go back
change my lottery ticket from last week to 67-81-138?,”
Alice asks with a look of awe in her eyes. Bob is loving
the moment, but he is not the greatest magician, and
cannot keep his mouth shut about the secret to his tricks.
”No Alice, look, the photons I gave you were actually entangled
with photons that I kept for myself. I did a series
of polarization measurements, and recorded my results.
My polarization measurements tell me how to divide up
your experimental results so that we can see interference
or not, but I cannot change the position at which any
photon actually landed,” Bob explains. He shows her by
plotting all of the results for which he measured horizontal
OR vertical (orthogonal directions) and they observe
the large mountain peak. He then does the same with
all results of +45◦ and −45◦, and they observe the same
mountain peak. Of course plotting all of the results together
regardless of polarization also gives the mountain
peak, as Alice had already observed. So Bob was not able
to alter the past, it is just that he had more information
than Alice.

thanks eagle...the above confirms/reassures what i said per my understanding of the experiment ...past cannot be changed...

however, in this particular paper/page, they left out the second point...it seems

i.e. the information that Bob has cannot be sent to Alice faster than the speed of light..


the Stefan paper quotes wheeler...and i agree only partially with wheeler...

It
is wrong to speak of the ”route ” of the photon in the experiment of the beam splitter. It is wrong to
attribute a tangibility to the photon in all its travel from the point of entry to its last instant of flight.

the "partial" tangibility is introduced by the fact that we can spot/stop the photon at any point on its path by using the formula time = distance/velocity of light

 

[sanpkl]

perhaps there is a "dimension" in addition to time-space, through which quantum mechanics operates? the "probability wave function" operates in that dimension.

when we try to measure a photon's position, the wave function collapses, the photon moves back into the time-space dimension

 

[eaglelake]

perhaps there is a "dimension" in addition to time-space, through which quantum mechanics operates? the "probability wave function" operates in that dimension.

when we try to measure a photon's position, the wave function collapses, the photon moves back into the time-space dimension

Your word “perhaps” says it all. We are forced to speculate about what is “really happening” because Quantum Mechanics tells us nothing about how the photon gets through the apparatus.

A quantum experiment consists of the entire apparatus, including the photon and the measuring device, as well as the experimental result. Everything is set up in space-time.

Photon detection is a real event that we do observe. But, we have no evidence for any other dimension that might help us understand “what is really happening” to the photon before it is detected. Any discussion of photon behavior prior to detection is pure speculation.

We do know that the wave function is defined in a Hilbert space. But as far as we know, it is not a part of the experiment; no one has ever observed the wave function “moving through” the apparatus. Nor has anyone observed its collapse when the photon is detected.

Please forgive me for being so harsh, but it does seem futile to suggest explanations that have no verifiable evidence to support them. But, you are not alone. There are lots of other bright people expending much time, effort, and brainpower in such endeavors.

Best wishes.