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

1. Feb 9, 2010

### sanpkl

Delayed choice in half silvered mirror or double slit

For now, I am focussing only on the half silvered mirror:

there are two paths/arms that are created.

My question is:

1. If a detector/obstruction is placed on either of the two paths after the photon has passed but before its hit the final detector

- does the interference disappear? ....i would think ofcourse it does

2. If a detector/obstruction is placed after it has hit the final detector but not been measured yet

- does the interference disappear?

Please let me know if there are any clarifications required or if you want to make some corrections to the above experiment

2. Feb 9, 2010

### Cthugha

Re: Delayed choice in half silvered mirror or double slit

I suppose you assume a delayed choice quantum eraser setup like the one used by Kim, Kulik, Shih and Scully (http://arxiv.org/abs/quant-ph/9903047) and when you say "the photon" you only talk about the idler and leave the signal unaltered on its way to detector D0.

Regarding 1):
If you create a situation where you block one arm and can be sure that the photon has already passed the position where you put the block, the interference pattern will still be there if you repeatedly remove and insert the block after the photon has passed - unless of course there is a large difference between the length of both paths.

Regarding 2):
As soon as the photon hits the detector, this is a measurement. It does not really matter, whether you write a number on a hard disk or whether you look at the results. So this case is basically the same as 1).

Last edited: Feb 9, 2010
3. Feb 9, 2010

### SpectraCat

Re: Delayed choice in half silvered mirror or double slit

Yeah ... some clarification is needed ... how do you make an interferometer with just a single half-silvered mirror? You need at least some other regular mirrors in there. Have you looked at Wheeler's original delayed-choice thought experiment? Is that what you are asking about? You need to show us a picture or your setup, or a least give a more detailed description.

That said, I think I can answer your question #2: There is no distinction between a photon "hitting the detector" and "being measured" in QM. They are the same event.

4. Feb 9, 2010

### sanpkl

Re: Delayed choice in half silvered mirror or double slit

Spectra, Cthugha, and others who are interested:

Sorry, I need to describe the setup.
I am talking about 2 experimental setups.
1. Half silvered mirror = Mach–Zehnder interferometer = the link below
http://en.wikipedia.org/wiki/Mach-Zehnder_interferometer
2. Delayed choice quantum eraser = the link below
http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser
3. Double slit delayed choice, we can simply make one setup up where a detector is placed at different point in time during the movement of the photon from the slits to the screen.

First question: Are the three setups essentially the same as far as the conceptual questions we discuss in this thread are concerned?

Second Question: My understanding is that even if one of the paths is blocked or detector is placed after the photon has passed. The interference pattern WILL disappear. This is contrary to what Cthugha is saying.

Next questions: can follow after we come to understanding of the second question.

Also I have posting a new thread on a different topic. Here is an attempt to hypothesize as to what is happening (travel mode of the photon) during the experiment. Look forward to your responses. However let try to stick to the different topics in different threads to reduce confusion.

5. Feb 9, 2010

### sanpkl

This is in reference to the two experiments:
1. Half silvered mirror = Mach–Zehnder interferometer = the link below
http://en.wikipedia.org/wiki/Mach-Zehnder_interferometer
2. Delayed choice quantum eraser = the link below
http://en.wikipedia.org/wiki/Delayed_choice_quantum_eraser

The attempt of this post is to hypothesize on how the photon travels after it
- Exits the double slit or after it hits the FIRST half silvered mirror

Hypothesis 1:
The photon becomes a wave and half wave goes through one slit/path and half through the other

Hypothesis 2:
The photon has both particle and wave at SAME time.
The photon takes one of the two paths and the wave splits into half. Half wave goes through one slit/path and half through the other.

Now to test Hypothesis 1 (or actually even 2) :

Lets say we put a detector on either path :
1. Are we ever able to measure any property of the wave that would suggest its half a wave?
For example intensity is half or some other property is half?
I think the answer is: we either measure single particle or no particle, zero or one but no half.
Is that correct?

Now to test hypothesis 2:

I read somewhere that now they are able to (vaguely?) figure out which slit the photon went through *without* disturbing the interference pattern which is equal to without collapsing the wave function (?).

a) Can someone shed light on that experiment?

b) Also is it true that interference phenomena also works with Bucky balls (a bunch of carbon atoms)?

6. Feb 9, 2010

### sanpkl

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

if i were to summarize the above in a few lines then it would be somthing like:

1. Are we ever able to measure/notice the split in the wave (function) after it has gone through the double slit or the half silvered mirror?

2. Are we vaguely able to tell which path the photon took without collapsing the wave (function)?

7. Feb 9, 2010

### SpectraCat

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

The results of this experiment are known ... you can even find them on wikipedia. They don't shed any light on the questions you are trying to ask. I would also say that your first hypothesis is stated in a non-standard way. Your second hypothesis sounds like a re-phrasing of the deBroglie-Bohm interpretation, except for that stuff about "half-waves" .. I don't know what those are.

I am still unclear on what you are talking about with the half wave stuff. Your description of the possible results of a particle detection are correct; only discrete, whole particles are observed.

I am pretty sure what you describe is impossible based on the laws of QM. You are gonna need more than a "vague" recollection of such an experiment to proceed here. Please describe it more completely, and/or provide a reference.

Yes, the double-slit experiment has been performed for Bucky-balls, and interference is observed.

8. Feb 9, 2010

### ZapperZ

Staff Emeritus
Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

Before you do your "hypothesizing", maybe you should first read this set of experimental results:

1. J.J. Thorn's et al. at http://people.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf
2. T.L. Dimitrova and A. Weis, Am. J. Phys. v.76, p.137 (2008) at http://doc.rero.ch/lm.php?url=1000,43,2,20080918095145-UZ/weis_wpd.pdf
3. https://www.physicsforums.com/showpost.php?p=1246862&postcount=37

Zz.

9. Feb 9, 2010

### Cthugha

Re: Delayed choice in half silvered mirror or double slit

No, they are pretty different. A Mach-Zehnder interferometer measures single photon interferences. A delayed choice quantum eraser measures two-photon interferences. This means you create two photons with a fixed phase relationship and create interferences in the joint detection probability. Accordingly you will only see interference patterns in the COINCIDENCE DETECTIONS of two detectors in a DCQE setup, but never on one detector alone.

Well, this question is not exactly trivial. You will need not just one photon, but a lot of photons to build up the interference pattern, so there are several possibilities.

a) You have a stream of photons present. In this case blocking one arm will of course prevent further build-up of the interference pattern.

b) You repeatedly fire single photons and place the block in one arm after you are sure the photon has passed and remove it again before the next photon arrives. As a result there will be some interference pattern. However, the path differences between both arms must be very small as the duration of a single photon pulse in the time domain is usually very short - somewhere in the femtosecond range. If the difference in travel time between both arms is longer than that, you will never see interference - no matter whether one arm is blocked or not.

10. Feb 9, 2010

### Cthugha

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

Well you cannot do that without disturbing the interference pattern, but you can do it without destroying it. In fact distinguishability $$D=|P_A -P_B|$$ of the two possible paths the photon can take (where $$P_A$$ is the probability that the photon took one path and $$P_B$$ is the probability that it took the other path) and the visibility V of the interference pattern are related by the Englert–Greenberger duality relation:
$$D^2+V^2\le1$$

So you can have some knowledge of the photon path (for example a 75% chance to go one way and a 25% chance to go the other way) at the cost of reduced visibility of the interference pattern. However, you cannot get both at the same time with arbitrary precision.

11. Feb 9, 2010

### sanpkl

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

Spectra cat below is one of the article which is claiming both which way and interference pattern at same time, however its not the article i saw earlier ..still searching for it..

http://www.dipankarhome.com/Non%20Classical%20interence%20and%20which%20path%20information%20in%20a%20gamma%20angular%20correlation%20experiment%20using%20a%20heavy%20ion%20orbiting%20reaction.pdf [Broken]

Last edited by a moderator: May 4, 2017
12. Feb 9, 2010

### sanpkl

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

Cthugha, thanks for clarifing that.....the 75%, 25% thing. I got it. thats what that article meant. I understand it now from you. thanks.

I am still thinking about your comment that interference pattern will stay....will get back a bit later...

13. Feb 9, 2010

### sanpkl

Re: Delayed choice in half silvered mirror or double slit

Cthuga,

i did not understand what you meant by path difference should be very small in the below quote. I thought the paths are same length in the Mach-Zehnder_interferometer.

http://en.wikipedia.org/wiki/Mach-Zehnder_interferometer

lets assume each path is 8 light minutes long (i.e. about the distance between sun and earth).

after say about 1 light minute distance we place a block on one of the paths however we place it after 1 light minute...i.e. after the photon has supposed crossed that point...

and we keep doing it (after every individual photon has passed) as you suggested in the quote below....

my understanding is that we won't see a interference pattern.

2. Now a modifications to the experiment....we remove a detector (erase which way information) after a photon has crossed a certain point...say...placed at 1 light minute distance...

in this case, my understanding is that....interference pattern willl happen. of course we have to do it photon at a time to build the pattern.

thus if we erase which way information , even after a photon has crossed the point (where we setup our appratus to determine which way information) the interference pattern will happen.

in short: if we can surely erase ( or vice versa...i.e. bring back )which way infromation even after the photon has passed....we will

1. get interfernce pattern if which way info is erased
2. no get interfernce pattern if which way info is recreated

this is true even after the photon has crossed the "observation point".

______

b) You repeatedly fire single photons and place the block in one arm after you are sure the photon has passed and remove it again before the next photon arrives. As a result there will be some interference pattern. However, the path differences between both arms must be very small as the duration of a single photon pulse in the time domain is usually very short - somewhere in the femtosecond range. If the difference in travel time between both arms is longer than that, you will never see interference - no matter whether one arm is blocked or not.[/QUOTE]

14. Feb 9, 2010

### SpectraCat

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

I haven't had time to read that paper in detail, but I have three observations after skimming it.

1) The authors of the paper are interpreting someone else's data and experiment ... that is fine in principle, but it means that their logic should be carefully checked before accepting their conclusions, because they aren't bringing anything new to the table except an interpretation. In this case, their interpretation is based on a whole slew of extrapolations and justifications as described on page 35, none of which I am prepared to concede are correct without careful thought.

2) I am not sure if this really qualifies as an example of what you are talking about ... I did not see any clear explanations in there of why the gamma ray bursts emitted by the carbon nuclei should provide "which path" information in any context analogous to a double-slit experiment.

3) The following quotation from their conclusions on p. 36 is puzzling, and makes me wonder how well they understand what they are talking about: (italics are the authors' emphasis).

"We emphasize that here this particular feature is consistent with the formalism of quantum mechanics. Note that the interference here cannot be explained by using any classical picture. This experiment therefore highlights that it is classical wave-like interference and “which path” information which are mutually exclusive and not any interference in general."

Last edited by a moderator: May 4, 2017
15. Feb 9, 2010

### Cthugha

Re: Delayed choice in half silvered mirror or double slit

You can build it this way, but it is not necessarily so. You can create small differences. These are usually not important, but they can be when it comes to short pulses.

Why should we not? As the photon has already passed you will get no which-way information by putting the block there.

No, putting and removing a position detector there will not work. As soon as a photon interacts in an irreversible way with a detector giving which-way information you will not be able to restore the interference pattern. In order to do so you need a reversible which-way marker. Usually you have photons of known polarization and use a plate causing a rotation of the polarization as a which way marker. Now by measuring the polarization at the detector you will know, which way the photon went and there will be no interference pattern. However, you can recover indistinguishability of both paths again by inserting a filter in the recombined beam, which transmits both polarizations with 50% probability and absorbs them with the same probability. In this case the which-way information has been erases again and the pattern will appear.

16. Feb 9, 2010

### sanpkl

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

SpectraCat,

We can ignore/skip/discard the below article. Cthuga has clarified the issue....namely you can change the probablities between the slits and this will disturb the interference pattern.

However we cannot get both (namely which way and interference pattern) with high precision.

So we can close this particular point.

Thanks,

San

17. Feb 9, 2010

### sanpkl

Re: Delayed choice in half silvered mirror or double slit

cthugha, i agree with all of what you wrote below.

i have a few questions regarding the reversible which way marker.

let abbreviate....i.e. RWWM = reversible which way marker

pls make modifications/corrections/caveats where required

1. the RWWM keeps the **ability/information** to find (or erase) which way information

2. however which way information can only be obtained after the photon has hit the screen/detector?

3. if at any point which way information is actually obtained the wave function would collapase and is **irreversible**

4. thus essentially the RWWM just keeps the ability but not the actual information on which path the photon took....till the photon actually hits the detector/screen

5. the actual path the photon took can be determined after it has hit the screen (and of course if the RWWM has not been used to erase the info)

6. i did not understand the part where something (entangled photons?) have to be compared in the coincidence counter to arrive at results/which way info.

18. Feb 9, 2010

### sanpkl

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

I am revising/modifying 4 since i think its not correct

4. thus essentially the RWWM just keeps the ability however if we make use of that ability then the wavefunction would collapse irreversibly....

4 a) can we find which way info before the photon hits the detector via the RWWM?

19. Feb 10, 2010

### Cthugha

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

1):
Yes, I think so.

2):
Yes, however, one should note that any irreversible interaction can be considered as a detector, whether it is the screen or some random dust particle in the air, which absorbs the photon.

3):
In principle yes. But let me emphasize that it does not matter, whether you (as a person) really obtain the information or not. It is enough that some irreversible interaction happened, which could enable you to obtain the information.

4):
Basically yes. There are interpretations of QM, which do not describe this act as a wave function collapse, but the observable physics stay the same. SO I agree.

4)a):
If you consider each irreversible interaction as a detector then you cannot get COMPLETE which way information before the photon hit the detector. However, you can of course get some which-way information for example by weak measurements or simply by setting up the experiment in such a way that the photon is more likely to go one way.

5):
Ok.

6):
The quantum eraser experiments are a bit more difficult than common double slit interference experiments. Here you entangle two photons and have one interact with a detector. Now you can choose, whether you can get an interference pattern or not by making a choice on what you do with the other photon after the first one has been detected. However, what is often not told is that the interference cannot be seen as a pattern on the screen where the first photon is detected, but in the joint count rates of both photons. Basically this is a kind of subsampling giving results like:

If I detected the other photon at this special position at the detector, the corresponding simultaneous detections of the first photon at the first detector will show an interference pattern. If I detected the other photon at a different position at the detector, the corresponding simultaneous detections of the first photon at the first detector will show a different interference pattern. If I superpose all of these patterns the result will be washed out and show no pattern at all. So in DCQE experiments you will see interferences only in measurements of the joint detection rates of two entangled photons.

20. Feb 10, 2010

### DrChinese

Re: Hypothesizing on photon mode of travel in double slit or similar experimental set

For anyone interested, here is a reference on the above:

http://arxiv.org/abs/0807.5079" [Broken]

"A recent experiment performed by S. S. Afshar et al. has been interpreted as a violation of Bohr's complementarity principle between interference visibility and which-path information in a two-path interferometer. We have reproduced this experiment, using true single-photon pulses propagating in a two-path wavefront- splitting interferometer realized with a Fresnel's biprism, and followed by a grating with adjustable transmitting slits. The measured values of interference visibility V and which-path information, characterized by the distinguishability parameter D, are found to obey the complementarity relation V^2+D^2=<1. This result demonstrates that the experiment can be perfectly explained by the Copenhagen interpretation of quantum mechanics. "

Last edited by a moderator: May 4, 2017