Double Slit Experiment and Quantum Eraser

[DylanLou]

Hello everybody,

I am absolutely a novice in physics and although I generally have a good grasp of math I am pretty sure my knowledge of it is quite far from the one required by quantum physics. However, I am very interested to its main concepts and for this reason I am reading an introductory book on the subject: "How to teach quantum physics to your dog" by Chad Orzel which I think is a very good source for novices to learn at least the basics theories.
However, I couldn't properly understand the quantum eraser experiment and, so I was wondering if you guys could help me with that.

Surfing on the internet I saw that there are many different versions of the experiment and the most discussed one seems to be "the delayed choice quantum eraser". However this is the version explained on the book (or at least that is my understanding of it) for which I couldn't find any other explanation online: If we cast a beam of light polarized at 45° through two slits, one where we put a vertical polarizer and the other where we put a horizontal polarizer, there is a 50% possibility for each photon to pass through the right slit and a 50% possibility to pass through the left slit. When we do this we don't see the production of an interference pattern.

I couldn't understand why should it be "unexpected" not to observe an interference pattern in this case.

Namely, if the photon will pass through one of the two slits but not the another one (as I understood is the case when we put polarizers) it is like saying that we are "forcing" the photon to pass through just one slit and, therefore there will be no interference with "the photon with itself" due to its wave nature (as it happens when there are no polarizers). So it seems to me pretty natural that we don't see an interference pattern (it is a distruptive effect of our measurement device that again "forces" the photon to pass from just one slits when it normally would have passed from both).

If the photon will still pass through both the slits even with polarizers (assuming that this is possible which I don't know) then the "two" photons (it is still the same one but sort of "duplicated" for the effect of its wave nature) will be one horizontally polarized and the other vertically polarized and, so there will be no interference between the two waves and again no interference pattern.

As you noticed, my doubts are related to the part before the actual experiment of "quantum erasure" that happens when we put another polarizer at 45° degree before the detector and we see the reoccurence of the interference pattern.

Thank you all in advance for your answers.

 

[StevieTNZ]

If the photon will still pass through both the slits even with polarizers (assuming that this is possible which I don't know) then the "two" photons (it is still the same one but sort of "duplicated" for the effect of its wave nature) will be one horizontally polarized and the other vertically polarized and, so there will be no interference between the two waves and again no interference pattern.

IF the photon passes through both slits, then it is in a superposition of V and H polarisation.

As you noticed, my doubts are related to the part before the actual experiment of "quantum erasure" that happens when we put another polarizer at 45° degree before the detector and we see the reoccurence of the interference pattern.

With the extra 45 degree polariser, we 'erase' whether the photon came from the H polarised slit, or the V polarised slit. Therefore, we are unable to tell which path it came along - it is in superposition of going along both paths -> leads to interference. Note that there are two interference patterns: one fringe and one anti-fringe. Adding both fringes together produces a scatter pattern (the pattern seen if the photon went along one path).

Let me know if my answers help/don't help/need clarification.

 

[DylanLou]

Thank you Stevie!

So, if I got it right, the photon passes through both the polarizers and it goes into a superposition state of V and H polarization (thus producing an interference pattern) but this only happens when there is something (the extra polarizer in this case) that erases the information we got on its path. Otherwise the simple fact that we can measure/observe which slit it went through will lead the photon behave as a "normal" particle and the interference pattern will not occur. Is that right?

And, that also means that two waves with different polarization still can interfer with each other, don't they?

 

[zonde]

However this is the version explained on the book (or at least that is my understanding of it) for which I couldn't find any other explanation online: If we cast a beam of light polarized at 45° through two slits, one where we put a vertical polarizer and the other where we put a horizontal polarizer, there is a 50% possibility for each photon to pass through the right slit and a 50% possibility to pass through the left slit. When we do this we don't see the production of an interference pattern.

This experiments sounds similar to Quantum eraser experiment

I couldn't understand why should it be "unexpected" not to observe an interference pattern in this case.

Namely, if the photon will pass through one of the two slits but not the another one (as I understood is the case when we put polarizers) it is like saying that we are "forcing" the photon to pass through just one slit and, therefore there will be no interference with "the photon with itself" due to its wave nature (as it happens when there are no polarizers). So it seems to me pretty natural that we don't see an interference pattern (it is a distruptive effect of our measurement device that again "forces" the photon to pass from just one slits when it normally would have passed from both).

If the photon will still pass through both the slits even with polarizers (assuming that this is possible which I don't know) then the "two" photons (it is still the same one but sort of "duplicated" for the effect of its wave nature) will be one horizontally polarized and the other vertically polarized and, so there will be no interference between the two waves and again no interference pattern.

As you noticed, my doubts are related to the part before the actual experiment of "quantum erasure" that happens when we put another polarizer at 45° degree before the detector and we see the reoccurence of the interference pattern.

Thank you all in advance for your answers.

Can't comment much about "unexpectedness" however. It the author of the book thought that horizontally polarized light should coherently overlap with vertically polarized light we could be interested about his argumentation.
Your argumentation seems to be something loosely along these lines - oscillations of H and V photons don't overlap spatially so there is no interference, right?

Of course if don't have convincing argumentation we can accept as experimental fact that H and V photons do not "overlap" and continue our reasoning from here.

 

[SergioPL]

I agree with Zonde since the interference of the photon passing through both slits pattern behaves quite much as an electromagnetic field and therefore if the polarization on both slits is perpendicular, it would not be interference at all.

 

[DylanLou]

Thank you guys,

This experiments sounds similar to Quantum eraser experiment

It is different, the one you posted involves entangled photons which are sent into two different directions.

It the author of the book thought that horizontally polarized light should coherently overlap with vertically polarized light we could be interested about his argumentation.

Well, I would not confidently say that this is what the author of the book thinks. As I said I am a complete novice to the topic so it is very likely that if there is something that happens strange or wrong it is due to a mistake in my interpretation and not to the author. However, the book doesn't address the question of interference or overlapping between H and V polarized light.

I agree with Zonde since the interference of the photon passing through both slits pattern behaves quite much as an electromagnetic field and therefore if the polarization on both slits is perpendicular, it would not be interference at all.

This was my second question indeed. So, V polarized light doens't interfer with H polarized light and doesn't produce any interference pattern? (but in that case it would be completely normal not to observe interference pattern in the experiment)

 

[zonde]

This was my second question indeed. So, V polarized light doens't interfer with H polarized light and doesn't produce any interference pattern? (but in that case it would be completely normal not to observe interference pattern in the experiment)

Unfortunately I don't know about any reference to experiment demonstrating that in straight forward fashion. So maybe someone else will write something about that.
I am assuming that like a working hypothesis and it seems fine so far but it would be much nicer to get some reference about experiment that tested this.

 

[micky_gta]

The way they explain quantum eraser in Wikipedia and from a video I saw is a little different.

First put some type of measuring device at the slits to find out what slit(s) the photons go through (for the sake of this explanation I will call it a digital camera)

Second we conduct the experiment and will notice that the photons go through the slits and land on the photo paper to produce a 2 slit pattern (this is what's expected when recording what slits they go through) not an interference pattern.

Now once the experiment is finished you can safely remove part of the evidence (the resulting image on the photo paper - two slit pattern), the photo paper, and place it safely in a vault or in another building or another country( actually anywhere in the universe).

We then go back to our digital camera that 'recorded' what slits the photons went through. At this point we can use the digital camera to prove objectively what slits the photons went through.

Now take a big hammer and smash the **** out of this camera so that it is non functional and cannot be used to prove anything about the experiment.

Then we go back and get the 'test results' of the photo paper of the original experiment and... wait for it... yes! an interference patter! hope this helps!

 

[Cthugha]

The way they explain quantum eraser in Wikipedia and from a video I saw is a little different.
[...]

Now take a big hammer and smash the **** out of this camera so that it is non functional and cannot be used to prove anything about the experiment.

Then we go back and get the 'test results' of the photo paper of the original experiment and... wait for it... yes! an interference patter! hope this helps!

That explanation is not only different, but also completely wrong. Smashing your measurement device does not change anything and does not affect detector recordings in the past or interference patterns seen somewhere else. In these forums it is a good rule that references should be peer reviewed and "some video" obviously does not qualify. Let me guess: it was either "doctor quantum" or "what the bleep do we know" or some similar crackpot nonsense, right?

 

[DylanLou]

The way they explain quantum eraser in Wikipedia and from a video I saw is a little different.

First put some type of measuring device at the slits to find out what slit(s) the photons go through (for the sake of this explanation I will call it a digital camera)

Second we conduct the experiment and will notice that the photons go through the slits and land on the photo paper to produce a 2 slit pattern (this is what's expected when recording what slits they go through) not an interference pattern.

Now once the experiment is finished you can safely remove part of the evidence (the resulting image on the photo paper - two slit pattern), the photo paper, and place it safely in a vault or in another building or another country( actually anywhere in the universe).

We then go back to our digital camera that 'recorded' what slits the photons went through. At this point we can use the digital camera to prove objectively what slits the photons went through.

Now take a big hammer and smash the **** out of this camera so that it is non functional and cannot be used to prove anything about the experiment.

Then we go back and get the 'test results' of the photo paper of the original experiment and... wait for it... yes! an interference patter! hope this helps!

If I understood correctly what your reference says is that the photo paper we had from the past experiment will "magically" change from a scatter pattern to an interference pattern right?

Again, I am a novice but I think this is incorrect and not the sense of the quantum eraser experiment. It is impossible for a PAST experiment to change result!

However, going back to the formulation of the experiment that I posted it seems to me that either one case or the other:

1) two waves H and V polarized can interfer with each other and, therefore the only reason why we don't see an interference pattern is that we "let the wavefunction" to collapse when we try to observe which slit the photon went through (and, in that case the experiment succeed to demonstrate the active nature of a measurment on quantum physics) or,

2) two waves two waves H and V polarized cannot interfer with each other and so it is obvious that we will not see an interference pattern (and the experiment is not an evidence of the active nature of the measurement).

Of course, I still think that there is something that I am missing, because the author seems to be someone who knows what he is talking about and, so if it is true that two waves differently polarized do not interfer with each other there must still be a reason why it is "unexpected" not to see interference pattern.

 

[micky_gta]

DylanLou you are wrong on this "It is impossible for a PAST experiment to change result! " YES that's how weird the quantum eraser and double slit experiment is. Ask you teacher! :)

Cthugha you got a lot to learn about reality. Unfortunately our science is based on many presumptions that cannot be proven. The NEW quantum physics is revealing this. Most physics books are going to be thrown out and rewritten. Just ask Leonard Susskind and many other teachers at Harvard and Stanford universities.

 

[micky_gta]

BTW just read this what do you think it means (from wikipedia)

Delayed choice and quantum eraser variations
The delayed-choice experiment and the quantum eraser are sophisticated variations of the double-slit with particle detectors placed not at the slits but elsewhere in the apparatus. The first demonstrates that extracting "which path" information after a particle passes through the slits can seem to retroactively alter its previous behavior at the slits.

>>> The second demonstrates that wave behavior can be restored by erasing or otherwise making permanently unavailable the "which path" information.(smashing the **** out of the camera) <<<

MAGIC! ;)

 

[StandardsGuy]

BTW just read this what do you think it means (from wikipedia)

Your explanation is much different that Wikis. It suggests that the past is changed, but you have done two different experiments. I want you to perform this 'experiment' and then tell us the result (and how much you spent on the camera). We will only accept your answer if you post a picture of the results and the smashed camera.

 

[DylanLou]

BTW just read this what do you think it means (from wikipedia)

Delayed choice and quantum eraser variations
The delayed-choice experiment and the quantum eraser are sophisticated variations of the double-slit with particle detectors placed not at the slits but elsewhere in the apparatus. The first demonstrates that extracting "which path" information after a particle passes through the slits can seem to retroactively alter its previous behavior at the slits.

>>> The second demonstrates that wave behavior can be restored by erasing or otherwise making permanently unavailable the "which path" information.(smashing the **** out of the camera) <<<

MAGIC! ;)

I don't know anything about the delayed-choice experiment, so I cannot say anything on that.

In the case of the quantum eraser, however, I think you are misinterpreting what it tells us: what is surprising is that when you put a device (such as the ulterior 45° polarizer in my formulation of the experiment) that will cancel out again every information you can obtain on the "which-path", you can see when you perform the experiment again the re-emergence of the interference pattern.

This demonstrates that the disappereance of the interference was not due to an effect of our measurement device but to the fact that it is impossible for us to gain at the same time information on the which path and to have an interference pattern.

To put it in another terms: when we try to measure which path the photon went through the photon will effectively behave like a particle (in the classical sense) and will produce a scatter pattern. When we "erase" the which-path infomration we obtained by putting another polarizer that will act indeed as an "eraser" (because both H and V polarized light can pass through a 45° polarizer) by making for us impossible again to understand where the photon passed, the photon will again behave like a wave and will produce an interference pattern.

However, these are two subsequent experiment. It is not that when we put the extra polarizer the scatter pattern we had from the last experiment will be transformed in an interference pattern. there will be interference pattern if we make the experiment again with the extra polarizer.

It is not a matter of smashing out the measuring device. It works, from what I understood, as a limit, in some sort of way, to our experiments (and to our knowledge if you like it).

hope this is usefull.

 

[DylanLou]

Hi guys,

I managed to find an explanation of the experiment given by the author himself on his blog. I am sorry I didn't manage to find it before. Apparently he was answering a guy who had my same doubts about the quantum eraser experiment. In particular he too pointed out how the disappereance of an interference pattern when we put polarizers is perfectly explained by the fact that ortogonally polarized waves don't interfer with each other.

As it is apparent form the explanation given, the turning point is that the polarizers are putted after the slits. In that case an interference pattern should have still be seen (because the photon in superposition would have produced interference immediately after passing thruogh the slits and before "hitting" onto the polarizers, exactly as it is expected by a wave).

However this is the question (in bold) and the answer as they could be found at the link http://scienceblogs.com/principles/2009/08/physics_for_dragons.php

I don't get what's so weird about the Quantum Eraser, other than things that are already weird in simpler double-slit experiments in any case. Once you've accepted that a photon can interfere with itself, are the results of the quantum eraser experiment not simply what you'd expect? Even in classical physics, surely, light only interferes with light that's similarly polarised.

The quantum eraser experiment is used as an example in Chapter 3, which talks about superposition states and the Copenhagen Interpretation (you can watch a video version of the dog dialogue that goes with Chapter 3). The idea of the quantum eraser, for those who aren't familiar with it is that you take a double slit experiment and "tag" the light passing through each slit by putting a horizontal polarizer over one slit and a vertical polarizer over the other. This destroys the interference pattern that you get with a normal double slit. TYou can recover the pattern, though, by putting a polarizer after the slits that is aligned at 45 degrees from vertical, passing half of the light from each slit.

(You can do this yourself with a laser pointer and some cheap polarizing film.)

Now, it's true that, as Adrian notes, this is what you expect in the classical wave model of light: the waves from the two slits are orthogonally polarized, and do not produce a visible interference pattern. What's weird about the quantum eraser is that it does exactly the same thing when you send the light in one photon at a time: a single photon interferes with itself or not depending on the presence or absence of a polarizer after the slits. If you think of light as a particle, this is extremely odd: the particle either decides to take both paths and interfere, or to take only one path and not interfere, depending on something that it doesn't encounter until after it has passed the slits.

That's strange on a few levels, and highlights the fact that photons aren't really particles in the same way that squeaky chew toys are particles. They're a new kind of object, with some particle and some wave properties at the same time. And that's why the quantum eraser is weird.

After having thought about it, this seems to me to clear my doubts and to explain what is weird and unexpected about the quantum eraser experiment. But, perhaps, you guys have a different opinion.

 

[Cthugha]

Cthugha you got a lot to learn about reality. Unfortunately our science is based on many presumptions that cannot be proven. The NEW quantum physics is revealing this. Most physics books are going to be thrown out and rewritten. Just ask Leonard Susskind and many other teachers at Harvard and Stanford universities.

This is a response which I would typically report. Comments like "all old physics is nonsense and we have to rewrite everything" are just the kind of new age crackpot claims that are notallowed here. And I do not know what I have to learn about reality. I have already performed a quantum eraser experiment myself. Have you?

BTW just read this what do you think it means (from wikipedia)

Delayed choice and quantum eraser variations
The delayed-choice experiment and the quantum eraser are sophisticated variations of the double-slit with particle detectors placed not at the slits but elsewhere in the apparatus. The first demonstrates that extracting "which path" information after a particle passes through the slits can seem to retroactively alter its previous behavior at the slits.

>>> The second demonstrates that wave behavior can be restored by erasing or otherwise making permanently unavailable the "which path" information.(smashing the **** out of the camera) <<<

Making permanently available means that you erase the possibility to perform a measurement that would give you which-way information. In other words, you put a horizontal and a vertical polarizer in front of the two double slits and noe have the possibility to acquire which-way info. Now you place another polarizer at 45 degrees between horizontal and vertical polarization behind the slits. Light from both slits has the same chance of passing this polarizer and there is no which way information left. If you now perform a measurement using the light from the last polarizer you will get an interference pattern. That is all there is. Smashing devices does not help and does not alter the past. Besides, that wikipedia article is badly written just like many of them. Better have a look at real delayed choice experiments and proposals like in:
V. Jacques et al., Science 315 966-968 (2007)
R. Ionicioiu and D. R. Terno, Phys. Rev. Lett. 107, 230406 (2011)
Y.-H. Kim et al., Phys. Rev. Lett. 84, 1–5 (2000)

So, do you have any peer reviewed evidence backing up your claim?

After having thought about it, this seems to me to clear my doubts and to explain what is weird and unexpected about the quantum eraser experiment. But, perhaps, you guys have a different opinion.

Just to give some additional perspective, I personally think it is instructive to place the quantum eraser experiments in their historical context. As explained well in the review article of Marian O. Scully, Berthold-Georg Englert & Herbert Walther ("Quantum optical tests of complementarity", Nature 351, 111 - 116 (1991)), the original intent of quantum eraser experiments was to clarify the link between co plementarity and uncertainty. Most thought experiments on complementarity can be reduced to the fact that you cannot get perfect which-way information and interference pattern visibility simultaneously due to the uncertainty principle. Any attempt to find out which slit a particle went through would disturb it so much that the interference pattern gets lost. This is obviously not the case in the quantum eraser setup, so back in its time it aimed at the question whether complementarity or uncertainty is more fundamental. That sparked quite a debate going back and forth.

 

[zonde]

As it is apparent form the explanation given, the turning point is that the polarizers are putted after the slits. In that case an interference pattern should have still be seen (because the photon in superposition would have produced interference immediately after passing thruogh the slits and before "hitting" onto the polarizers, exactly as it is expected by a wave).

No, polarizers are placed before slits but before detection there is another polarizer at 45° so that H and V modes are converted (with 50% loss) into the same polarization.

 

[micky_gta]

BTW it's not the detectors themselves that are changing the outcome of the experiments. The detectors can be switched on and the double slit pattern will still show up. Its the 'knowing' (usually by a human) that changes the outcome. In other words the information that the detectors can pick up must be sent somewhere so a person can read/see and know the information of where the particle passed.

 

[DrChinese]

BTW it's not the detectors themselves that are changing the outcome of the experiments. The detectors can be switched on and the double slit pattern will still show up. Its the 'knowing' (usually by a human) that changes the outcome. In other words the information that the detectors can pick up must be sent somewhere so a person can read/see and know the information of where the particle passed.

This is a very poor description of what is going on. It is true (by definition) that you can claim ANY behavior is not real unless a human observes it. On the other hand, there is not the slightest evidence of this otherwise.

The standard criteria is the "possibility in principle" of knowing the which-path information, regardless of whether or not anyone is able to actually determine such. If you could potentially know which-slit info, there is NO interference. This is true even if the observing human has not even hooked up anything to actually learn the outcome.

 

[Cthugha]

BTW it's not the detectors themselves that are changing the outcome of the experiments. The detectors can be switched on and the double slit pattern will still show up. Its the 'knowing' (usually by a human) that changes the outcome. In other words the information that the detectors can pick up must be sent somewhere so a person can read/see and know the information of where the particle passed.

This is still wrong and on the level of crackpot sites like bottomlayer. For the last time: Post some peer reviewed evidence that there is some truth to your claims or stop posting wrong stuff. Hint: You will not post peer-reviewed evidence as there is none. Or maybe you can just explain where exactly the flaw in the Ionicioiu or Kim papers is when they explain why there is obviously no retrocausality.

No, polarizers are placed before slits but before detection there is another polarizer at 45° so that H and V modes are converted (with 50% loss) into the same polarization.

In principle both will work as long as the polarizers are placed so close to the slit position that actually only light going through one slit will pass through one polarizer, but light from the other slit will not. But indeed it is quite typical to place them before the slits.

 

[zonde]

In principle both will work as long as the polarizers are placed so close to the slit position that actually only light going through one slit will pass through one polarizer, but light from the other slit will not. But indeed it is quite typical to place them before the slits.

You are right of course.
But I suppose that in this sentence from authors explanation:
"The idea of the quantum eraser, for those who aren't familiar with it is that you take a double slit experiment and "tag" the light passing through each slit by putting a horizontal polarizer over one slit and a vertical polarizer over the other."
"over" means before, right? ... like a cap, when we look from direction of source. You too are not native English speaking person but I suppose it's not so much English as common visualization of setup.

 

[USeptim]

I don't know anything about the delayed-choice experiment, so I cannot say anything on that.

In the case of the quantum eraser, however, I think you are misinterpreting what it tells us: what is surprising is that when you put a device (such as the ulterior 45° polarizer in my formulation of the experiment) that will cancel out again every information you can obtain on the "which-path", you can see when you perform the experiment again the re-emergence of the interference pattern.

This demonstrates that the disappereance of the interference was not due to an effect of our measurement device but to the fact that it is impossible for us to gain at the same time information on the which path and to have an interference pattern.

To put it in another terms: when we try to measure which path the photon went through the photon will effectively behave like a particle (in the classical sense) and will produce a scatter pattern. When we "erase" the which-path infomration we obtained by putting another polarizer that will act indeed as an "eraser" (because both H and V polarized light can pass through a 45° polarizer) by making for us impossible again to understand where the photon passed, the photon will again behave like a wave and will produce an interference pattern.

The quantum eraser demonstrates what I though, that the photon behaves like classical electromagnetic field until it is absorbed or scattered by interaction with some particle, then it shows its quantum nature. By the way, I'm SergioPL, I've changed my used because I prefer not to use my name.
The case is that when two paths are merged in the eraser both decrease its intensity by the same factor 1/2 and both get the same polarization so the interference patter appears again.

 

[micky_gta]

This one isn't bad << link removed by Moderator >>

 

[DylanLou]

No, polarizers are placed before slits but before detection there is another polarizer at 45° so that H and V modes are converted (with 50% loss) into the same polarization.

In principle both will work as long as the polarizers are placed so close to the slit position that actually only light going through one slit will pass through one polarizer, but light from the other slit will not. But indeed it is quite typical to place them before the slits.

But if you place polarizers before the slits then it will be "normal" not to observe an interference pattern. The photon will pass just through one slit instead of two as it would have done without polarizers. Namely, it would be our measurement device which would have force the photon to go through just one slits or, there will be no interference pattern because after coming out from the slits it will be in a superposition of H and V polarization which doesn't produce any interference.

In that case we will go back to the same exact situation as before: the disappereance of the interference pattern is due to our measurement device.

If it is true that polarizers are placed after the slits instead (but from what I interpreted, far enough that the photon will still be able to go into superposition and interfere with itself), then the disappeareance of the interference pattern cannot be explained by an effect of our measurement device. It will therefore, succeed to explain how it is fundamentally (that is in a way that does not depend by the device used but by the reality itself) impossible for us to determine the position and to have interference pattern at the same time. And this is the point that the author tries to convey in the book.

Please note that I am talking about the double slit experiment before we place the extra 45° polarizer before the detector. The point is whether is "normal" (that is explanable in the classical way) or not to observe the disappereance of the interference pattern when we put the two polarizers at the slits.

Am I missing something?

 

[Cthugha]

This one isn't bad [removed crackpot link

Especially the written version is a bad misrepresentation of Walborn's PRA paper. For example they do not even mention that the interference pattern will only show up in coincidence counting, but never in one arm of the experiment alone. The final remarks on the importance of consciousness are not tenable. Also, you do not really think a video linking to a website called "genesismission" can be taken seriously, do you?

"The idea of the quantum eraser, for those who aren't familiar with it is that you take a double slit experiment and "tag" the light passing through each slit by putting a horizontal polarizer over one slit and a vertical polarizer over the other."
"over" means before, right? ... like a cap, when we look from direction of source. You too are not native English speaking person but I suppose it's not so much English as common visualization of setup.

You are right. I am not a native English speaker, too, but I would also interpret the way you do.

But if you place polarizers before the slits then it will be "normal" not to observe an interference pattern. The photon will pass just through one slit instead of two as it would have done without polarizers. Namely, it would be our measurement device which would have force the photon to go through just one slits or, there will be no interference pattern because after coming out from the slits it will be in a superposition of H and V polarization which doesn't produce any interference.

Oh, I think I get your misunderstanding now. In some sense the photon will always go through both slits at once. Whether or not you see an interference pattern just depends on whether the several possible pathways that could lead to a detection event at some position are in principle distinguishable or not. If you place the polarizers in front of the slits, the photon will go through both slits and the probability amplitude for detecting a photon at some detector position will be the probabiklity amplitudes of the processes of having a horizontally polarized photon which went through slit 1 and a vertically polarized one which went through slit 2. If you performed a measurement now, you would just get a single slit diffraction pattern and could in principle backtrack which photon went through which slit. However the superposition of both ways being taken breaks down only at the moment you perform a measurement which allows you to erase any which-way marker by placing the additional polarizer. This breakdown at the time of measurement is analogous to wavefunction collapse in the famous Schrödinger cat situation.

If you are interested in the ontology of the situation, I can only say that it is quite difficult to say what really happened to the photon in between. In standard qm formulation it does not have too well defined properties in between measurements. However, there are also different interpretations physically indistinguishable from that standard one which "divide" the photon into an ontological particle and a guiding wave. However, I am no expert on that and there are others who can say more about that.

If it is true that polarizers are placed after the slits instead (but from what I interpreted, far enough that the photon will still be able to go into superposition and interfere with itself), then the disappeareance of the interference pattern cannot be explained by an effect of our measurement device. It will therefore, succeed to explain how it is fundamentally (that is in a way that does not depend by the device used but by the reality itself) impossible for us to determine the position and to have interference pattern at the same time.

Well, yes. It is fundamentally impossible to have both perfectly at the same time (a bit of which way information at a reduced interference pattern visibility works, though). This is the complementarity principle and the original quantum eraser experiments were indeed designed to test it.

 

[DylanLou]

Whether or not you see an interference pattern just depends on whether the several possible pathways that could lead to a detection event at some position are in principle distinguishable or not.

I agree, this is indeed the sense of the experiment.

But what I am saying is that if you place the polarizers before the slits this explanation doesn't work because the disappereance of the pattern will be dependent on an active distruptive effect of the device used not on the distinguishability in principle of the possible pathway (as it happens instead when the polarizers are placed after the slits).

Namely, with the polarizers before the slits, the photon will still pass trhrough both the slits (and the polarizers) but then (for the effect of the polarizers) it will be in a superposition H and V polarized and, so will not produce interference for this reason.

 

[Cthugha]

But what I am saying is that if you place the polarizers before the slits this explanation doesn't work because the disappereance of the pattern will be dependent on an active distruptive effect of the device used not on the distinguishability in principle of the possible pathway (as it happens instead when the polarizers are placed after the slits).

This depends a bit on what you exactly mean by device. The polarizers are not measurement devices as the change to the beam they introduce is completely reversible and they do not have any active disruptive effect. I do not see where there can be any difference whether you place the polarizers in front behind or inside the slits as no measurement is happening.

 

[DylanLou]

This depends a bit on what you exactly mean by device. The polarizers are not measurement devices as the change to the beam they introduce is completely reversible and they do not have any active disruptive effect. I do not see where there can be any difference whether you place the polarizers in front behind or inside the slits as no measurement is happening.

As I see it, but of course I might be wrong, they have an active effect in this case:

Case 1 (polarizers over the slits): the photon will pass through both of them, overcoming the slits, and (exactly because it passed through the polarizers) it will find itself in a superposition state of H and V polarization. Therefore it will not interfere with itself.

Case 2 (polarizers after the slits): the photon will pass through both the slits in superposition. For the effect of its wave nature it will be interfereing with itself immediately after its passage through the slits. Therefore, at the moment when it hits the polarizers the interference should be already happening but then we find that we don't record any interference. That can only means that the photon passed through just one slit (I don't like this terminology but it is as it "knew" before that we were trying to measure through which slit it passed and, therefore "decided" to behave like a particle in the classical sense).

I think this is the sense of the experiment that the author is trying to convey.

 

[Cthugha]

No, you cannot draw that conclusion. In both cases you can restore the interference pattern using another 45° polarizer.

Why should it make any difference whether the light beam (or single photon) passes through the double slit first and meets the polarizers afterwards? In both cases you attach a "label" saying horizontal polarization to slit 1 and one saying vertical polarization to slit 2.

You seem to have the opinion that the interference of two light beams is something that happens and vanishes suddenly. That is not the case. For interference to happen you need two mutually coherent light beams which fields can add up. For orthogonally polarized light fields, the fields obviously cannot add up to some altered intensity because - loosely speaking - the direction in which the oscillations take place are also orthogonal and there is no component pointing in the same direction for both beams. As you "erase" the which-way information by rotating the polarization, you return to a state in which the field oscillations of both fields are aligned into the same direction and therefore interference can occur.

 

[DylanLou]

No, you cannot draw that conclusion. In both cases you can restore the interference pattern using another 45° polarizer.

Why should it make any difference whether the light beam (or single photon) passes through the double slit first and meets the polarizers afterwards? In both cases you attach a "label" saying horizontal polarization to slit 1 and one saying vertical polarization to slit 2.

You seem to have the opinion that the interference of two light beams is something that happens and vanishes suddenly. That is not the case. For interference to happen you need two mutually coherent light beams which fields can add up. For orthogonally polarized light fields, the fields obviously cannot add up to some altered intensity because - loosely speaking - the direction in which the oscillations take place are also orthogonal and there is no component pointing in the same direction for both beams. As you "erase" the which-way information by rotating the polarization, you return to a state in which the field oscillations of both fields are aligned into the same direction and therefore interference can occur.

Ok I can definitely agree with this. Only, in this case we go back to the original question: what is "weird" about the quantum eraser experiment?

The explanation you posted is similar to the one I gave at the beginning:

I couldn't understand why should it be "unexpected" not to observe an interference pattern in this case.

If the photon will still pass through both the slits even with polarizers (assuming that this is possible which I don't know) then the "two" photons (it is still the same one but sort of "duplicated" for the effect of its wave nature) will be one horizontally polarized and the other vertically polarized and, so there will be no interference between the two waves and again no interference pattern.

I mean, if it is like this the appereance and the disappereance of the wave pattern can be easily explained by classical physics:

with 2 polarizers: there is no interference because the waves are ortogonally polarized.

with 3 polarizers (the first two plus the extra one): there is interference because the extra polarizer led again the waves to have the same polarization.

 

[Cthugha]

Well, I do not consider the quantum eraser to be that weird, so I cannot tell. It may be a bit surprising that it works as is supposed classically even on the single photon level. It might be a bit uncomforting if you think of a photon as being similar to a small ball flying through space, but that is one of the worst analogies one could think of anyway - but it is nevertheless an analogy one meets on a regular basis.

It gets a bit more surprising when throwing in delayed choice, but that is a different story.

 

[micky_gta]

Here you go! http://xenophilius.wordpress.com/2012/04/23/quantum-physics-mimics-spooky-action-into-the-past/

 

[Cthugha]

What do you want to tell us using that link? It is a pretty bad pop-sci summary of a scientific paper. If you had read the article instead of posting some mediocre summary, you would have found the following statement in the conclusion area:

"If one viewed the quantum state as a real physical object, one could get the paradoxical situation that future actions seem to have an influence on past and already irrevocably recorded events. However, there is never a paradox if the quantum state is viewed as no more
than a `catalogue of our knowledge'[2]. Then the state is a probability list for all possible measurement outcomes, the relative temporal order of the three observers' events is irrelevant and no physical interactions whatsoever between these events, especially into the past, are necessary to explain the delayed-choice entanglement swapping."

No indications of changing the past or similar things happening here.

This is again a very good example for why it is a good practice to use links to the peer-reviewed publications as the foundation for a debate and not some second-hand summaries.

 

[Nadav]

This is still wrong and on the level of crackpot sites like bottomlayer. For the last time: Post some peer reviewed evidence that there is some truth to your claims or stop posting wrong stuff. Hint: You will not post peer-reviewed evidence as there is none. Or maybe you can just explain where exactly the flaw in the Ionicioiu or Kim papers is when they explain why there is obviously no retrocausality.



are you serious? before saying stuff you better check them! have you even read Kim papers?
and i quote:
"
In conclusion, we have realized a quantum eraser ex-
periment of the type proposed in ref. [3]. The experimen-
tal results demonstrate the possibility of observing both
particle-like and wave-like behavior of a light quantum
via quantum mechanical entanglement. 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."

-A Delayed Choice Quantum Eraser
Yoon-Ho Kim, R. Yu, S.P. Kulik∗ , and Y.H. Shih
Department of Physics, University of Maryland, Baltimore County,
Baltimore, MD 21250

 

[Nadav]

"even after the registration"