# Double Slit Experiment and Quantum Eraser

1. Apr 21, 2012

### 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.

2. Apr 21, 2012

### StevieTNZ

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

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.

3. Apr 22, 2012

### 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?

4. Apr 22, 2012

### zonde

This experiments sounds similar to Quantum eraser experiment

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.

5. Apr 22, 2012

### 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.

6. Apr 22, 2012

### DylanLou

Thank you guys,

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

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.

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)

7. Apr 22, 2012

### zonde

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.

8. Apr 22, 2012

### 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!

9. Apr 22, 2012

### Cthugha

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?

10. Apr 22, 2012

### DylanLou

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.

11. Apr 22, 2012

### 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.

12. Apr 22, 2012

### 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!!! ;)

13. Apr 22, 2012

### StandardsGuy

14. Apr 22, 2012

### DylanLou

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.

15. Apr 22, 2012

### 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 [Broken]

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.

Last edited by a moderator: May 5, 2017
16. Apr 23, 2012

### Cthugha

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?

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?

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.

Last edited: Apr 23, 2012
17. Apr 23, 2012

### zonde

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.

18. Apr 23, 2012

### 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.

19. Apr 23, 2012

### DrChinese

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.

20. Apr 23, 2012

### Cthugha

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.

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.

Last edited: Apr 23, 2012