Measuring one slit in double slit experiment

[cryptist]

In double slit experiment, what if we put a detector on one slit, instead of both slits. Does the interference pattern appear or not?

I think, if superposition principle is true and particles passing through both slits at the same time, measuring one slit will be enough to collapse its wave function and interference pattern will be disappear. Otherwise, superposition principle will be wrong, isn't it?

 

[arkajad]

If you will put a detector on one slit and no other detectors whatsoever, then this detector will react, once in a while, and you will gete only a sequence of records of this one detector. From just one sequence how will you able to tell whether there is interference or not? The very existence (or not) of the other slit will have practically no influence on this one detector.

To test the interfence you put the detectors far enough beyond the slits, not in the slits.

 

[cryptist]

It does not matter where we put detectors far beyond or in or any other place, wherever you put, if you get an information about the state of the particle, particle's interference pattern disappears. Please, see delayed choice exp:

http://grad.physics.sunysb.edu/~amarch/

I think I could not explain what I mean; Let's we send a beam of electrons. (not one electron, a beam; so that we can see interference better. In fact I can see interference from one electron also but, anyway..) In the experiment, there are two detectors. One for left slit, one for right slit. I am saying that let's remove left slit detector. We can still know electron pass which way.(If we detect electron then it means it passed right slit, if not then it passed left slit.) In both case, interference pattern should be disappear according to the superposition principle.

Is there any experiment like this? Can you give a link if there is, please?

 

[arkajad]

With one detector you will never see any interference. You will have just counts of electrons registered by this detector. That's all. In fact, in order to discuss the problem you will have to state explicitly, perhaps with a drawing, a descriptive geometry an the idea of the experiments. Because the devil likes to hide in the details.

 

[cryptist]

In the link that I gave, please see which way marker section. In the picture there are two detectors; QWP1 and QWP2. Let's remove QWP2. Wouldn't be sufficient one detector? After all if detector detects electron, then we know electron passed from 1st slit. (therefore interference pattern disapears)

But in other case? If QWP1 did not detect any electron, then this means electron passed from 2nd slit. (So we can know this information without QWP2) Then what would be? Interference pattern should disappear, because we know which slit that electron passed. Right? If yes, can you give any link of this experiment if there is?

However, if superposition principle is true, then QWP1 would have to detect electron at all times. (Since according to superposition principle electron passes both slits at the same time)

I hope I could explain what I mean.

 

[arkajad]

Can you explain how the two photons are "entangled"? That is: can you write down the two-photon state that is supposed to be created? Or point me to a paper or a book where it is clearly explained and written as a formula? The devil is in the details.

 

[cryptist]

Because of the angular momentum conservations. It depends on actually how do you want to create them. Here is a link;

http://www.physics.ohio-state.edu/~wilkins/writing/Assign/topics/Q-trans-prl.pdf

But, why are you asking these? What is the relevance?

 

[arkajad]

From my own experience with computer simulations of similar experiments, if you remove QWP2, then the interference pattern will be messed up to some degree but not completely. To what degree will depend on the details.

 

[venton]

The interference pattern will disappear if you have a detector on one slit. As soon as you can work out which slit it went through (ie you didn't detect it, it went through the other slit) the interference pattern will disappear and you will see a dot on the detector. This is definitely what I have read in several books on the subject.

 

[arkajad]

Whether the interference pattern will disappear or not depends on how effective the detector is. Each detector is characterized by its discriminating capability ranging, say, from 0 to infinity.

 

[cryptist]

Yes, my professor also said that and I convinced, unfortunately interference pattern will disappear, since we have the information of which slit electron passed.

However, then there is something very very odd. That means even without measuring it, an electron somehow know that we have the information or not. Electron knows a detector will not detect itself and by this we will get information.

Then maybe electron knows the space and communicate with the others!

 

[arkajad]

It is not that the electron knows. The wave function knows and it acts as one entity.

 

[venton]

However, then there is something very very odd. That means even without measuring it, an electron somehow know that we have the information or not. Electron knows a detector will not detect itself and by this we will get information.

Precisely - quantum physics is very odd - no doubt about it. No-one knows why it happens.