Variation of the double slit experiment

[nickelite]

Hello!

I have a question?

Does anyone know if scientists have tried to run the electrons through the double slit, record the particles that go through each slit, and before looking at the results, looking at the pattern on the screen. And THEN doing the EXACT OPPOSITE of what the screen showed?

i.e. if the screen shows a wave like pattern, look at the results. If the screen shows a particle like impact, erasing the results?

Thanks!

 

[StevieTNZ]

I'm a bit unsure of what you're asking; perhaps you should clarify but from what I gather you're asking I'll respond to what I'm thinking you are:

I don't think the screen pattern would be a wave directly - hidden in the scatter pattern (the pattern produced when a particle goes through only one slit) you'd find two interference patterns.

Do you mean having which-way information available, then erasing it, after the system has hit the screen? A good explanation of the results you'll obtain can be found in 'The Demon and the Quantum' (latest is 2nd edition). I have had email contact with Paul Kwiat, one of the experimenters who, with Anton Zeilinger, realized the first quantum eraser experiment, who says that the quantum system, even with which-way info available, did go through both slits.

I agree with him, as if the particle had indeed gone through either slit a photon would be in only one cavity, and by the time it comes to erasure you'd have 100% of the time a click (rather than 50% constructive/50% destructive interference of the photons when erasure occurs). Of course you need to do the relevant things to obtain the interference patterns (fringes & anti-fringes) in the scatter pattern on the screen.

 

[nickelite]

Hello Stevie,

Thanks for answering. I'll look for the book you mentioned it seems very interesting. I hadn't heard about those results but I'll look into them.

Regarding my question, let me further explain myself.

I have read about the double slit experiment, and done it myself. To see the wave-like behavior of light. There are 2 possible outcomes to the experiment:

- a wave-like pattern of interference in the screen
- an imprint of two vertical bands, characteristic of particles.

When scientists made electrons go through the slits they found that the screen had a wave-like interference pattern, thus implying that electrons were waves. They thought electrons were interfering with each other and decided to shoot them one at a time. The result was the same.

Then they questioned whether the electron was going through one slit or the other and decided to use a device to detect when the electron goes through either one. I have read (and this part of the experiment I haven't done) that when they detected the electrons passing through the slit and recorded the result, the pattern on the screen reverted to the imprint of vertical bands, as if the electrons knew they were being watched. I know it sounds illogical but the information I have found is consistent (I found it searching through the Internet, not experimenting), I still need to do the experiment myself to test this, but I don't have the means to do it right know.

Anyway the result of the experiment seemed to indicate that the scientists' state of consciousness regarding the measurements, altered the result of the experiment. You can see a detailed explanation here http://www.youtube.com/watch?v=Bgnuib0z0vI"

In the video, they explain that the scientists did the following:

1. they shot the electrons through the slits
2. Recorded which slit they went through
3. Decided whether or not to look at the results of the recordings or erase them.
4. Look at the screen to see the pattern.

When they decided to look at the evidence, they found the two vertical bands.
And when they decided to erase it they found a wave like pattern.

They started to believe that the knowledge they had of whether the electrons went through one slit or the other, altered the result.

So my question is:

¿Couldn't the experiment be more conclusive if they looked at the screen before watching the results? And so if they found a wave like pattern, THEN look at the results, and if they found two vertical bands, THEN erase the results.

Thanks!

 

[DrChinese]

...
When they decided to look at the evidence, they found the two vertical bands.
And when they decided to erase it they found a wave like pattern.

They started to believe that the knowledge they had of whether the electrons went through one slit or the other, altered the result.

So my question is:

¿Couldn't the experiment be more conclusive if they looked at the screen before watching the results? And so if they found a wave like pattern, THEN look at the results, and if they found two vertical bands, THEN erase the results.

Thanks!

I assure you, the pattern did not change when they looked at the results. I think one of the best to picture the situation is this:

Take a double slit setup using light instead of electrons (which still yields similar patterns). Place one polarizer over the left slit at angle L, another over the right slit at angle R. When L-R=0, there IS interference. When L-R=90 degrees, there is NO interference. Obviously, the presence or absence of a polarizer does not change anything unless which slit information is gained. That becomes progressively more feasible as L-R goes from 0 to 90 degrees. In other words: you *could* know which path (100%) certain) by checking the polarization of the detected photon when L-R=90 (crossed), even if you didn't.

In other words, the mere possibility of knowing which slit will change things, even if you do nothing to learn that information. No human is required. I think, since we can never be sure, can we?

 

[StevieTNZ]

I assure you, the pattern did not change when they looked at the results. I think one of the best to picture the situation is this:

Take a double slit setup using light instead of electrons (which still yields similar patterns). Place one polarizer over the left slit at angle L, another over the right slit at angle R. When L-R=0, there IS interference. When L-R=90 degrees, there is NO interference. Obviously, the presence or absence of a polarizer does not change anything unless which slit information is gained. That becomes progressively more feasible as L-R goes from 0 to 90 degrees. In other words: you *could* know which path (100%) certain) by checking the polarization of the detected photon when L-R=90 (crossed), even if you didn't.

In other words, the mere possibility of knowing which slit will change things, even if you do nothing to learn that information. No human is required. I think, since we can never be sure, can we?

I'm guessing whether we choose to look at the results or not, even if which-way info is available, as Paul Kwiat says in the experiment devised by Marlan Scully, the quantum system did go through both slits and that the interference patterns are hidden in the scatter pattern. Only when we can be sure which the particle followed will the scatter pattern be the final pattern, rather than two interference patterns. In say this in light of the fact at erasure, 50% of the time destructive interference occurs which suggests there was no particle in either cavity as if there were one in either, a click would occur - but according to Marlan this shouldn't happen within standard quantum mechanics.