question about double-slit experiment

[gib65]

Concerning the souble-slit experiment with one particle fired at a time...

 

If we put a detection device in one of the slits so that we can detect the presence of the particle or not, can it be setup so that the particle still passes through the slit, or does the detection device essentially block the slit entirely?

[Cap'n Refsmmat]

The particle can still pass through the slit, as far as I understand it.

[iNow]

Concerning the souble-slit experiment with one particle fired at a time...

 

If we put a detection device in one of the slits so that we can detect the presence of the particle or not, can it be setup so that the particle still passes through the slit, or does the detection device essentially block the slit entirely?

 

It can, and has, been done. In fact, they put a detector at both slits, and when single photons were fired with these detectors in place, the interference pattern went away. Huh? An interference pattern is created when you don't know which slit the photon went through, but not when you do know? Yup.

 

You know what else is odd though? Without those detectors, even single photons create an interference pattern. How does a single photon interfere with itself?

 

Here's a cool article:

http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/SinglePhotonInterference/SinglePhotonInterference.html

And another:

http://ophelia.princeton.edu/~page/single_photon.html

[gib65]

It can, and has, been done. In fact, they put a detector at both slits, and when single photons were fired with these detectors in place, the interference pattern went away. Huh? An interference pattern is created when you don't know which slit the photon went through, but not when you do know? Yup.

 

You know what else is odd though? Without those detectors, even single photons create an interference pattern. How does a single photon interfere with itself?

 

Here's a cool article:

http://www.fas.harvard.edu/~scdiroff/lds/QuantumRelativity/SinglePhotonInterference/SinglePhotonInterference.html

And another:

http://ophelia.princeton.edu/~page/single_photon.html

 

Yup, it is cool stuff. I'm writing a paper on it right now and I'm mentioning all those things.

 

So it goes through even with the detection device present. How is this detection device setup? I imagine you could have magnets at the edges of the slits, and so if you fire charged particles at the slits, you could detect them by a slight "pull" on the magnets. It would have to be super sensitive though. Is this how they do it?

[iNow]

I imagine you could have magnets at the edges of the slits, and so if you fire charged particles at the slits, you could detect them by a slight "pull" on the magnets. It would have to be super sensitive though. Is this how they do it?

 

 

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

To make the "which-way" detector, a quarter wave plate (QWP) is put in front of each slit. This device is a special crystal that can change linearly polarized light into circularly polarized light. The two wave plates are set so that given a photon with a particular linear polarization, one wave plate would change it to right circular polarization while the other would change it to left circular polarization.

 

With this configuration, it is possible to figure out which slit the s photon went through, without disturbing the s photon in any way. Because the s and p photons are an entangled pair, if we measure the polarization of p to be x we can be sure that the polarization of s before the quarter wave plates was y. QWP 1, which precedes slit 1, will change a y polarized photon to a right circularly polarized photon while QWP 2 will change it to a left circularly polarized photon. Therefore, by measuring the polarization of the s photon at the detector, we could determine which slit it went through.

Emphasis mine.

 

In case you might be suspicious of the quarter wave plates, it is worth noting that given a beam of light incident on a double slit, changing the polarization of the light has no effect whatsoever on the interference pattern. The pattern will remain the same for an x polarized beam, a y polarized beam, a left or a right circularly polarized beam.

[Pangloss]

I'm confused... the above posts seem to suggest that detection is possible without stopping the interference pattern from appearing. But if I read my Brian Greene right, he seems to say that the detection equipment either works, in which case the particle is affected and the interference pattern doesn't appear, or the detection equipment FAILS to detect the particle (and answer the question of which slit it travelled through), in which case the interference pattern appears.

 

Am I misunderstanding something here? He goes to great lengths in The Elegent Universe talking about how Heisenburg shows that while the energy used in detection can travel down to zero, at that point it no longer affects the particle, but can no longer detect the particle either, therefore you either modify the particle in some way (and answer your question), or you do not -- there is no way to detect the particle and fail to modify its behavior (and eliminate the interference pattern).

 

Am I just a victim of a book that doesn't explain the subject completely enough, or lacks recent information? (Or perhaps I didn't read it thoroughly enough?)

 

Thanks.

[Klaynos]

Pangloss, if you read the article linked to above (which I've just done) you cannot determine the slit the photon goes through AND get an interference pattern using their setup. When they setup so this happens the pattern disappears. But they do have a detection method where photons passing through the slit are not themselves interrogated, but their entangled partner.

[Pangloss]

Thanks for the quick reply. Is this the article you mean?

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

 

The article seems to say that they can determine which slit the particle went through by measuring the location of its entangled partner. But when it does so it stops the interference pattern, which is then restored by deleting the entangled partner.

 

But then how do they know which slit the particle passed through? Aren't they actually just capturing one potential path? I thought the whole point of this is that the particle passes through EVERY potential path. ALL of them. Wasn't that Feinman's contribution? Has that been disproven?

[swansont]

Thanks for the quick reply. Is this the article you mean?

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

 

The article seems to say that they can determine which slit the particle went through by measuring the location of its entangled partner. But when it does so it stops the interference pattern, which is then restored by deleting the entangled partner.

 

But then how do they know which slit the particle passed through? Aren't they actually just capturing one potential path? I thought the whole point of this is that the particle passes through EVERY potential path. ALL of them. Wasn't that Feinman's contribution? Has that been disproven?

 

If the particles are entangled, you know what the polarization of the second photon should be when you detect the first. The QWPs give different answers, depending on the slit, so that measuring that circular polarization tells you which slit it went through.

 

The photon travels all paths, but most contributions interfere and sum to zero.

[Pangloss]

Oh right... that cancelling-out explains why macroscopic materials behave in Newtonian manner, if memory serves (but please correct me if I'm wrong).

 

I think I understand now. Thanks for the quick answers, folks.

[bombus]

Concerning the souble-slit experiment with one particle fired at a time...

 

If we put a detection device in one of the slits so that we can detect the presence of the particle or not, can it be setup so that the particle still passes through the slit, or does the detection device essentially block the slit entirely?

 

This explains it (from wiki, but correct):

 

Quantum version of experiment

 

By the 1920s, various other experiments (such as the photoelectric effect) had demonstrated that light interacts with matter only in discrete, "quantum"-sized packets called photons.

 

If sunlight is replaced with a light source that is capable of producing just one photon at a time, and the screen is sensitive enough to detect a single photon, Young's experiment can, in theory, be performed one photon at a time with identical results.

 

If either slit is covered, the individual photons hitting the screen, over time, create a pattern with a single peak. But if both slits are left open, the pattern of photons hitting the screen, over time, again becomes a series of light and dark fringes. This result seems to both confirm and contradict the wave theory. On the one hand, the interference pattern confirms that light still behaves much like a wave, even though we send it one particle at a time. On the other hand, each time a photon with a certain energy is emitted, the screen detects a photon with the same energy. Under the Copenhagen interpretation of quantum theory, an individual photon is seen as passing through both slits at once, and interfering with itself, producing the interference pattern.

 

A remarkable result follows from a variation of the double-slit experiment, in which detectors are placed in each of the two slits, in an attempt to determine which slit the photon passes through on its way to the screen. Placing a detector even in just one of the slits will result in the disappearance of the interference pattern. The detection of a photon involves a physical interaction between the photon and the detector of the sort that physically changes the detector. (If nothing changed in the detector, it would not detect anything.) If two photons of the same frequency were emitted at the same time they would be coherent. If they went through two unobstructed slits then they would remain coherent and arriving at the screen at the same time but laterally displaced from each other they would exhibit interference. However, if one or both of them were to encounter a detector, then they would fall out of step with each other, that is, they would decohere. They would then arrive at the screen at slightly different times and could not interfere because the first to arrive would have already interacted with the screen before the second got there. If only one photon is involved, it must be detected at one or the other detector, and its continued path goes forward only from the slit where it was detected.

 

The Copenhagen interpretation posits the existence of probability waves which describe the likelihood of finding the particle at a given location. Until the particle is detected at any location along this probability wave, it effectively exists at every point. Thus, when the particle could be passing through either of the two slits, it will actually pass through both, and so an interference pattern results. But if the particle is detected at one of the two slits, then it can no longer be passing through both—its presence must become manifested at one or the other, and so no interference pattern appears.

 

This is similar to the path integral formulation of quantum mechanics provided by Richard Feynman. (Feynman stresses that this is merely a mathematical description, not an attempt to describe some "real" process that we cannot see.) In the path integral formulation, a particle such as a photon takes every possible path through space-time to get from point A to point B. In the double-slit experiment, point A might be the emitter, and point B the screen upon which the interference pattern appears, and a particle takes every possible path; through both slits at once; to get from A to B. When a detector is placed at one of the slits, the situation changes, and we now have a different point B. Point B is now at the detector, and a new path proceeds from the detector to the screen. In this eventuality there is only empty space between (B =) A' and the new terminus B', no double slit in the way, and so an interference pattern no longer appears.

 

The rest is here: http://en.wikipedia.org/wiki/Double_slit#Quantum_version_of_experiment