Two-slit Light Experiment at Home
Most recent answer: 01/15/2013
Q:
I hope you answer questions from old guys like me..... After reviewing a lot of information on the "double slit" experiment I setup my own and found it worked just as expected in its most simple configuration. My question is about how to collapse the wave function and see the interference pattern disappear. On one site it claims that the collapse occurs when a light is shone on the beam as it leaves the slits since this will allow the path taken to be observed. I tried shining another laser at the slits as the light exited, but the interference pattern remained. Another site (http://www.youtube.com/watch?v=R-6St1rDbzo) recommended placing polarizing filters on the slits as the light exited, then positioning another filter at 45 degrees to demonstrate a "Quantum Eraser" effect (however the explanation at http://answers.yahoo.com/question/index?qid=20090116053436AA8P1Ct claims this is not a quantum effect), This worked fine, although it still worked when I positioned the filters before the light passed through the slits, thus showing (it appears) that the filters were not actually detecting which path the photons took as they went through the slit.
Can you please explain how I can (at home) setup some way to detect the "which path" information? Can you also explain why the two methods I used did not work as I expected and finally can you tell me if (as this site claims http://www.users.csbsju.edu/~frioux/polarize/POLAR-sup.pdf) whether the three polarizer experiment is a quantum effect or not?
Thanks.
Malcolm
- Malcolm Leitch (age 60)
La Verkin, Utah, USA
- Malcolm Leitch (age 60)
La Verkin, Utah, USA
A:
First, the easier parts. So long as the light going through each of the two slits is orthogonally polarized, the interference between them will be lost. This can be described in a simple classical way. The classical light intensity goes as the square of the electric field E. If E=E1+E2, the E2=E12+E22+2E1.E2. If E1 and E2 are orthogonal, E1.E2=0, so there's no interference.
You can make the light coming from the slits orthogonal either with polarizers just in front or just behind the slits. The 45° polarizer business is also classical. The portion of the fields that survive passage through that polarizer are no longer orthogonal, so interference is restored. You can think of light going through one slit turned 90° from the other as partially made up of field rotated +45° to the other and partly +135°. You can think of the original one as made up of +45°and -45° components. The two +45° parts give one interference pattern and the -45° and +135° parts give an opposite pattern. The cross terms are between orthogonal parts and give no pattern. So the combination shows no interference pattern. If you remove the -45° and +135° parts, you're left with just one pattern, which you can see because there's no cancellation.
OK, so much for the classical part. You'd like some way to do which-way detection and destroy the interference without doing some obvious classical move like rotating the polarization. To be really quantum mechanical, you need some "entanglement", the "spooky correlations at a distance" that made Einstein so uncomfortable. Say that your light source generates entangled photon pairs, heading opposite ways with zero net momentum. Now if you measure where the backward going photon ended up, you have information on which way the forward going one went, including if it went through one or the other slit. So the two-slit interference is lost. What's weird about this is that it happens even if the detector for the backward photon is far away, not allowing any signal from it to propagate to the forward photon before it has been detected.
Mike W.
You can make the light coming from the slits orthogonal either with polarizers just in front or just behind the slits. The 45° polarizer business is also classical. The portion of the fields that survive passage through that polarizer are no longer orthogonal, so interference is restored. You can think of light going through one slit turned 90° from the other as partially made up of field rotated +45° to the other and partly +135°. You can think of the original one as made up of +45°and -45° components. The two +45° parts give one interference pattern and the -45° and +135° parts give an opposite pattern. The cross terms are between orthogonal parts and give no pattern. So the combination shows no interference pattern. If you remove the -45° and +135° parts, you're left with just one pattern, which you can see because there's no cancellation.
OK, so much for the classical part. You'd like some way to do which-way detection and destroy the interference without doing some obvious classical move like rotating the polarization. To be really quantum mechanical, you need some "entanglement", the "spooky correlations at a distance" that made Einstein so uncomfortable. Say that your light source generates entangled photon pairs, heading opposite ways with zero net momentum. Now if you measure where the backward going photon ended up, you have information on which way the forward going one went, including if it went through one or the other slit. So the two-slit interference is lost. What's weird about this is that it happens even if the detector for the backward photon is far away, not allowing any signal from it to propagate to the forward photon before it has been detected.
Mike W.
(published on 01/15/2013)