How to measure which slit a photon went through

[michelcolman]

The double slit experiment is often described like this:
(I'll describe the photon version, but of course the electron one is the same)

"We fire individual photons at a screen with two slits. An interference pattern appears. But when we try to look at the slits to figure out which slit the photon went through, the interference pattern disappears!"

But how do we 'look at' those slits? How do you figure out whether a photon (or electron) went throug a particular slit or not? You could let it interfere with an electron which absorbs the photon and emits a new one, generating some current while doing so. But it's rather obvious that this changes the whole experiment, there's really no mystery! Are there any other ways of 'looking' at those photons that don't completely obviously and unmysteriously change the whole setup?

Otherwise, it's a bit like saying "hey, this tennis ball gun is shooting all balls in the same direction, but when I just 'look at' the balls by hitting them with a tennis racket, they go all over the place!"

Is there something I'm missing? Or is that just all there is to it?

[lightarrow]

There is no need of an interaction between light and something else: "which way information" is enough to destroy interference.
See "Mach-Zehnder interferometer":
http://en.wikipedia.org/wiki/Mach%E2%80%93Zehnder_interferometer

Referring to the first picture up right, without an obstacle in the position where is the "sample" there is interference and you will have 100% of the light in detector 1 and 0% in detector 2 (the first beam splitter, that is the one closer to the light source, is the analogous to the two slits).

With an obstacle (totally absorbing light) in the position of the "sample": detector 1 will fire 25% of the times, and so detector 2. Explaining this result with the "particle" paradigma is the "mistery": if the photon, after the first beam splitter, takes the route to the right, why should detector 2 fire at all?

[zaybu]

For an easy explanation see:

http://soi.blogspot.com/2011/02/two-slit-experiment.html