Double Slit Experiment with Interference Patterns

[daniel_i_l]

If you take the normal double slit experiment but shine light on one of the slits so that it's possible to tell which slit each electron went through (by detecting scattered photons) then the "fringes" disappear amd you get constant light. If before the scattered photons hit the photon detector -but after the electrons hit the screen- the information they hold is destroyed (but passing them through a lense for example) then the fringes "should" reappear because now there's no way to tell which slit they passed through but there's no visible (or measurable) difference because there're now two fringe patterens (where one is shifted a little from the other) which can no longer be tild apart. (Is that right so far)
My question is, why are there two fringe patterens? I understand why there's one in the middle (by middle I mean one who has a fringe in in the middle of the screen which is the kind of pattern you see in the standerd experiment) by where did the shifted one come from?
Thanks.

 

[cesiumfrog]

Not sure I follow exactly, but I'd say it comes from the lense you used to erase the photon's information.. if your measurement truly erases the information, you'll probably find the measurement does still have two outcomes (they're just perpendicular to the question of "which slit"). To distinguish the two sets of fringes, you need to distinguish those perpendicular measurments. It kindof makes sense, since otherwise you could try to fool causality (trying to measure the electrons before deciding whether to erase the which-path information), but this way you need to use the result of the erasure measurement as a token (or receipt, to prove to the universe that you've paid the price and destroyed the path information) to get back a path-interference pattern. If you then study the erasure measurement, you'll see the results correspond to a possible phase difference between the possible photon paths through each slit, and when the phase difference occurs it offsets the pattern.

 

[daniel_i_l]

if your measurement truly erases the information, you'll probably find the measurement does still have two outcomes (they're just perpendicular to the question of "which slit").

Could you explain that please? I think that that's exactly what I don't understand.
thanks

 

[cesiumfrog]

Kindof like in the Stern-Gerlach (sp?) experiment, you can measure spin "up" or "down". Or you can rotate the whole apparatus, and measure "left" or "right". If you measure left/right, you have erased the up/down information. Although you do know something, since left & right can presumably be written as two (different!) superpositions of up & down.

I assume you're studying the delayed choice quantum erasor, so study closely the apparatus in the original paper (all five detectors).