# Quantum Eraser

1. Feb 17, 2012

### palleon

Hi, I'm new here, and not *really* a physics student, just a guy trying to wrap his head around science by reading lots of books! Stuck on a well known theory, which in my head does not make sense, and hoping someone can clarify...

1. The problem statement, all variables and given/known data

A quantum Eraser - this works by adding a vertical polarized lens over one slit of a "2-slit interference pattern creator" and a horizontal polarized lens over the other slit. When added, the interference pattern vanishes and just has one "clump". Replacing with a 50/50 45o lens to both slits, the interference pattern reappears.

The books all tell me this is because you can not measure the effect, and this experience attempts to do that, but by doing so, stops it working. The vertical lens only allows through a vertical particle and so on with the other.

2. Relevant equations

Surely all this means is that when the photon attempts to "go through both slits at the same time" it is either in a horizontal or vertical state, and so one of the lenses stops that (as it cant be in both states at once). When the 50/50 lens goes on, if the photon can go through one lens it can also go through the other at the same time.

3. The attempt at a solution

This says to me that its not a case of measuring it at all that stops it working, its just you cant put a square peg into a triangular hole.

As the entire book hinges on this "cant measure" rule, so far to me it all appears to be based on a misunderstanding... therefore the whole quantum thing is collapsing for me.

Please, what key part of this am I getting wrong?

Thank you so much.

2. Feb 17, 2012

### StevieTNZ

If the photon did obtain a V or H polarisation by going through either slit, then it takes on a path until the 45/135 degree polariser erases that polarisation. But then getting interference, you'd be changing a definite past.

I think the photon would be best described as a superposition of V and H polarisations after the slit (as would be indicated by QM). Only half of the photons fired towards the slit make it through, and then a subsequent 1/2 of those would make it through the 45 or 135 degree polariser.

The possibility of measuring a V or H polarisation after the photon obtains 45 or 135 degree polarisations is now zero, resulting in the fringe and anti-fringe interference patterns.