Hi. If we place two perpendicularly oriented linear polarizers at a double slit, there will be no interference pattern. In the wave picture, this is trivial: Two waves with perpendicular polarization cannot interfere. In a quantum picture, I've often seen the interpretation that the sheer possibility of finding out which path a photon took (by measuring its polarization) destroys the interference, even if this measurement hasn't been performed. Now there's those infamous delayed choice quantum eraser experiments: Apparently one can measure the path information (thereby destroying interference) and erase it to restore the interference patter (using some rather shady post-processing). This appears to me as if the actual performance of a measurement of the path information destroys interference, and apparently it can be undone. So what exactly destroys interference: Is it the sheer possibility of measuring the path information or does it need an actual measurement? Is the "measurement and erasure" procedure in a quantum eraser experiment even a measurement in the orthodox (Kopenhagen) sense, i.e. is it a projection, or is it more of a unitary time evolution?