A simple quantum eraser

An optical two-slit experiment with a vertical polaroid placed before one slit and a horizontal polaroid before the other one will not display interference because the path through each slit is labeled by orthogonal states of polarization. The label may be removed and the interference restored by placing a polaroid oriented at 45 with respect to the vertical somewhere between the two-slit screen and the detection screen. This result has important implications for the conceptual structure of quantum mechanics. (orig.)