Quantum Eraser

Quantum eraser is a 45-degree polarizer with respect to quantum markers. When it is placed behind the quantum markers, the interference pattern will be restored. In classical wave theory, the previous two perpendicular polarized light now have the same polarization after the passing through the 45-degree polarizer. In quantum theory, the path information of a photon is erased by the third polarizer, meaning a polarization measurement will no longer tell which slit the photon passes through, so the photon will interfere with itself again.