Quantum Markers

Quantum markers are two perpendicular polarizers. When they are placed in front of each slit of the double slits, the light would not generate an interference pattern when it passes the double slits. In classical wave theory, this is because two perpendicular polarized light will not interfere with each other. In quantum theory, a photon will pass through either of the slits with certain probability. As it only passes one slit, it will no longer interfere with itself. This is what “quantum marker” means here: the path information of the photon is marked by the polarizer in front of the slit.