Which-path information
= Which-path information
If the two arms have internal marker states $|a\rangle,|b\rangle$, an interference cross term is multiplied by $\langle a|b\rangle$. Orthogonal markers remove interference from polarization-insensitive detection even if nobody reads the markers. Coherently making the two markers identical erases that distinguishing information and restores the cross term.
// Target: quantum-measurement.bigb