Which-path information

If the two arms have internal marker states $|a⟩,|b⟩$, an interference cross term is multiplied by $⟨a|b⟩$. 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