Which-way information and interference: erasure restores fringes
In a double-slit quantum eraser, entangled "which-way" tags destroy interference. When the eraser restores indistinguishability (by erasing which-slit information), fringes reappear — even when the erasure decision is made after the photon hits the detector (delayed choice). The pattern emerges only in post-selected coincidences, preserving causality.