A photon travels both paths of a Mach-Zehnder interferometer. When "which-path" information is available, interference vanishes. When that information is erased (even after detection), fringes reappear in coincidence counting — the quantum eraser.
|ψ⟩ = (1/√2)(|upper⟩ + e^(iφ)|lower⟩). Distinguishability D and fringe visibility V obey D²+V²≤1 (Englert 1996). Slider "which-path info" interpolates D from 0 (full interference) to 1 (particle-like).