Which-Path Information
complementarity · D² + V² ≤ 1 · Welcher-Weg
Physics: Wave-particle duality is quantified by Englert's (1996) duality relation: D² + V² ≤ 1, where D = path distinguishability and V = fringe visibility. D=1 → no interference; D=0 → full fringes. Partial which-path information partially destroys coherence. Mechanism: entanglement between photon and which-path detector causes decoherence of the interference term. Quantum eraser: erasing the which-path information restores fringes (but cannot signal FTL). Feynman: "If you can tell which slit the electron went through, there is no interference." Scully et al. 1991 quantum eraser with microwave cavity detectors.