Quantum Eraser: Which-Path

Double-slit experiment: Each photon passes through both slits simultaneously (superposition) and lands at a position sampled from I(x) = I₀·cos²(πd·x/λL). The interference pattern builds up statistically — one dot at a time.

Which-path measurement: If we tag each photon with which slit it passed through (e.g., via polarization), the superposition collapses. The photon NOW has a definite path, and I(x) = I₀ (no fringes). The act of measurement — not the apparatus — destroys interference.

Quantum eraser: After tagging, if we erase the which-path information before detection (e.g., via a polarizer at 45°), the interference pattern returns! Causality is preserved: you can't erase retroactively in a useful way without the "signal" photon.

Key formula: Visibility V = (I_max - I_min)/(I_max + I_min) = √(1 - D²) where D is the distinguishability (which-path information). V² + D² = 1 — a complementarity relation.