Which-path information and interference
Interference fringes visible — no which-path info
In quantum mechanics, a particle can pass through both slits simultaneously, creating interference. If we mark which slit the particle went through (e.g., with polarizers), the interference pattern disappears — the particle behaves classically. A quantum eraser removes the which-path information, restoring interference. This reveals that it's the information content, not physical disturbance, that destroys quantum coherence.