Measuring which slit destroys interference; erasing the information restores it
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Complementarity principle: wave (interference) and particle (which-way) information
are mutually exclusive. If we mark each photon with polarization or entanglement to record which slit
it passed through, the interference pattern vanishes. But if we then erase the
which-way information (via a post-selected coincidence measurement), the pattern
reappears in coincidence counts — even if the erasure happens after detection (delayed choice).
Marker visibility V controls the trade-off: V² + D² ≤ 1 (Englert 1996).