Editing Openai/6897769e-4ee4-800f-aba5-69cca34f701c (section)

==== In interference (double-slit, Mach–Zehnder) with single photons, each photon is still emitted and later absorbed at a detector pixel, but between source and detector the field evolves as a coherent wave. Interference fringes build up one detection at a time — the iconic “wave/particle” story confirmed since the 1980s. wp.optics.arizona.edu<ref>{{cite web|title=wp.optics.arizona.edu|url=https://wp.optics.arizona.edu/opti544/wp-content/uploads/sites/54/2021/01/Lecture-01252021.pdf|publisher=wp.optics.arizona.edu|access-date=2025-12-16}}</ref> ====

Why can light traverse glass or air without “obvious” absorption while still getting a phase shift? Because the refractive index arises from coherent, near-resonant scattering (virtual excitations of bound electrons) that don’t leave which-way information. It’s a collective, phase-only response (dispersion) — no energy permanently deposited, so coherence survives. This is standard Lorentz-oscillator/refractive-index physics. cpb.iphy.ac.cn<ref>{{cite web|title=cpb.iphy.ac.cn|url=https://cpb.iphy.ac.cn/article/2019/1976/cpb_28_3_030303.html|publisher=cpb.iphy.ac.cn|access-date=2025-12-16}}</ref>arXiv<ref>{{cite web|title=arXiv|url=https://arxiv.org/abs/quant-ph/0105127|publisher=arxiv.org|access-date=2025-12-16}}</ref>

If you deliberately introduce real scattering/absorption (which leaves a trace in the environment), interference fades in exact agreement with complementarity relations like Englert’s inequality V2+D2≤1V^2 + D^2 \le 1V2+D2≤1 and with decoherence theory (Zurek). arXiv<ref>{{cite web|title=arXiv|url=https://arxiv.org/pdf/1604.08316|publisher=arxiv.org|access-date=2025-12-16}}</ref>

Take-home: In wave-like experiments, the path is dominated by coherent (phase-only) interactions; the only truly irreversible absorption event is at detection (or at any intentional which-way marker).