Two-photon states meet polarization-gradient metasurfaces for nanometric, low-dose lateral-displacement metrology

Accurate displacement sensing is indispensable in advanced semiconductor lithography. Conventional coherent-light-based approaches are hindered by photon budget limitations, slowing down in-situ measurements. In a recent study, Chen et al. introduced a polarization-gradient metasurface integrated with two-photon quantum interference to achieve equivalent precision with only ~3% of the photons required by classical methods. This work represents a decisive step in merging metasurfaces with quantum resources, paving the way for high-speed, low-noise, and integration-ready displacement metrology.