A projection-free approach toward mapping the structured polarization fields

Abstract

We present a projection-free method for mapping two-dimensional polarization distributions using Hong-Ou -Mandel (HOM) interference. Conventional polarization characterization techniques, such as Stokes polarimetry, rely on sequential intensity measurements under multiple polarization projections, making their accuracy and sensitivity susceptible to the extinction ratio, calibration errors, and stability of the polarization analysis optics. Our approach overcomes these limitations by exploiting the sensitivity of two-photon bunching to polarization indistinguishability at a balanced beam splitter. Using a HOM interferometer driven by a high-brightness spontaneous parametric down-conversion photon-pair source at 810 nm, we introduced a birefringent vortex waveplate with spatially varying polarization rotations in one interferometer arm and measured the resulting coincidence counts with a high signal-to-noise ratio, enabling projection-free characterization of the sample-induced polarization transformations. This configuration maps spatially dependent polarization variations directly onto coincidence counts, providing a projection-free reconstruction of the polarization distribution. We demonstrate high-fidelity ($\sim$95$\%$) reconstruction of the spatial polarization pattern with an angular resolution of approximately $\sim0.4^\circ$. The use an estimator that saturates the Cramer-Rao bound, computed from the Fisher information, can improve resolution further at the cost of longer acquisition. The proposed quantum-optical technique offers a simple, scalable, and high-precision framework for characterizing structured polarization fields in birefringent materials.