Quantum imaging of the fast-axis angle using Hong–Ou–Mandel interference

Two-photon interference in a Hong–Ou–Mandel (HOM) interferometer can be used as a quantum sensing mechanism due to the sensitivity of the interference dip to perturbations of the photon indistinguishability. In particular, recent studies have generalized this concept to microscopy setups, but the sensitivity to optical path differences constrains its application to samples with thickness variation typically below a few micrometers if tracking changes in the coincidences at a fixed delay. Extending the concept to polarization microscopy and circumventing this limitation, this manuscript explores the use of a narrowband photon pair source with coherence length >1 mm to broaden the HOM dip. Thus, realistic sample-thickness variations introduce negligible temporal distinguishability, and changes in coincidence rate at the dip center are then dominated by sample-induced polarization effects. To compute the polarization rotation, we develop a statistical model for the interferometer, derive the Fisher information, and establish a maximum-likelihood estimator for the local fast-axis angle. Recording dip and baseline frames at each sample position via raster scanning, the experimental results validate the framework, agreeing with classical polarized-intensity images while demonstrating operation close to the maximum-precision regime allowed by the statistical model and insensitivity to layer thickness. Overall, this proof-of-concept demonstrates thickness-insensitive quantum mapping of the local fast-axis angle, which may motivate future developments toward more comprehensive birefringence characterization and low-damage imaging of photosensitive samples, while the present proof-of-concept focuses on the half-wave-like regime, where the local fast-axis angle can be estimated with a single-parameter model.