Singh, SandeepSandeepSinghKumar, VimleshVimleshKumarSamanta, G. K.G. K.Samanta2025-08-222025-08-222024-01-012331-8422https://doi.org/10.48550/arXiv.2401.11853https://d8.irins.org/handle/IITG2025/18534Hong-Ou-Mandel (HOM) interferometry has emerged as a valuable tool for quantum sensing applications, particularly in measuring physical parameters that influence the relative optical delay between pair photons. Unlike classical techniques, HOM-based quantum sensors offer higher resolution due to their intrinsic dispersion cancellation property. Despite this advantage, achieving precise measurements of optical delay crucial for practical applications often involves time-consuming integration and post-processing with traditional statistical methods. To address this challenge, our recent work focused on optimizing optical delay measurements in a time-efficient manner. By carefully selecting the length of a 1 mm periodically-poled KTP (PPKTP) crystal for pair photon generation, we achieved a remarkable group index measurement precision of ∼6.75×10−6 per centimeter of sample length, surpassing the previous maximum precision by over 400%. These current measurements maintain fast detection and high photon counts, which are essential for practical quantum sensing applications. The HOM-based method, while limiting the measurement range, can be extended by compensating for photon delay using an optical delay stage. As a proof-of-principle, we measured the group index variation of PPKTP over a temperature range up to 200∘C with a precision in the range of one part per million (∼10−6). This advancement not only contributes to quantum sensing but also holds promising implications for high-precision and long-range measurements in quantum optical coherence tomography.en-USe-Printe-Print123456789/615