A novel high-precision refractive index measurement scheme based on entangled coherent states and parity detection

Traditional intensity-based refractive index measurement methods are constrained by the classical diffraction limit and the shot noise limit, which severely restricts the improvement of measurement precision. To address this issue, a novel quantum measurement scheme integrating entangled coherent states (ECS) and parity detection (PD) is proposed. Taking advantage of the non-classical correlation of quantum entanglement, the scheme constructs a dual-mode entangled coherent state light source and realizes high-fidelity signal demodulation through a customized parity detection system. Theoretical derivation and numerical simulation results demonstrate that the measurement resolution of the proposed scheme breaks through the Rayleigh limit, achieving a $ \sqrt{N} $-fold improvement compared with the traditional coherent state measurement method in the full loss range. In lossless scenarios and cases with loss rates below 10%, the sensitivity surpasses the shot noise limit. Finally, the experimental challenges faced by the experiment are also elaborated in detail. This quantum measurement architecture provides a new technical pathway for precision optical detection, biosensing, and other fields, exhibiting significant practical value and broad application prospects.