Metrology of Optical Activity with Entangled Orbital Angular Momentum Photons

Quantum metrology exploits entanglement to improve the precision of physical measurements beyond the shot-noise limit of classical optics. We present a framework for measuring the optical activity of chiral solutions using photons entangled in their orbital angular momentum (OAM). We show optical rotatory dispersion (ORD) can be formulated as a phase-estimation problem. Here the optical rotation of a medium is imprinted on the interference pattern of entangled OAM photons detected in coincidence. While polarization-entangled light experiments have demonstrated improved ORD estimation, we extend this strategy by harnessing the higher-dimensional entanglement available in OAM states. The precision of the entangled-OAM signal, quantified by the Fisher information, exceeds both classical and polarization-entangled benchmarks. Our analysis identifies an optimal OAM number governed by the Gouy phase and the conditions under which entanglement provides a quantum advantage. These results connect chiroptical spectroscopy with the broader task of quantum-enhanced phase estimation using structured light.