Quantum-enhanced Sagnac sensing

We present a simple and robust design for a squeezing-enhanced Sagnac interferometer that significantly surpasses the the shot-noise limit of sensitivity in rotational sensing. By strategically placing an optical parametric amplifier (OPA) inside the Sagnac loop, light is automatically squeezed in both forward and backward directions of the loop, which enhances the detectability of a small phase. For measuring the squeezed quadrature, we explore two approaches: parametric homodyne and direct detection. In direct detection, the intensity of the output dark port is measured directly, which is very simple, but necessitates photo-detectors of near-unit efficiency. With parametric homodyne, an additional OPA is placed at the Sagnac output to amplify the quadrature of interest above the classical noise limit, eliminating the need for near-ideal photo-detectors. Our analysis demonstrates super-classical sensitivity under most realistic conditions of loss and detector inefficiency, thereby leveraging leveraging the resources of squeezing and the principles of SU(1,1) interference, while maintaining compatibility with standard Sagnac configurations.