Optical Sensing Near the Quantum Limit with Enhanced Dynamic Range by Resolving the Spectra of Interfering Photons

Optical sensing schemes based on two-photon interference offer a powerful platform for precision metrology, but are usually limited by a trade-off between dynamic range and measurement precision. Here, we overcome this limitation by resolving the frequency of two photons impinging on a beam splitter and combining these measurements with a new estimation strategy. This increases the usable dynamic range by up to a factor of 20 compared with conventional frequency nonresolved schemes. We implement the method with independent photon pair sources and characterize its performance in the presence of finite-resolution frequency-resolved detectors, which approach the quantum limit in the lossless regime. Our approach enables scan-free, nanometer resolution depth sensing over millimeter scale distances, with potential applications in biological and nanomaterial imaging.