Sample-half-inserted quantum interferometer

Quantum metrology often relies on the Hong-Ou-Mandel (HOM) interferometer, a foundational tool in quantum optics that uses two-photon interference to measure temporal intervals with high precision. However, standard HOM interferometry is limited by low Fisher information per trial, requiring tens of thousands of repetitions and massive photon counts to achieve high sensitivity. This paper introduces the sample-half-inserted HOM (SHOM) interferometer, a configuration that transforms a previously considered experimental artifact into a powerful metrological resource.

In a conventional HOM setup, a sample is placed fully in one arm of the interferometer, causing a simple shift in the interference dip. The authors propose an asymmetric configuration where the sample is only partially inserted into the signal arm. This modification creates a distinctive dip-bump-dip interference structure. By analyzing this structure, the researchers demonstrate that the Fisher information—a measure of the information content regarding the parameter being estimated—is enhanced by five orders of magnitude compared to the standard full-insertion method.

Experimental validation was performed by measuring the optical path difference of a glass sample under varying temperatures. The SHOM configuration achieved an average precision of 4.09 nm (13.63 as) and an accuracy of 1.22 nm (4.07 as) using only 10^7 photons. This represents a two-order-of-magnitude improvement in precision while requiring 175 times fewer trials than previous state-of-the-art attosecond-resolution HOM experiments. This technique provides a robust, phase-insensitive strategy for applications such as thin-film monitoring in semiconductor fabrication, surface profiling, and quantum optical coherence tomography.