Attosecond delay metrology beyond the photon coherence time with spectrally resolved Hong-Ou-Mandel interferometry

Abstract Hong–Ou–Mandel (HOM) interferometry enables delay estimation at the quantum precision limit but is traditionally constrained to path differences within the coherence time of the interfering photons. Here, we demonstrate single-shot path-delay sensing using spectrally resolved HOM interference, achieving a precision near the measurement Cramér–Rao bound over an operating range of approximately ±300 μm, removing the conventional dynamic-range limitation imposed by the photon coherence window. By extracting delay information from the spectral interference fringes of spectrally entangled photon pairs, we retain near-optimal sensitivity over an operational range exceeding the photon coherence time by over two orders of magnitude. Using one million detected photon pairs, we achieve a time-delay precision of 13 attosecond (4 nm), while real-time operation (at 1 Hz) with 2000 detected photons yields 330 attosecond (100 nm) precision. Because the estimator relies on fringe periodicity rather than absolute coincidence rates, the method is intrinsically robust to variations in interference visibility, eliminating the need for recalibration. As a practical demonstration, we measure the thickness of a nominally 300 μm transmissive target with nanometer-scale precision. These results mark a significant step towards deploying quantum-enhanced HOM interferometry for high-precision displacement sensing in real-world applications.