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arXiv:2506.03098 · Advanced Quantum Technologies

High‐Precision Measurement of Time Delay with Frequency‐Resolved Hong‐Ou‐Mandel Interference of Weak Coherent States

Francesco Di Lena, Fabrizio Sgobba, Danilo Triggiani, Andrea Andrisani, Cosmo Lupo, Piergiorgio Daniele, Gennaro Fratta, Giulia Acconcia, Ivan Rech, Luigi Santamaria Amato

Published Nov 28, 2025• 1 citation
arXivHTMLPDF

Abstract

ABSTRACT We demonstrate a scheme for high‐precision measurements of time delay based on frequency‐resolved Hong‐Ou‐Mandel (HOM) interference. Our approach is applied to weak coherent states and exploits an array of single‐photon avalanche diodes (SPADs). Unlike conventional HOM experiments, our setup enables high‐precision measurements, producing an uncertainty per coincidence of about ps even for photons separated by delays up to ps so much greater than their coherence time, where ordinary non‐resolved HOM fails. This result confirms our newly developed theoretical predictions that consider, differently from previous theoretical results, a finite frequency resolution in the detection. We compare the performance of this scheme against the conventional non‐resolved case. Experimental data align well with the predictions of quantum estimation theory, demonstrating a significant reduction in the uncertainty. Due to the physics of the frequency‐resolved HOM effect, the gain in precision is particularly high when the estimated time delay is much longer than the coherence time.

DOI
10.1002/qute.202500636

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ABSTRACT We demonstrate a scheme for high‐precision measurements of time delay based on frequency‐resolved Hong‐Ou‐Mandel (HOM) interference. Our approach is applied to weak coherent states and exploits an array of single‐photon avalanche diodes (SPADs). Unlike conventional HOM experiments, our setup enables high‐precision measurements, producing an uncertainty per coincidence of about ps even for photons separated by delays up to ps so much greater than their coherence time, where ordinary non‐resolved HOM fails. This result confirms our newly developed theoretical predictions that consider, differently from previous theoretical results, a finite frequency resolution in the detection. We compare the performance of this scheme against the conventional non‐resolved case. Experimental data align well with the predictions of quantum estimation theory, demonstrating a significant reduction in the uncertainty. Due to the physics of the frequency‐resolved HOM effect, the gain in precision is particularly high when the estimated time delay is much longer than the coherence time.
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-measurement path-delay sensing at the measurement Cramer-Rao bound using spectrally resolved HOM interference, thereby removing the conventional dynamic-range limitation imposed by the photon coherence window, with no scanning required for calibration. 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 20 attosecond (6 nm), while real-time operation (at 1 Hz) yields 330 attosecond (100 nm) precision. Because the estimator relies on fringe periodicity rather than absolute coincidence rates, the method is intrinsically robust to photon losses and variations in interference visibility, eliminating the need for recalibration. As a practical demonstration, we measure the thickness of a 300 um transmissive target with nanometer-scale precision. These results mark a significant step towards deploying quantum-limited measurements in real-world sensing applications using HOM interferometry.
Abstract Hong–Ou–Mandel (HOM) interference, bunching of two indistinguishable photons on a balanced beam‐splitter, has emerged as a promising tool for quantum sensing. There is a need for wide spectral‐bandwidth photon pairs (for high‐resolution sensing) with high brightness (for fast sensing). Here, the generation of photon‐pairs with flexible spectral‐bandwidth even using single‐frequency, continuous‐wave diode laser enabling high‐precision, real‐time sensing is showed. Using 1‐mm‐long periodically‐poled KTP crystal, degenerate, photon‐pairs with spectral‐bandwidth of 163.42±1.68 nm are produced resulting in a HOM‐dip width of 4.01±0.04 m to measure a displacement of 60 nm, and sufficiently high brightness to enable the measurement of vibrations with amplitude of 205 ± 0.75 nm and frequency of 8 Hz. Fisher‐information (FI) and maximum likelihood estimation enables optical delay measurements as small as 4.97 nm with precision (Cramér–Rao bound) and accuracy of 0.89 and 0.54 nm, respectively, therefore showing HOM sensing capability for real‐time, precision‐augmented, in‐field quantum sensing applications.
Abstract We demonstrate an experimental scheme for high-precision position measurements based on transverse-momentum-resolved two-photon interferometry with independent photons and single photon avalanche diode (SPAD) arrays. Our scheme extends the operative range of Hong–Ou–Mandel interferometry beyond its intrinsic constraints due to photons indistinguishability, paving the way to applications in high-resolution imaging. We assess the experimental results against the ultimate precision bounds as determined by quantum estimation theory. Our experiment ultimately proves that transverse-momentum resolved measurements of fourth-order correlations in the fields can be employed to overcome spatial distinguishability between independent photons. The relevance of our results extends beyond sensing and imaging towards quantum information processing, as we show that partial photon distinguishability and entanglement impurity are not necessarily a nuisance in a technique that relies on two-photon interference.
Multiphoton interference effects can be measured with a single detector when two input photons are temporally well separated when compared with the dead time of the single-photon avalanche detector. Here we experimentally demonstrate that the Hong-Ou-Mandel interference effect can be observed with a single non-photon-number resolving detector via a time-delayed coincidence measurement of successive electrical signals from the detector. The two-photon interference experiment is performed by utilizing temporally well-separated pairwise weak coherent pulses, and the interference fringes are successfully measured with high visibility in the range of the limited upper bound for the weak coherent photon source.
In this paper, we revisit the well-known Hong–Ou–Mandel (HOM) effect in which two photons, which meet at a beamsplitter, can interfere destructively, leading to null in coincidence counts. In a standard HOM measurement, the coincidence counts across the two output ports of the beamsplitter are monitored as the temporal delay between the two photons prior to the beamsplitter is varied, resulting in the well-known HOM dip. We show, both theoretically and experimentally, that by leaving the delay fixed at a particular value while relying on spectrally resolved coincidence photon counting, we can reconstruct the HOM dip, which would have been obtained through a standard delay-scanning, non-spectrally resolved HOM measurement. We show that our numerical reconstruction procedure exhibits a novel dispersion cancellation effect, to all orders. We discuss how our present work can lead to a drastic reduction in the time required to acquire a HOM interferogram, and specifically discuss how this could be of particular importance for the implementation of efficient quantum-optical coherence tomography devices.
We examine the precision limits of Hong-Ou-Mandel (HOM) timing measurements, as well as precision limits applying to generalized two-photon measurements. As a special case we consider the use of two-photon measurements using photons with variable bandwidths and frequency correlations. When the photon bandwidths are not equal, maximizing the measurement precision involves a tradeoff between high interference visibility and strong frequency anticorrelations, with the optimal precision occurring when the photons share nonmaximal frequency anticorrelations. We show that a generalized measurement has precision limits that are qualitatively similar to those of the HOM measurement whenever the generalized measurement is insensitive to the net delay of both photons. By examining the performance of states with more general frequency distributions, our analysis allows for engineering of the joint spectral amplitude for use in realistic situations, in which both photons may not have ideal spectral properties.