Abstract Hong–Ou–Mandel (HOM) dip from a biphoton source in a two-photon interferometer provides a myriad of quantum tools for quantum communication and sensing. But the stringent requirements for spatial coherence between the photon pair makes it prohibitively difficult to observe high-fidelity HOM dip in long-distance free-space implementations, e.g., for the photon pairs involved in quantum communication need to match the two path lengths within a few 10 s of micron because of the short coherence width of the two-photon wave-packet. While many techniques for the pathlength balancing of two interferometric arms have been studied and applied extensively, such balancing is further complicated in presence of turbulence in long-distance free-space propagation. On a table-top testbed with each arm of a Mach–Zehnder interferometer (MZI) $$\sim 10$$ ∼ 10 -meter long in our turbulent atmospheric simulator for qubit (TASQ), we have employed below-threshold lasers to achieve classical low-coherence interferometry and show that we could narrow down the interrogation window for the pathlength balancing to obtain interference peak at the center. This method allowed us to reduce the interrogation distance for searching the HOM dip by four orders of magnitude, i.e., from tens of centimeters to a few tens of micron. Such pathlength balancing allowed realizing the HOM dip within $$160\,\upmu \textrm{m}$$ 160 μ m distance from the peak of the classical interference fringe. This technique could be helpful in a wide variety of long-pathlength interference-based quantum sensing.
Abstract Hong–Ou–Mandel (HOM) dip from a biphoton source in a two-photon interferometer provides a myriad of quantum tools for quantum communication and sensing. But the stringent requirements for spatial coherence between the photon pair makes it prohibitively difficult to observe high-fidelity HOM dip in long-distance free-space implementations, e.g., for the photon pairs involved in quantum communication need to match the two path lengths within a few 10 s of micron because of the short coherence width of the two-photon wave-packet. While many techniques for the pathlength balancing of two interferometric arms have been studied and applied extensively, such balancing is further complicated in presence of turbulence in long-distance free-space propagation. On a table-top testbed with each arm of a Mach–Zehnder interferometer (MZI) ∼10 ∼ 10 -meter long in our turbulent atmospheric simulator for qubit (TASQ), we have employed below-threshold lasers to achieve classical low-coherence interferometry and show that we could narrow down the interrogation window for the pathlength balancing to obtain interference peak at the center. This method allowed us to reduce the interrogation distance for searching the HOM dip by four orders of magnitude, i.e., from tens of centimeters to a few tens of micron. Such pathlength balancing allowed realizing the HOM dip within 160\upmum 160 μ m distance from the peak of the classical interference fringe. This technique could be helpful in a wide variety of long-pathlength interference-based quantum sensing.
Two-photon interference, commonly known as Hong-Ou-Mandel (HOM) interference [1] , has attracted a great deal of interest in quantum sensing, including the precision timing measurements [2] and quantum optical coherence tomography [3] . While the dip of the HOM interference is the direct measure of the purity of the single-photon, the width of the HOM dip relates to the coherence time of the interfering photons. The broad consensus for using the HOM interferometer for sensing application remained the HOM dip width as the limiting factor for the precision measurements. As a consequence, the ultra-broadband photon sources have long been hailed as a vital prerequisite for ultra-precise HOM interferometry. Here, we have studied the variation in the width of the HOM dip with the crystal length for the continuous-wave pump laser and measured the HOM dip as low as 8.2 ± 0.2 µm corresponding to the spectral width of the single-photon source of 79.6±1.9 nm useful for sensitive sensing applications.
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.
Two-photon interference in a Hong–Ou–Mandel (HOM) interferometer can be used as a quantum sensing mechanism due to the sensitivity of the interference dip to perturbations of the photon indistinguishability. In particular, recent studies have generalized this concept to microscopy setups, but the sensitivity to optical path differences constrains its application to samples with thickness variation typically below a few micrometers if tracking changes in the coincidences at a fixed delay. Extending the concept to polarization microscopy and circumventing this limitation, this manuscript explores the use of a narrowband photon pair source with coherence length >1 mm to broaden the HOM dip. Thus, realistic sample-thickness variations introduce negligible temporal distinguishability, and changes in coincidence rate at the dip center are then dominated by sample-induced polarization effects. To compute the polarization rotation, we develop a statistical model for the interferometer, derive the Fisher information, and establish a maximum-likelihood estimator for the local fast-axis angle. Recording dip and baseline frames at each sample position via raster scanning, the experimental results validate the framework, agreeing with classical polarized-intensity images while demonstrating operation close to the maximum-precision regime allowed by the statistical model and insensitivity to layer thickness. Overall, this proof-of-concept demonstrates thickness-insensitive quantum mapping of the local fast-axis angle, which may motivate future developments toward more comprehensive birefringence characterization and low-damage imaging of photosensitive samples, while the present proof-of-concept focuses on the half-wave-like regime, where the local fast-axis angle can be estimated with a single-parameter model.
The Hong–Ou–Mandel (HOM) interference of two light beams having different longitudinal spatial coherence properties is investigated theoretically and experimentally. The normalized second-order correlation function ( g ( 2 ) ) is determined for the interfering photons from two sources of different angular widths using Feynman’s path integral theory. We find that the difference in angular width of the sources has an explicit impact on the HOM interference pattern, which can be quantified through the visibility and full width at half maxima of the HOM dip. The effect of distinguishability of the interfering longitudinally spatially coherent beams on the HOM dip is verified experimentally and is analogous to non-classical HOM interference. The magnitude of the angular width of beams manifested through the difference in angular width has a significant impact. Along with the difference of two sources, this HOM scheme is sensitive to the angular spectrum width of each source. The enhanced sensitivity can be useful in the remote sensing of objects and beams in metrological applications. This work can play a significant role in fundamental and applied physics.
Interference has long been used for precision measurement of path-length changes. Since the advent of the laser, interference has become one of the most versatile tools in metrology. Specifically, ultra-short laser pulses allow unprecedented resolution in absolute length measurements. While ultra-short laser pulses lead to high resolution, for example in white-light interferometry, they are very susceptible to dispersion. \n \nQuantum resources have been proposed to overcome some of the problems related to distortions in the interferometric signal. For example, the Hong-Ou-Mandel (HOM) interferometer relies on frequency-entangled photon pairs and features automatic even-order dispersion cancellation and high interference visibility resilient to unbalanced loss. Quantum-OCT is a technique based on HOM interferometry, that promises to overcome Optical Coherence Tomography (OCT) a classical imaging technique based on low coherence light. Furthermore, straightforward modifications of the HOM interferometer can display several different interferometric signals, including the HOM peak, quantum beating, and phase super-resolution. However, the quantum resources required are hard to produce and dim, leading to long integration times and single-photon counting. \n \nIn this thesis, we introduce the theory behind Chirped-Pulse Interferometry (CPI), a new technique that combines all the advantages of Q-OCT, including even-order dispersion cancellation, but without the need for any quantum resources. We then experimentally implement CPI and demonstrate all the important characteristics shared by the HOM interferometer, but at dramatically larger signal levels. We show how CPI can be used to measure dispersion cancelled axial profiles of an optical sample and show the improvement in resolution over white-light interferometry. Finally, we show that by modifying CPI in analogous ways to HOM, CPI can also be made to produce interferometric signal identical to the HOM peak, quantum beating, and phase super-resolution.
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.