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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Sub-$\mu\mathrm{m}$ Axial Precision Depth Imaging with Entangled Two-Colour Hong-Ou-Mandel Microscopy

Cyril Torre, Alex McMillan, Jorge Monroy-Ruz, Jonathan C. F. Matthews

Published Jun 26, 2023• 1 citation

Abstract

Hong-Ou-Mandel (HOM) interference results in a modified probability of bunching events when indistinguishable photons with a relative timing offset interact at a beamsplitter. Lyons et al. recently achieved attosecond precision in time delay measurement by applying a classical Fisher information analysis to HOM interference [1]. More recently an increase in the measurement precision has been shown by detuning the central wavelengths of the signal and idler from an entangled two-colour photon pair source and applied in temperature drift detection for optical fibres [2]. Here we exploit the timing resolution of the HOM interference to implement a robust 3D imaging microscope with tuneable sensitivity from millimetre to sub-micrometre.

DOI: 10.1109/cleo/europe-eqec57999.2023.10231753

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Sub-

\mu\mathrm{m}

Axial Precision Depth Imaging with Entangled Two-Colour Hong-Ou-Mandel Microscopy

Hong-Ou-Mandel (HOM) interference results in a modified probability of bunching events when indistinguishable photons with a relative timing offset interact at a beamsplitter. Lyons et al. recently achieved attosecond precision in time delay measurement by applying a classical Fisher information analysis to HOM interference [1]. More recently an increase in the measurement precision has been shown by detuning the central wavelengths of the signal and idler from an entangled two-colour photon pair source and applied in temperature drift detection for optical fibres [2]. Here we exploit the timing resolution of the HOM interference to implement a robust 3D imaging microscope with tuneable sensitivity from millimetre to sub-micrometre.

The quantum interference of two wavelength-entangled photons overlapping at a beamsplitter results in an oscillating interference pattern. The frequency of the beat note is dependent on the wavelength separation of the entangled photons but is robust to wavelength scale perturbations that can limit the practicality of standard interferometry. Here we use two-colour entanglement interferometry to evaluate the variation in thickness of a semi-transparent sample in combination with two-dimensional raster scanning. The axial precision and the dynamic range of the microscope are actively controlled by adjusting the wavelength separation of the entangled photon pairs. Sub-

μm

precision is reported using up to

12.3~nm

of detuning and

\sim10^4

detected photon pairs.

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.

Hong-Ou-Mandel (HOM) interference, i.e. the bunching of indistinguishable photons at a beam splitter is a staple of quantum optics and lies at the heart of many quantum sensing approaches and recent optical quantum computers. Although originally proposed as a method for sensing micron-scale variations in photon propagation path lengths and despite the detection of photon bunching using camera technologies, the technique is still to be extended to the imaging domain. We report a full-field, scan-free, quantum imaging technique that exploits HOM interference to reconstruct the surface depth profile of transparent samples. We measure both the bunched and anti-bunched photon-pair distributions at the HOM interferometer output which are combined to provide a lower-noise image of the sample. This approach demonstrates the possibility of HOM microscopy as a tool for label-free imaging of transparent samples in the very low photon regime.