Metasurfaces provide a promising route for structuring light and generating holograms with designed amplitude, phase, and polarization profiles, leading to a versatile platform for integrating and constructing optical components beyond the conventional ones. At the same time, incorporating coincidence in imaging allows a high signal-to-noise ratio for imaging in very low light levels. As beneficial from the recent development in both metasurfaces and single-photon avalanche diode (SPAD) cameras, we combine the polarization-sensitive capability of metasurfaces with Hong-Ou-Mandel (HOM)-type interference in generating images with tailor-made two-photon interference and polarization coincidence signatures. By using orthogonal linear-polarized photons as incidence, correlated, anticorrelated, and uncorrelated polarization coincidence features can be observed within the same image from the pairwise second-order coherence statistics across different pixels of the image. Our work adds polarization to the demonstrated amplitude and phase sensitivity in the domain of "HOM microscopy" and can be useful for biological and security applications.
Metasurfaces provide a promising route for structuring light and generating holograms with designed amplitude, phase, and polarization profiles, leading to a versatile platform for integrating and constructing optical components beyond the conventional ones. At the same time, incorporating coincidence in imaging allows a high signal-to-noise ratio for imaging in very low light levels. As beneficial from the recent development in both metasurfaces and single-photon avalanche diode (SPAD) cameras, we combine the polarization-sensitive capability of metasurfaces with Hong-Ou-Mandel (HOM)-type interference in generating images with tailor-made two-photon interference and polarization coincidence signatures. By using orthogonal linear-polarized photons as incidence, correlated, anticorrelated, and uncorrelated polarization coincidence features can be observed within the same image from the pairwise second-order coherence statistics across different pixels of the image. Our work adds polarization to the demonstrated amplitude and phase sensitivity in the domain of "HOM microscopy" and can be useful for biological and security 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 works 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 centre 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 at a maximum-precision regime and insensitiveness to layer thickness. Overall, the approach enclosed provides a quantum-based quantitative imaging of birefringent structures, which can motivate further advantageous applications, including enhanced signal-to-noise ratio and lower damage imaging of photosensitive samples.
Two-photon interference is an important effect that is tightly related to the quantum nature of light. Recently, it has been shown that the photon bunching from the Hong-Ou-Mandel (HOM) effect can be used for quantum imaging in which sample properties (reflection/transmission amplitude, phase delay, or polarization) can be characterized at the pixel-by-pixel level. In this work, we perform Jones matrix imaging for an unknown object based on two-photon interference. By using a reference metasurface with panels of known polarization responses in pairwise coincidence measurements, the object's polarization responses at each pixel can be retrieved from the dependence of the coincidence visibility as a function of the reference polarization. The post-selection of coincidence images with specific reference polarization in our approach eliminates the need in switching the incident polarization and thus parallelized optical measurements for Jones matrix characterization. The parallelization in preparing input states, prevalent in any quantum algorithms, is an advantage of adopting two-photon interference in Jones matrix imaging. We believe our work points to the usage of metasurfaces in biological and medical imaging in the quantum optical regime.
Interference experiments like the light and electron double-slit are crucially important to quantum mechanics since they illustrate the core principle of wave-particle duality, by demonstrating the wave-like aspect of particle phenomena. In such experiments, a device is typically required to manipulate the particle-wave dual phenomenon, producing controllable interference. Metasurfaces, with their highly flexible configurability, can serve as more versatile versions of such devices. They are powerful tools capable of modulating a broader category of effects beyond classical interference, such as the quantum Hong-Ou-Mandel (HOM) effect. The HOM effect is caused by interference taking place not between coherent waves from distinct sources which intersect in real space, but rather between parallel processes indistinguishable to the measurement apparatus. Existing research has highlighted the outsized importance of measurement in HOM interferometry, by showing that the occurrence of process interference depends only on indistinguishability at the detectors and nowhere else. We are therefore interested in how quantum measurement occurs in HOM, and how metasurfaces can be used to enhance the HOM effect. In this thesis, we extend a formulation of quantum trajectory theory in two-eigenstate systems to 𝑁-state systems, producing a three-state formalism which serves as a possible description of coincidence pair detection in HOM experiments. We then develop an analytic model of polarisation HOM interferometry capturing both the width and the 𝑔2 ratio of the HOM dip; finally, we design and fabricate a metasurface that exhibits polarisation filtering with respect to two-photon coincidence, whereupon HOM photon bunching occurs only at select metasurface orientations relative to photon polarisation. Our three-state quantum state collapse model offers a promising explanation for how state measurement takes place in an HOM interferometer, where the interferometer output is a superimposition of three two-photon eigenstates. Meanwhile, our interferometer model and experiments provide evidence of successful polarisation filtering in HOM interferometry.
Optical metasurfaces open new avenues for the precise wavefront control of light for integrated quantum technology. Here, we demonstrate a hybrid integrated quantum photonic system that is capable of entangling and disentangling two-photon spin states at a dielectric metasurface. Via the interference of single-photon pairs at a nanostructured dielectric metasurface, a path-entangled two-photon NOON state with circular polarization that exhibits a quantum HOM interference visibility of 86 ± 4% is generated. Furthermore, we demonstrate nonclassicality andphase sensitivity in a metasurface-based interferometer with a fringe visibility of 86.8 ± 1.1% in the coincidence counts. This high visibility proves the metasurface-induced path entanglement inside the interferometer. Our findings provide a promising way to develop hybrid-integrated quantum technology operating in the high-dimensional mode space in various applications, such as imaging, sensing, and computing.
Hong-Ou-Mandel (HOM) interference with two independent photons of different wavelengths is a primary tool for controlled transfer of quantum information across a hybrid quantum network such as the quantum internet, in which the diverse material nodes operate at different resonant frequencies. The key challenge of realizing two-photon interference in the frequency domain is the need for linear frequency conversion between spectrally distinct photons, which, in contrast to frequency mixing in nonlinear optical processes, is free from multiphoton noise arising in strong laser fields. Here, we present a linear HOM scheme based on a lossless three-wave mixing between two photons of different frequencies in the laser-controlled tripod-type atoms, where the parametric interaction between photons is triggered by the induced coherence between two metastable states of the atoms [S. Petrosyan et al., Phys. Rev. A 105, 052606 (2022)]. We give the general derivation of the HOM effect and examine it for two single-pulse photons and in the important case of time-bin encoded single photons, which are well protected from decoherence in optical fibers. We show that two-photon coincidence measurements display the HOM-type ``dip and peak'' fringes according to the number of time bins in the incident photon state, allowing full recovery of the original quantum information. An essential feature of our scheme is the neutrality of the HOM effect implementation to the polarization indistinguishability of photons, which makes our model flexible for use in hybrid quantum networks.
Metasurfaces, with their capability to control all possible dimensions of light, have become integral to quantum optical applications, including quantum state generation, operation, and tomography. We utilize a metasurface to generate polarization–hologram hybrid entanglement between a signal–idler photon pair to construct a quantum hologram. The properties of the quantum hologram can be revealed by collapsing the polarization degree of freedom of the idler photon, inducing interference between two holographic states of the signal photon as a meaningful and selective erasure of the holographic content. On the contrary, interference disappears when the idler photon is detected without observing polarization. This process can be further interpreted as a quantum holographic eraser, where the erasing action is visualized with erased contents in holograms. Our construction of a polarization–hologram hybrid entangled state with metasurfaces will be useful for quantum communication with enhanced robustness, anticounterfeiting applications through the additional quantum degrees of freedom or phase difference between two holographic states, and as an emerging platform for exploring fundamental quantum concepts for entanglement and nonlocality.