Quantum imaging using entangled photon pairs from spontaneous parametric down-conversion (SPDC) provides fundamental advantages over classical phase imaging in the study of transparent object, such as the operation at ultra-low photon flux and high signal-to-noise ratio [1], [2]. Metasurfaces supporting angle-dependent resonances have recently been employed to achieve phase imaging by extracting phase gradients of incident light [3]. These nanoscale metasurfaces indicate significant potential to replace bulky differential interference contrast microscopes, offering an opportunity for integration into ultra-compact biomedical devices. Together with recent development on metasurface-based quantum light source, this opens up the potential for developing a compact quantum phase imaging device, where metasurfaces serve both generation and detection functions.
Quantum imaging using entangled photon pairs from spontaneous parametric down-conversion (SPDC) provides fundamental advantages over classical phase imaging in the study of transparent object, such as the operation at ultra-low photon flux and high signal-to-noise ratio [1], [2]. Metasurfaces supporting angle-dependent resonances have recently been employed to achieve phase imaging by extracting phase gradients of incident light [3]. These nanoscale metasurfaces indicate significant potential to replace bulky differential interference contrast microscopes, offering an opportunity for integration into ultra-compact biomedical devices. Together with recent development on metasurface-based quantum light source, this opens up the potential for developing a compact quantum phase imaging device, where metasurfaces serve both generation and detection functions.
The quantum correction of photon pairs inspired the development of quantum ghost imaging principles. It offers fundamental advantages compared to conventional imaging, including the operation at ultra-low photon flux and the potential for higher resolution [1]. Over the last year, it was demonstrated experimentally that nonlinear metasurfaces with a thickness of only a few hundred nanometers can facilitate strongly enhanced generation of photon pairs through spontaneous parametric down-conversion (SPDC). Furthermore, the quantum photon state can be tailored to feature spatial [2] and spectral [3] entanglement. However, the potential of metasurface-based photon-pair sources for quantum imaging was not previously investigated.
Quantum phase imaging enables the analysis of transparent samples with thickness and refractive index variations in scenarios requiring precise measurements under low-light conditions. Here, we present a compact quantum phase-gradient imaging system integrating a lithium niobate (LiNbO3) metasurface for generating spatially entangled photon pairs and a silicon (Si) metasurface for phase gradient extraction. By leveraging nonlocal resonances, the LiNbO3 metasurface enables efficient spontaneous parametric down-conversion (SPDC) with all-optically angularly tunable emission, while the Si metasurface provides a nearly linear optical transfer function (OTF) that differentiates the photon wavefunction and extracts phase gradients.Experimental proof-of-concept results demonstrate the imaging of up to 25~rad/mm phase gradients, achieving 89% similarity with the reference values. The pixel resolution of the system can be potentially enhanced by orders of magnitude by increasing the metasurface dimensions and resonance quality factor.Our work showcases the application of metasurfaces in both generating and detecting quantum states and establishes a new paradigm for portable quantum phase-gradient imaging, with potential applications in quantum sensing, microscopy, and LiDAR technology.
Phase contrast microscopy is essential in optical imaging, but traditional systems are bulky and limited by single-phase modulation, hindering visualization of complex specimens.Metasurfaces, with their subwavelength structures, offer compact integration and versatile light field control.Quantum imaging further enhances performance by leveraging nonclassical photon correlations to suppress classical noise, improve contrast, and enable multifunctional processing.Here, we report, a dual-mode bright-dark phase contrast imaging scheme enabled by quantum metasurface synergy.By combining polarization entanglement from quantum light sources with metasurface phase modulation, our method achieves highcontrast, adaptive bright-dark phase contrast imaging of transparent samples with arbitrary phase distributions under low-light conditions.Compared to classical weak light imaging, quantum illumination suppresses environmental noise at equivalent photon flux, improving the contrast from 0.22 to 0.81.Moreover, quantum polarization entanglement enables remote switching to ensure high-contrast imaging in at least one mode, supporting dynamic observation of biological cells with complex phase structures.This capability is especially valuable in scenarios where physical intervention is difficult, such as in miniaturized systems, in vivo platforms, or extreme environments.Overall, the proposed scheme offers an efficient noninvasive solution for biomedical imaging with strong potential in life science applications.
The recent advent of optical metasurfaces, ultra-thin structures composed of nanoresonators facilitating strong light concentration [1–3], opens the path towards efficient \nfrequency conversion [4–7] in a material thousand times thinner than a human hair [8]. \nThere is a remarkable capacity to individually shape each nanoresonator of the metasurface to spatially and spectrally control the conversion process with unprecedented \nnanoscale resolution, facilitating an ultimate flexibility to selectively convert, focus, \nand image different colours with a single metasurface. The strong enhancement of the \nnonlinear processes in dielectric nanoresonators is largely due to the absence of material \nabsorption and the excitation of Mie-type bulk resonances [2]. The highest conversion \nefficiency to date has been achieved employing III-V semiconductor nanostructures, \nsuch as AlGaAs which is a non-centrosymmetric material with high quadratic nonlinear \nsusceptibility. In particular, second-harmonic generation efficiencies up to 10−4 have \nbeen recently demonstrated [8–12], six orders of magnitude higher than in plasmonics. \nSuch capabilities can make an immense fundamental and practical impact, including \nthe quantum state generation in nonlinear metasurfaces as we discuss in this chapter. \nThe quantum state of correlated photon-pairs is the essential building block for photon entanglement [13, 14], which underpins many quantum applications, including secure networks, enhanced measurement and lithography, and quantum information processing [15]. One of the most versatile techniques for the generation of correlated photons is the process of spontaneous parametric down-conversion (SPDC) [16, 17], see \nFig. 1. It allows for an arbitrary choice of energy and momentum correlations between \nthe generated photons, robust operation at room temperature, as well as for spatial and \ntemporal coherence between simultaneously pumped multiple SPDC sources. Alternative approaches based on atom-like single photon emitters, such as solid-state fluorescent atomic defects [18], quantum dots [19, 20], and 2D host materials [21, 22], have \nreached a high degree of frequency indistinguishability, purity and brightness [19, 20]. \nHowever, this comes with the expense of operation at cryogenic temperatures and lack \nof spatial coherence between multiple quantum emitters. These features might limit \npossible applications and reduce the potential for device scalability. Furthermore, the \nsmall size of the atomic sources often requires complex schemes aimed at coupling to \noptical nanoantennas and improving the photon extraction efficiency [21]. \nThe miniaturization of SPDC quantum-light sources to micro and nanoscale dimensions is a continuing quest, as it enables denser integration of functional quantum \ndevices. Traditionally, bulky cm-sized crystals were utilized for SPDC, entailing the \ndifficulty of aligning multiple optical elements after the SPDC crystal, while offering \nrelatively low photon-pair rates [17]. As a first step of miniaturization, SPDC was \nrealized in low-index-contrast waveguides, which allowed confining light down to several square micrometers transversely to the propagation direction, significantly enhancing the conversion efficiency [23]. However, this approach still requires centimetres \nof propagation length, which makes the on-chip integration with other elements challenging [24]. The introduction of high-index contrast waveguides and ring resonators \nallowed for shrinking the sizes necessary for SPDC to millimetres [25], and to tens \nof micrometers [26]. However, further miniaturisation down to the metasurfaces composed of nanoresonators requires conceptually different approaches.
Photon pairs generated by spontaneous parametric down-conversion (SPDC) are playing a pivotal role in many quantum applications such as quantum computing, quantum imaging and quantum communication. Metasurfaces, which are two-dimensional arrays of nanostructures, have been studied intensively to enhance SPDC and provide additional functionalities over the bulky optical systems [1]. Van der Waals (vdWs) layered materials have emerged as promising platform for quantum light sources, owing to their unique properties [2], [3]. For example, vdWs materials offer high refractive index, a high nonlinear susceptibility, and compatibility with on-chip integration.
Phase reconstruction plays a pivotal role in biology, medical imaging, and wavefront sensing. However, multiple measurements and adjustments are usually required for conventional schemes, which inevitably reduces the quality of phase imaging. Here, based on multi-channel metasurface and quantum entanglement source, a simple and integrated quantum analog operation system is proposed to realize quantitative phase reconstruction with a high signal-to-noise ratio (SNR) under a low signal photon level. Without additional measurements and adjustments, four differential images necessary for the phase reconstruction are captured simultaneously. The non-local correlation of entangled photon pairs enables to remotely manipulate working modes of the system. Besides, the consistency of entangled photon pairs in time domain makes it possible to achieve a high SNR imaging by trigger detection. The results may potentially empower the application of metasurfaces in optical chip, wave function reconstruction, and label-free biology imaging.