Quantum phase gradient imaging using a nonlocal metasurface system

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 paradigm for portable quantum phase-gradient imaging, with potential applications in quantum sensing, microscopy, and LiDAR technology.