Quantum Phase Gradient Imaging Achieves Precise Measurements Using a Nonlocal Metasurface System

Researchers are developing increasingly sophisticated methods for visualising transparent samples and extracting detailed information about their structure, and a team led by Jinliang Ren, Katsuya Tanaka, and Sukhorukov from the Australian National University now presents a significant advance in this field. They have created a compact phase imaging system that accurately measures phase gradients, changes in the optical path of light, using a novel combination of nanoscale materials called metasurfaces. This system generates entangled photons and then extracts phase information through spatial correlations, achieving high-resolution imaging without the need for mechanical adjustments. The results demonstrate the ability to resolve phase gradients with exceptional fidelity, paving the way for portable, high-precision imaging with potential applications in microscopy, LiDAR, and other areas requiring detailed structural analysis.

Metasurface Generation of Entangled Photon Pairs

Scientists are pioneering the use of specifically designed metasurfaces, artificial nanostructures, to generate and manipulate entangled photon pairs, a crucial resource for emerging quantum technologies. A key focus is image differentiation, the process of identifying edges and gradients within an image, which the team aims to achieve by harnessing the unique properties of the generated entangled photons and carefully designing the metasurface. This work bridges the gap between the fundamental principles of quantum optics and the practical applications of classical optics.

The research involves designing and fabricating metasurfaces with specific nonlinear optical properties, enabling efficient conversion of light into entangled photon pairs. Materials like lithium niobate are frequently used due to their strong response to light. Scientists are controlling the properties of the generated entangled photons, including their polarization and spatial characteristics, to tailor them for specific applications. Crucially, the team aims to tune the properties of the entangled photons, such as their wavelength and spatial mode, to optimize performance. The team explores various quantum imaging techniques, including ghost imaging, which creates an image without directly illuminating the object, and quantum illumination, which improves object detection in noisy environments. They are specifically investigating how entangled photons can be used to perform image differentiation, achieving this by designing metasurfaces that spatially correlate the entangled photons in a way that highlights edges and gradients. This interdisciplinary approach combines optics, nanotechnology, quantum physics, and signal processing.

LiNbO3 and Silicon Metasurface Phase Imaging

Scientists engineered a compact phase imaging system by integrating lithium niobate (LiNbO3) and silicon (Si) metasurfaces, achieving a breakthrough in quantum imaging capabilities. The LiNbO3 metasurface efficiently generates spatially entangled photon pairs through spontaneous parametric down-conversion, enabling tunable emission with angular dispersion. The Si metasurface extracts phase gradients via spatial correlations using a nearly linear optical transfer function. This dual-metasurface approach combines ghost imaging and all-optical scanning protocols, reconstructing phase gradients without mechanical tuning, a significant advancement over traditional methods. To demonstrate the system’s capabilities, researchers reconstructed phase gradients from an S-shaped object by rescaling coincidence data into an inverse Gaussian distribution, revealing a linear relationship between fitted and original data. Applying these fitting parameters enabled reconstruction of the phase gradient, achieving 88% image similarity when compared to the target phase gradient via a mode overlap method.

Quantum Metasurface Enables Precise Phase Imaging

Scientists have developed a novel system for phase-gradient imaging that leverages the unique properties of quantum light and compact metasurfaces, achieving a breakthrough in high-sensitivity, low-light measurements. The research demonstrates an all-optical approach to reconstructing phase gradients without mechanical tuning, opening new possibilities for applications in sensing, microscopy, and LiDAR. The core of the system utilizes a lithium niobate metasurface to efficiently generate spatially entangled photon pairs through spontaneous parametric down-conversion, creating a stable and versatile quantum light source. For phase-gradient extraction, the team designed a silicon metasurface that performs first-order differentiation of the single-photon wavefield, directly retrieving the phase gradient through two-photon correlation measurements.

Experiments demonstrate the system’s ability to resolve phase gradients up to 25 rad/mm with 88% fidelity, using a compact 6×3-pixel proof-of-concept setup. This high fidelity confirms the precision of the phase reconstruction process. The resolution of the system is primarily limited by the quality factor of the compact design, leveraging nonlocal resonances and quantum correlations. The system operates on the principle of quantum ghost imaging, offering advantages such as reduced noise at low light levels. Single-photon wave packets are prepared at different spatial positions and, after propagating through a phase object, acquire spatially varying phase modulations. The silicon metasurface then imposes a linear optical transfer function, effectively taking a spatial derivative of the phase in real space. This innovative approach allows for precise phase-gradient imaging with a compact and all-optically tunable system, representing a significant advancement in the field of quantum imaging.

Compact Metasurface Imaging Resolves Phase Gradients

This research demonstrates a compact phase imaging system built with lithium niobate and silicon metasurfaces, capable of resolving phase gradients with high fidelity. By integrating these metasurfaces, the team achieved spatially resolved phase gradient reconstruction without the need for mechanical tuning, a significant advancement for portable imaging technologies. The system generates entangled photon pairs and then extracts phase information through spatial correlations, enabling the reconstruction of phase gradients up to 25 rad/mm with 88% fidelity using a small 6×3 pixel setup. The work establishes a new approach to phase-gradient imaging, leveraging nonlocal resonances and correlations within the metasurface design. The resolution of the system is primarily limited by the quality factor of the compact design, as confirmed by theoretical analysis and experimental results. While the current demonstration utilizes a limited pixel count, the principles established pave the way for higher resolution imaging systems with potential applications in microscopy and LiDAR.