A new quantum metrology technique that can measure displacements with sub-diffraction limit precision has been demonstrated by researchers from the Chinese Academy of Sciences, Fudan University and other institutions. This innovative method, published in Engineering, offers a simplified and robust approach to achieving nanoscale resolution, which is critical for applications such as nanofabrication, weak force sensing, and high-resolution microscopy.

Traditional optical nanometry methods often rely on complex setups involving precision nanostructure fabrication, multi-beam interferometry, or intricate post-processing algorithms. These methods can be limited by practical constraints such as cost, complexity, and susceptibility to environmental noise. The new technique, however, leverages a metasurface substrate with a mode-conversion function to achieve high precision without these drawbacks.

The research team designed a metasurface that converts an incident Gaussian beam into higher-order transverse electromagnetic (TEM) modes. By analyzing the displacement within an extended quantum framework and calculating the Fisher information for nanoscale displacements, the researchers demonstrated that the position of the metasurface could be detected with high accuracy, even below the diffraction limit. In their experimental setup, the metasurface was mounted on a one-dimensional displacement nanostage, allowing for transverse displacements. The incident Gaussian beam was converted into higher-order TEM modes, which were then detected using optical sensors.

The study’s experimental results showed a precision of approximately 1/290 000 of the diffraction limit, with an achievable resolution of 2.2 pm and an experimental demonstration of 1 nm resolution. This level of precision was maintained even when the displacement was below the diffraction limit, as confirmed by the Fisher information calculations. The metasurface used in the experiment was fabricated using laser-induced birefringence nanopores within silica glass, with an average nanopore diameter ranging from 35 to 45 nm. This method emphasized high robustness by creating meta-atoms within a transparent substrate.

The researchers also explored the effects of spot size and rotation angle on measurement accuracy. They found that smaller spot sizes resulted in more concentrated power and higher conversion efficiency, which in turn improved measurement precision. Additionally, the optimal incident angle was identified as a critical factor for achieving high conversion efficiency and accurate displacement measurements.

This quantum metrology technique offers several advantages over traditional methods. It simplifies the optical architecture, enhances robustness, and reduces cost while maintaining high precision. The method’s versatility allows for mode-conversion patterns to be fabricated on various surfaces, making it suitable for a wide range of applications. The researchers propose that this technology could be integrated into practical applications such as real-time wafer stage monitoring in extreme ultraviolet (EUV) lithography, providing a cost-effective and high-precision alternative to conventional interferometric techniques.

Future work will focus on expanding this technique to larger-range, multi-degree-of-freedom displacement measurements and further enhancing its accuracy by incorporating advanced quantum measurement strategies. This research not only provides a valuable tool for ultra-precision displacement sensing but also paves the way for broader advancements in quantum-enhanced optical measurement technologies.

The paper “Sub-Diffraction Limit Quantum Metrology for Nanofabrication,” is authored by Wenyi Ye,Yang Li,Lianwei Chen,Mingbo Pu,Zheting Meng,Yuanjian Huang,Hengshuo Guo,Xiaoyin Li,Yinghui Guo,Xiong Li,Yun Long,Emmanuel Stratakis,Xiangang Luo. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.04.010. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.