Harnessing the power of special light-based technologies has long promised stable and efficient ways to control how light moves through devices. But one major limitation has been the need for slow and careful changes to maintain something called “adiabaticity” — a condition where light moves through a system without jumping unpredictably between different energy levels. A recent study offers an important advancement by identifying the shortest possible time these changes can happen, opening the door to smaller and faster optical devices.
Professor Tao Li and Dr. Wange Song, from Nanjing University have developed a method to reach this shortest limit — known as the adiabatic infimum, the minimum time or length needed to preserve stability in a system — using specially designed light-guiding structures made from lithium niobate, a crystal material often used in high-performance optics. Their work is published in the peer-reviewed journal Nature Communications.
The research centers on a process known as topological pumping, a way of transferring light or other particles from one location to another in a system by carefully adjusting conditions over time. What makes this process special is its topological nature — meaning it relies on the overall structure of the system rather than its specific details, which helps it remain stable even when imperfections are present. Normally, this transfer requires slow adjustments to keep the system adiabatic, but the team discovered a way to speed up the process by optimizing the path that controls how the system evolves.
To make this work, Professor Li and Dr. Song focused on the shape of what’s called a modulation loop, the path along which key system properties are varied to guide the light. Central to their approach was minimizing the Berry connection, a mathematical concept that describes how the quantum state of light shifts as it moves through the system. This connection determines how likely the light is to jump into unwanted states. By finding a way to reduce it along the path, the researchers made it possible for the system to evolve faster while still staying stable.
The team tested this using two versions of their design, both based on the Rice-Mele model, a simplified framework often used to study systems with two alternating parts, like a chain of optical waveguides. One design followed the traditional loop, while the other used their optimized version, referred to as the INFI loop — short for “infimum” — representing the most efficient route. In the standard setup, the light only moved cleanly when the device was relatively long. In contrast, the INFI loop achieved the same result over a much shorter distance. “We approach the adiabatic infimum by minimizing the effective Berry connection along the loop,” Professor Li said, highlighting how this smarter route avoided problematic areas in the system’s configuration.
These results weren’t just theoretical. The researchers built the devices on chips using thin layers of lithium niobate, known as a thin-film lithium niobate-on-insulator platform, which is a technology that combines good optical properties with the ability to make compact devices. They injected light into the waveguide structures — tiny paths that guide light along specific routes — and observed how it moved. In the traditional design, the light failed to stay on track unless the path was long enough. But in the new design, the light followed its intended route over a much shorter length, confirming the success of the approach.
“The adiabatic infimum accelerates the topological pumps from the constraints of slow evolution and facilitates the design of compact topological devices” Dr. Song added. In other words, this method lifts the restrictions caused by slow processes and enables much smaller devices with the same performance. This is especially important for lithium niobate platforms, which typically need more space because the material bends light less strongly than others like silicon.
The findings also provide new understanding of how light can be controlled in systems that are stable against imperfections. By rethinking how paths are shaped in the system’s configuration space — the abstract landscape that represents all possible settings — the team has shown that light can be pumped faster without losing reliability. This has broad potential in areas such as quantum computing, where light needs to move precisely and quickly, or in telecommunications and sensing technologies that require compact, reliable optical circuits.
Professor Li and Dr. Song’s study has not only improved the performance of these devices, but also offered new insights into the physics behind them. By reaching the adiabatic infimum, they’ve shown how careful design can push the limits of speed and efficiency in light-based systems — a step forward in developing the next generation of optical technologies.
Journal Reference
Wu S., Song W., Sun J., Li J., Lin Z., Liu X., Zhu S., Li T. “Approaching the adiabatic infimum of topological pumps on thin-film lithium niobate waveguides.” Nature Communications, 2024. DOI: https://doi.org/10.1038/s41467-024-54065-9
About the Authors
Tao Li is a professor at Nanjing University. He received the National Science Fund for Distinguished Young Scholars and the Excellent Young Scientists Fund. He has also received the “Wang Kuancheng” Education Fund from Hong Kong and has been selected as a leading talent in science and technology innovation by the Ministry of Science and Technology, as well as being part of the first cohort of Nanjing University’s “Dengfeng Project B.” He has been recognized five times for significant advancements in the field of optics in China. Professor Li has delivered over 50 invited talks at international conferences and has published more than 130 papers in journals such as Nature and its sub-journals, Phys. Rev. Lett., and Light Sci. Appl., with his work being cited over 12,000 times. Currently, he serves as a youth editorial board member for the Chinese Laser Press and an editorial board member for several other journals and publications, including Science Bulletin and ADI. He is also a council member of the Metamaterials Division of the Chinese Materials Research Society, the Jiangsu Physical Society, and the Jiangsu Optical Society.
Wange Song is an associate researcher at Nanjing University and a visiting scholar at the University of Hong Kong. He obtained his Bachelor’s degree in Materials Physics from Nanjing University in 2016 and his Ph.D. in Optical Engineering from the same university in 2021. He has been awarded the Wang Daheng Optics Award, the Jiangsu Province Young Optical Science and Technology Award, and has been selected as a Yuxiu Young Scholar at Nanjing University. Song’s research primarily focuses on micro-nano optics, and in recent years, he has made significant contributions in the fields of topological optics and non-Hermitian optical field manipulation. To date, he has published over 30 papers in international academic journals such as PRL (6 papers), Nat. Commun., and Sci. Adv. His research has been featured in Editor’s Suggestions and cover articles, and has been highlighted by Physics, PhysOrg and SPIE. He also serves as a youth editorial board member and guest editor for several academic journals.
