Integrated optical frequency combs, as a key technology in modern photonics, have attracted significant attention in recent years due to their high-dimensional modal properties in the time and frequency domains. This technology has demonstrated remarkable applications in classical optics, such as high-capacity communication, spectral analysis, and ultrafast ranging, while also enabling unprecedented capabilities for the generation and manipulation of high-dimensional quantum states in quantum optics. However, the development of optical frequency combs, particularly quantum frequency combs (QFCs), faces a fundamental challenge: decoherence of high-dimensional quantum states caused by structural perturbations, such as sharp bends. This decoherence effect severely limits the practical applications and scalability of QFCs, as interactions with the environment in complex structures disrupt quantum coherence.

Topological photonics provides an innovative solution to this problem by enabling robust light transport in complex structures through topological protection. This mechanism ensures that photons not only propagate unidirectionally in waveguides but also remain immune to structural defects and sharp turns. In particular, the topological edge states supported by valley photonic crystal (VPC) waveguides enable robust unidirectional transport of broadband optical signals along interfaces formed by photonic crystals with opposite valley Chern numbers. These waveguides feature low loss and compact size, making them particularly suitable for on-chip integration.

Based on this concept, the researcher proposed and experimentally validated an on-chip topological waveguide design for robust transmission of high-dimensional quantum states. Experimental results demonstrated that topological waveguides provide significant bandgap protection for quantum optical frequency combs, maintaining low loss and high fidelity even under sharp bends and structural perturbations. By measuring the time correlations and frequency entanglement of QFCs, the study confirmed the effectiveness of topological protection mechanisms for preserving quantum states. Relevant research results were recently published in Photonics Research, Volume 13, Issue 1, 2025. [Zhen Jiang, Hongwei Wang, Peng Xie, Yuechen Yang, Yang Shen, Bo Ji, Yanghe Chen, Yong Zhang, Lu Sun, Zheng Wang, Chun Jiang, Yikai Su, Guangqiang He, "On-chip topological transport of integrated optical frequency combs," Photonics Res. 13, 163 (2025)].

The research team designed VPC waveguides with a bandwidth exceeding 25 THz in the telecommunications range (as shown in Fig. 1). These waveguides were fabricated on silicon-on-insulator (SOI) wafers using high-precision electron beam lithography and inductively coupled plasma etching. The experimental setup (as shown in Fig. 2) systematically validated the topological waveguide's ability to protect quantum optical frequency combs. By using a silicon nitride microresonator with a free spectral range of 100 GHz, the researchers generated quantum frequency combs under low pump power conditions. Measurements of the time-correlation spectra and joint spectral intensity successfully demonstrated that high-dimensional quantum states with frequency entanglement remained robust during transmission through sharp bends, further confirming the critical role of VPC waveguides in the topological protection of high-dimensional quantum states.

Figure 1 (a) Schematic of the valley photonic crystal (VPC) waveguide supporting on-chip topological transport of optical frequency combs. (b) Dispersion relation of the topological waveguide. (c) Scanning electron microscopy (SEM) image of the straight topological waveguide and (d) Z-shaped topological waveguide. (e) Transmission spectra of the topological waveguide.

The team will further explore the applications of topological photonics in multimode frequency comb transmission, particularly in the multiplexing and dynamic control of multidimensional quantum states. Efforts will focus on developing highly integrated topological photonic structures and achieving seamless integration with existing photonic chip platforms. Additionally, the synergy between nonlinear optical effects and quantum state manipulation under the framework of topological protection will be investigated.

Figure 2 Experimental setup for the topological transport of optical frequency combs.