Exceptional points (EPs) are singularities in non-Hermitian systems, where k (k ≥ 2) eigenvalues and eigenstates coalesce. High-order EPs exhibit richer topological characteristics and better sensing performance than second-order EPs. Theory predicts even richer non-Hermitian topological phases for high-order EP geometries, such as lines or rings formed entirely by high-order EPs. However, experimental exploration of high-order EP geometries has hitherto proved difficult due to the demand for more degrees of freedom in the Hamiltonian’s parameter space or a higher level of symmetries. Here we observe a third-order exceptional line in an atomic-scale system. To this end, we use a nitrogen-vacancy spin in diamond and introduce multiple symmetries in the non-Hermitian Hamiltonian realized with the system. Furthermore, we show that the symmetries play an essential role in the occurrence of high-order EP geometries. Our approach can in future be further applied to explore high-order EP-related topological physics at the atomic scale and, potentially, for applications of high-order EPs in quantum technologies.

例外点 (EP) 是非厄米系统中的奇点，其中k ( k  ≥ 2) 个特征值和特征态合并。高阶EP比二阶EP表现出更丰富的拓扑特性和更好的传感性能。理论预测高阶 EP 几何形状会出现更丰富的非厄米拓扑相，例如完全由高阶 EP 形成的线或环。然而，由于需要哈密顿量参数空间中的更多自由度或更高水平的对称性，迄今为止，高阶 EP 几何形状的实验探索已被证明是困难的。在这里，我们观察到原子尺度系统中的三阶异常线。为此，我们在金刚石中使用氮空位自旋，并在系统实现的非厄米哈密顿量中引入多重对称性。此外，我们表明对称性在高阶 EP 几何形状的出现中起着至关重要的作用。我们的方法将来可以进一步应用于探索原子尺度上与高阶 EP 相关的拓扑物理，并有可能用于高阶 EP 在量子技术中的应用。