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The exceptional points (EPs) aroused from the non-Hermiticity bring richphenomena, such as exceptional nodal topologies, unidirectional invisibility,single-mode lasing, sensitivity enhancement and energy harvesting. Isolatedhigh-order EPs have been observed to exhibit richer topological characteristicsand better performance in sensing over 2nd-order EPs. Recently, high-order EPgeometries, such as lines or rings formed entirely by high order EPs, arepredicted to provide richer phenomena and advantages over stand-alonehigh-order EPs. However, experimental exploration of high-order EP geometriesis hitherto beyond reach due to the demand of more degrees of freedom in theHamiltonian's parameter space or a higher level of symmetries. Here we reportthe observation of the third-order exceptional line (EL) at the atomic scale.By introducing multiple symmetries, the emergence of the third-order EL hasbeen successfully realized with a single electron spin of nitrogen-vacancycenter in diamond. Furthermore, the behaviors of the EP structure underdifferent symmetries are systematically investigated. The symmetries are shownto play essential roles in the occurrence of high-order EPs and the related EPgeometries. Our work opens a new avenue to explore high-order EP-relatedtopological physics at the atomic scale and to the potential applications ofhigh-order EPs in quantum technologies.

Third-order exceptional line in a nitrogen-vacancy spin system