Single-photon emitters serve as building blocks for many emerging concepts in quantum photonics. The recent identification of bright, tunable and stable emitters in hexagonal boron nitride (hBN) has opened the door to quantum platforms operating across the infrared to ultraviolet spectrum. Although it is widely acknowledged that defects are responsible for single-photon emitters in hBN, crucial details regarding their origin, electronic levels and orbital involvement remain unknown. Here we employ a combination of resonant inelastic X-ray scattering and photoluminescence spectroscopy in defective hBN, unveiling an elementary excitation at 285 meV that gives rise to a plethora of harmonics correlated with single-photon emitters. We discuss the importance of N π* anti-bonding orbitals in shaping the electronic states of the emitters. The discovery of elementary excitations in hBN provides fundamental insights into quantum emission in low-dimensional materials, paving the way for future investigations in other platforms.

单光子发射器是量子光子学中许多新兴概念的构建模块。最近在六方氮化硼（hBN）中发现了明亮、可调谐且稳定的发射体，为跨红外到紫外光谱运行的量子平台打开了大门。尽管人们普遍认为缺陷是六方氮化硼中单光子发射体的原因，但有关其起源、电子能级和轨道参与的关键细节仍然未知。在这里，我们在有缺陷的 hBN 中结合使用了共振非弹性 X 射线散射和光致发光光谱，揭示了 285 meV 的基本激发，产生了与单光子发射器相关的大量谐波。我们讨论了 N π* 反键轨道在塑造发射体电子态中的重要性。六方氮化硼中基本激发的发现为低维材料中的量子发射提供了基本见解，为其他平台的未来研究铺平了道路。