The development of many quantum optical technologies depends on the availability of single quantum emitters with near-perfect coherence. Systematic improvement is limited by a lack of understanding of the microscopic energy flow at the single-emitter level and ultrafast timescales. Here we utilize a combination of fluorescence correlation spectroscopy and ultrafast spectroscopy to capture the sample-averaged dynamics of defects with single-particle sensitivity. We employ this approach to study heterogeneous emitters in two-dimensional hexagonal boron nitride. From milliseconds to nanoseconds, the translational, shelving, rotational and antibunching features are disentangled in time, which quantifies the normalized two-photon emission quantum yield. Leveraging the femtosecond resolution of this technique, we visualize electron–phonon coupling and discover the acceleration of polaronic formation on multi-electron excitation. Corroborated with theory, this translates to the photon fidelity characterization of cascaded emission efficiency and decoherence time. Our work provides a framework for ultrafast spectroscopy in heterogeneous emitters, opening new avenues of extreme-scale characterization for quantum applications.

许多量子光学技术的发展取决于具有近乎完美相干性的单量子发射器的可用性。由于缺乏对单发射器水平和超快时间尺度的微观能量流的理解，系统性的改进受到限制。在这里，我们利用荧光相关光谱和超快光谱的组合来捕获具有单粒子灵敏度的缺陷的样本平均动态。我们采用这种方法来研究二维六方氮化硼中的异质发射器。从毫秒到纳秒，平移、倾斜、旋转和反聚束特征在时间上相互关联，从而量化了归一化的双光子发射量子产率。利用该技术的飞秒分辨率，我们将电子-声子耦合可视化，并发现多电子激发时极化子形成的加速。经理论证实，这转化为级联发射效率和退相干时间的光子保真度表征。我们的工作为异质发射器中的超快光谱提供了一个框架，为量子应用的超尺度表征开辟了新途径。