  The development of many optical quantum technologies depends on the
availability of solid-state single quantum emitters with near-perfect optical
coherence. However, a standing issue that limits systematic improvement is the
significant sample heterogeneity and lack of mechanistic understanding of
microscopic energy flow at the single emitter level and ultrafast timescales.
Here we develop solution-phase single-particle pump-probe spectroscopy with
photon correlation detection that captures sample-averaged dynamics in single
molecules and/or defect states with unprecedented clarity at femtosecond
resolution. We apply this technique to single quantum emitters in
two-dimensional hexagonal boron nitride, which suffers from significant
heterogeneity and low quantum efficiency. From millisecond to nanosecond
timescales, the translation diffusion, metastable-state-related bunching
shoulders, rotational dynamics, and antibunching features are disentangled by
their distinct photon-correlation timescales, which collectively quantify the
normalized two-photon emission quantum yield. Leveraging its femtosecond
resolution, spectral selectivity and ultralow noise (two orders of magnitude
improvement over solid-state methods), we visualize electron-phonon coupling in
the time domain at the single defect level, and discover the acceleration of
polaronic formation driven by multi-electron excitation. Corroborated with
results from a theoretical polaron model, we show how this translates to
sample-averaged photon fidelity characterization of cascaded emission
efficiency and optical decoherence time. Our work provides a framework for
ultrafast spectroscopy in single emitters, molecules, or defects prone to
photoluminescence intermittency and heterogeneity, opening new avenues of
extreme-scale characterization and synthetic improvements for quantum
information applications.
