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The development of many optical quantum technologies depends on theavailability of solid-state single quantum emitters with near-perfect opticalcoherence. However, a standing issue that limits systematic improvement is thesignificant sample heterogeneity and lack of mechanistic understanding ofmicroscopic energy flow at the single emitter level and ultrafast timescales.Here we develop solution-phase single-particle pump-probe spectroscopy withphoton correlation detection that captures sample-averaged dynamics in singlemolecules and/or defect states with unprecedented clarity at femtosecondresolution. We apply this technique to single quantum emitters intwo-dimensional hexagonal boron nitride, which suffers from significantheterogeneity and low quantum efficiency. From millisecond to nanosecondtimescales, the translation diffusion, metastable-state-related bunchingshoulders, rotational dynamics, and antibunching features are disentangled bytheir distinct photon-correlation timescales, which collectively quantify thenormalized two-photon emission quantum yield. Leveraging its femtosecondresolution, spectral selectivity and ultralow noise (two orders of magnitudeimprovement over solid-state methods), we visualize electron-phonon coupling inthe time domain at the single defect level, and discover the acceleration ofpolaronic formation driven by multi-electron excitation. Corroborated withresults from a theoretical polaron model, we show how this translates tosample-averaged photon fidelity characterization of cascaded emissionefficiency and optical decoherence time. Our work provides a framework forultrafast spectroscopy in single emitters, molecules, or defects prone tophotoluminescence intermittency and heterogeneity, opening new avenues ofextreme-scale characterization and synthetic improvements for quantuminformation applications.

Solution-phase single-particle spectroscopy for probing multi-polaronic dynamics in quantum emitters at femtosecond resolution