Light-matter interactions in resonant media often occur on femtosecond timescales and are typically characterized using ultrafast laser systems. We employ continuous-wave (cw) Hong-Ou-Mandel (HOM) quantum interferometry using indistinguishable photon pairs generated via spontaneous parametric down-conversion to measure molecular dephasing times of the solvatochromic polymethine dye IR-797 across different solvents. Here variations in the surrounding solvent modify the electronic transition energy and linewidth of the dye, and these environment-induced changes directly influence the molecular dephasing time, which governs electronic coherence on femtosecond timescales. By inserting the dye solution into one arm of the interferometer, we observe solvent-induced modifications of the HOM interference dip arising from the coherent linear response of the medium. Fitting the coincidence profiles using a susceptibility-based Lorentzian model allows extraction of the dephasing time. At a concentration of $20\mu \mathrm{g}\text{/}\mathrm{ml}$, the retrieved coherence times range from 25 to 60 fs depending on the solvent. For comparison, classical absorption spectroscopy yields effective coherence times of 20–30 fs. These results establish cw-pumped HOM interferometry as a quantum-light approach to ultrafast spectroscopy at ultralow photon flux, enabling femtosecond-scale measurements without femtosecond laser sources. In addition, the technique inherently cancels even-order dispersion and offers promising applications in materials science, chemistry, and biological systems.

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• Light-matter interaction
• Nonlinear optics
• Photonics
• Quantum optics
• Ultrafast optics
• Ultrafast phenomena