We report an experimental method that combines quantum interferometry with molecular spectroscopy using indistinguishable photon pairs generated via spontaneous parametric down-conversion of continuous-wave (cw) pump radiation in a Hong-Ou-Mandel (HOM) setup. Introducing a resonant organic medium into one interferometer arm leads to measurable changes in the coincidence signal due to the coherence response of the medium, from which we determine a molecular dephasing time of $40.1\pm 0.7\text{fs}$ for concentrations ranging from 2 to $40\mu \mathrm{g}/\mathrm{mL}$. Unlike classical techniques, this approach leverages photon correlations with low photon flux, making it immune to even-order dispersion and insensitive to classical losses. The results align with a theoretical framework that incorporates the joint spectral properties of the photon pair and the molecular transmission response. This work extends the technique of Eshun et al. [J. Am. Chem. Soc. 143, 9070 (2021)] to the cw domain, demonstrating the potential of cw-pumped HOM interferometry for probing ultrafast molecular dynamics and highlighting its promise as a foundation for advanced quantum-light spectroscopic techniques in organic systems.

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• Molecular spectra
• Optical coherence
• Quantum optics
• Ultrafast optics
• Ultrafast phenomena