Two-dimensional fluorescence spectroscopy with entangled photons and time- and frequency-resolved two-photon coincidence detection

Recent theoretical studies highlight how nonclassical photon correlations in entangled photon pairs can selectively address nonlinear optical pathways. However, the resulting signals are typically too weak for practical time-resolved experiments. Here, we propose two-dimensional (2D) time-resolved fluorescence spectroscopy that exploits these correlations and operates with current single-photon detectors. The method provides two advantages over conventional 2D electronic spectroscopy: (i) It yields 2D spectra without phase-stable multipulse control, relying instead on heralded twin-photon correlations, and (ii) it simplifies spectra by isolating the contribution that is spectroscopically equivalent to stimulated emission, thereby suppressing ground-state bleaching and excited-state absorption. Numerical calculations for a natural pigment-protein complex-inspired trimer show that this pathway selectivity enables the extraction of rich information on energy transfer dynamics. These results indicate a feasible route to real-time observation of molecular dynamics using entangled photon pairs.