  Recent advances in techniques for generating quantum light have stimulated
research on novel spectroscopic measurements using quantum entangled photons.
One such spectroscopy technique utilizes non-classical correlations among
entangled photons to enable measurements with enhanced sensitivity and
selectivity. Here, we investigate spectroscopic measurement utilizing entangled
three photons generated through cascaded parametric down-conversion. In this
measurement, time-resolved entangled photon spectroscopy with monochromatic
pumping [J. Chem. Phys. 153, 051102 (2020).] is integrated with the
frequency-dispersed two-photon counting technique, which suppresses undesired
accidental photon counts in the detector and thus allows one to separate the
weak desired signal. This time-resolved frequency-dispersed two-photon counting
signal, which is a function of two frequencies, is shown to provide the same
information as that of coherent two-dimensional optical spectra. The spectral
distribution of the phase-matching function works as a frequency filter to
selectively resolve a specific region of the two-dimensional spectra, whereas
the excited-state dynamics under investigation are temporally resolved in the
time region longer than the entanglement time. The signal is not subject to
Fourier limitations on the joint temporal and spectral resolution, and
therefore, it is expected to be useful for investigating complex molecular
systems in which multiple electronic states are present within a narrow energy
range.
