  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. 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.
