  Quantum light is increasingly recognized as a promising resource for
developing optical measurement techniques. Particular attention has been paid
to enhancing the precision of the measurements beyond classical techniques by
using nonclassical correlations between quantum entangled photons. Recent
advances in quantum optics technology have made it possible to manipulate the
spectral and temporal properties of entangled photons, and the photon
correlations can facilitate the extraction of matter information with
relatively simple optical systems compared to conventional schemes. In these
respects, the applications of entangled photons to time-resolved spectroscopy
can open new avenues for unambiguously extracting information on dynamical
processes in complex molecular and materials systems. Here, we propose
time-resolved spectroscopy in which specific signal contributions are
selectively enhanced by harnessing the nonclassical correlations of entangled
photons. The entanglement time characterizes the mutual delay between an
entangled twin and determines the spectral distribution of the photon
correlations. The entanglement time plays a dual role as the knob for
controlling the accessible time region of dynamical processes and the degrees
of spectral selectivity. In this sense, the role of the entanglement time is
substantially equivalent to the temporal width of the classical laser pulse.
The results demonstrate that the application of quantum entangled photons to
time-resolved spectroscopy leads to monitoring dynamical processes in complex
molecular and materials systems by selectively extracting desired signal
contributions from congested spectra. We anticipate that more elaborately
engineered photon states would broaden the availability of quantum light
spectroscopy.
