Abstract
Absorption spectroscopy is one of the most fundamental and widely used spectroscopic methods. Its
sensitivity is usually limited by shot noise, which arises from the statistical fluctuation of the number of
photons. When entangled photon pairs are used as the light source, however, it is possible to suppress the
shot noise and increase the sensitivity of absorption spectroscopy beyond the shot-noise limit. Recently,
experimental realizations of ultrasensitive absorption spectroscopy using entangled photon pairs have
been reported by two groups including us. Furthermore, we demonstrated that ultrasensitive absorption
spectroscopy exhibits an increased capability to identify and quantify the chemical constitution of sample
solutions, which are the two principal purposes of using absorption spectroscopy as an analytical tool.
