Entangled two-photon absorption spectroscopy for optically forbidden transition detection

We theoretically propose a spectroscopic method for measuring optically forbidden states using entangled two-photon absorption (TPA). As a model system, we consider a diatomic molecular system consisting of three adiabatic potentials, namely, ground, intermediate, and excited states, where the intermediate state cannot be directly excited from the ground state. In our method, we pump the excited state using entangled TPA and indirectly measure the optically forbidden intermediate state through the photon emission from the excited state to the intermediate state. The condition required for this method is only that the transition rate between the excited and intermediate states is sufficiently high. Using our proposed method, we show that the optically forbidden state can be detected with a high degree of accuracy when highly efficient and selective TPA is realized.