Monitoring Wavepacket Dynamics at Conical Intersections by Entangled Two-Photon Absorption

In this theoretical study, we show how ultrafast electron- nuclear dynamics in a molecule can be monitored by two-photon absorption of time-energy entangled photon pairs generated by spontaneous parametric down-conversion with a narrow-band pump. The dynamics is tracked by a controllable signal-idler delay. Time correlation of the entangled photons enables an ultrafast measurement even with a monochromatic pump. The frequency anticorrelation of the photon pair allows the projection of the intermediate excited-state wavepacket onto an energy-selected vibrational level on a higher-lying electronic state with high energy resolution. Such joint high temporal and energy resolution is not possible with classical light. We demonstrate our scheme for a conical intersection model with four electronic states and two vibrational modes. The ultrafast nuclear wavepacket dynamics around the conical intersection is imprinted onto the time- and frequencyresolved entangled two-photon absorption signals. Electronic population dynamics is obtained by the frequency-integrated signal.