This book presents the latest theoretical studies giving new predictions and interpretations on the quantum correlation in molecular dynamics induced by ultrashort laser pulses. The author quantifies the amount of correlation in terms of entanglement by employing methods developed in quantum information science, in particular applied to the photoionization of a hydrogen molecule. It is also revealed that the photoelectron–ion correlation affects the vibrational dynamics of the molecular ion and induces the attosecond-level time delay in the molecular vibration. Furthermore, the book also presents how molecular vibration can couple to photons in a plasmoic nanocavity. Physicists and chemists interested in the ultrafast molecular dynamics would be the most relevant readers. They can learn how we can employ the quantum-information-science tools to understand the correlation in the molecular dynamics and why we should consider the correlation between the photoelectron and the molecular ion to describe the ion’s dynamics. They can also learn how to treat a molecule coupled to photons in a nanocavity. All the topics are related to the state-of-the-art experiments, and so, it is important to publish these results to enhance the understanding and to induce new experiments to confirm the theory presented.