Characterization of conical intersection in a cis-trans isomerization through nonclassicality and entanglement

In this paper we have characterized the conical intersection, an important topological feature of potential energy surfaces in a in cis-trans isomerization, in terms of the nonclassicality and quantum entanglement which are shown to be controlled by the torsion angle as a molecular parameter. In this context we have provided a quantitative measure of entanglement in terms of Wigner function matrix and compared with other standard measures, namely, von Neumann entropy and partial transpose of joint density matrix. It is shown that the entanglement in the ground state maximizes as the angle reaches the conical intersection point. The highly nonclassical nature of the conical intersection is shown in terms of the significant amount of squeezing for long ranges of the torsion angle and vibronic coupling. The region of high entanglement at and around the conical intersection is connected to the spectroscopically 'dark' time window recently detected in ultrafast transient optical spectroscopy experiments in molecular systems with torsional motion which is associated with the presence of conical intersection.