Dynamical Properties Of Entanglement Of Vibrations In Molecule H₂S

Quantum entanglement is one of the most significant features of quantum mechanics.It has the non-local nature of the quantum states so that it is ext(?)nsively applied to quantum information and quantum computing and becomes an important foundation of the process of quantum information,such as the realization of quantum teleportation,quantum key distribution,quantum dense coding,and quantum correction. In recent years,the preparation and measurement of the entanglement states has stimulated widespread interest and has become one of the key issues in the research and application of quantum information.Because the entropy function automatically contains all the statistical density matrix of the quantum system,it is a very sensitive operation and mesurement of quantum states so that it is used to describe the degree of the entanglement in quantum system.Furthermore,it is an important tool to explain the dynamical properties of quantum system and has many applications in the field of quantum information.Atom-field interation is one of the prominent subjects for generating controllabe entanglement.And quantum entanglement of two stretching vibrations in molecules has recently been studied.Their entanglement can be used in quantum information and quantum computing.This thesis investigates the dynimical behaviors of entanglement in above mentioned two typical models with the theory of entanglement entropy.Firstly,the evolution of entanglement between a two-level atom and the coherent field with varying frequency via singe-photon transition is studied in terms of the linear entropy.The field frequency varying with time in the forms of sine is considered. Changing amplitudeαand angular frequencyβof the field-frequency variation,we found that the evolution process of the linear entropy is more sensitive to the changes of the modulation amplitude.Then the entanglement features of two stretching vibrations in molecule H₂S is investigated for various intial states in terms of the reduced-density von Neumann entropy and the mean entropy defined by average over time.We found that the maximal and the mean entropies of an intial state with local-mode character are smaller than those with normal-model character,and the mean entropy displays a maximum near the position of the normal-to-local transtion.Therefore,the behaviour of the entanglement can be regarded as a dynamical signature of that transition.In addition,the larger entanglement state can be prepared by a quantum state with normal-mode character.We believe that these can be useful for quantum computing based on vibrational modes in molecule H₂S.