The Generation And Concentration For Quantum Entangled State

With the continuous development of science,quantum information science which combines quantum mechanics and information science,has developed rapidly.Quantum information science includes two parts,one is quantum communication,the other is quantum computer.Quantum communication,which has been proved to be absolutely safe in theory,has attracted much attention of many research institutes,even at the national level.Quantum computer,which has the unique ability of multi quantum state parallel computing,is much faster than the classical computer.It brings serious challenge to the existing classical encryption system.As an essential resource in quantum information,quantum entangled states play fundamental roles in quantum communication and quantum computers.Therefore,how to create entangled state is a key point.On the other hand,in the process of quantum communication,environmental noise will inevitably affect the entanglement,which may make the maximal entanglement degenerate into partially entangled states,which will seriously affect the quality and security of quantum communication.For the generation of entangled states,we esearched the coherent state and concatenated Greenberger-Horne-Zeilinger(C-GHZ)type state,which have attracted much interesting in current research.After analyzing their advantages and disadvantages,and the actual needs of quantum communication,we propose the C-GHZ coherent state and give a theoretical scheme on its generation.The scheme is based on linear optics,and can be implemented in the current experimental conditions.The scheme can be used to generate C-GHZ coherent state of arbitrary photon number.The Hybrid Entangled State(HES)has attracted much attention in current research.As the maximally entangled state will degrade to the pure less-entangled state in the noisy environment,we proposed three entanglement concentration protocols(ECPs)for Bell-type HES,W-type HES and Cluster-type HES,respectively.The three ECPs are completely based on the linear optics.Under the existing technical conditions,these ECPs are easy to realize.All the three ECPs have application potential in the long distance quantum communication.In addition,we also did some researches on the existing quantum communication technology.Bennett proposed the concept of the entanglement of formation to quantify entangled States.We proposed a two-step concurrence measurement protocol for the hyperentangled state.The scheme requires the weak cross-Kerr nonlinearity to construct the quantum nondemolition measurement(QND)and does not require the sophisticated CNOT gate operation.It is feasible in future experimental technology.