The Research On The Quantum Discord Properties About Several Models

In quantum information science,quantum entanglement has a wide range of applications,including quantum dense coding,quantum teleportation,quantum key distribution and so on.Quantum dense coding uses entangled state as a channel,which has an interesting non-classical effect in quantum communication.It can obtain a higher information capacity than the classical channel,reflecting the importance of quantum entanglement.Without the help of quantum entanglement,quantum dense coding cannot be effectively completed.Quantum dense coding is an important application of quantum information,so it becomes an important content to analyze and study the characteristics of dense coding in different models.The core content of this paper is how to determine the optimal dense coding capacity in the model.This thesis mainly includes six chapters,and the main work of this thesis is in chapter three to five.The first chapter introduces the development of quantum information,elucidate the importance of quantum entanglement,and reviews the development theory of quantum dense coding and quantum teleportation.The second chapter introduces some fundamental concepts related to quantum dense coding theories,such as Von Neumann entropy,DM interaction,decoherence.We also describes the spin chain system model and spin compression model,and gives a detailed introduction to different types of systems.In chapter 3,we use the spin compression model of double qubit to analyze the characteristics of quantum dense coding.Two qubit spin compression models can be divided into two types,namely one-axis twisting model and two-axis counter twisting model.Based on the Hamiltonian and eigenvectors of the two models,the effects of external magnetic fieldΩ,squeezing interaction μ and temperature T on dense coding capacity are analyzed.By analyzing the parameters Ω and T,the results show that the parameters and have a positive effect on the dense encoding capacity,while the temperature T has a negative effect on the dense encoding capacity.More importantly,by adjusting and optimizing these parameters,not only effective dense coding can be carried out in a proper range,but also the optimal dense coding can be achieved.In chapter 4,we study the optimal dense coding of the two-qubit anisotropic Heisenberg XXZ model under decoherence conditions,with the Werner state as the initial state.Our results show that the initial state purity r has a great influence on the initial value of the optimal dense encoding capacity.When the purity r of the initial state increases,the initial value of the optimal dense coding capacity also increases.In addition,the coupling coefficient and the DM interaction both have a strong influence on the frequency of the oscillation and the area of the t that the optimal dense coding is feasible(tf).Interestingly,the decoherence rate γ strongly affects the dense coding capacity.With the decrease ofγ,the area oftf(which will make the dense encoding capacity greater than 1)becomes more wider.When y=0,it will still allows a valid dense coding.So we can adjust the parameters to get a optimal dense coding.In chapter 5,we investigated the properties of quantum dense coding between two identical and spatially separated atoms in free space with different initial states.It shows that the dense coding capacity χ experienced a sharp decline firstly and then gradually increased to be one steady valuel with increasing t.The implementation of dense coding capability depends heavily on the initial state.It is worth noting that the dense encoding of the initial pure state in this system is not useful because the dense encoding capacity is always less than 1(a valid dense encoding capacity satisfies χ>1).In addition,for the initial entangled state and mixed state,the valid dense coding capacity can be obtained.The results show that there is a threshold,and the dense coding capacity is valid whent<tc.Tuning the atomic distance between the two atoms slightly broaden the valid dense coding region and improve the value of tc.Our results show that reducing the purity of the initial state not only broadens the coding region,but also prolongs the effective time for successful implementation of effective dense coding capacity.In chapter 6,By introducing the nonuniform magnetic field b and the decoherence parameters,the properties of quantum dense coding is investigated in detail with different initial states.We mainly investigated the influences of those parameters on dense coding capacity χin detail,such as the nonuniform magnetic field b and the anisotropic parameter Δ.It is show that the quantum dense coding properties are very complicated under different initial states.The dense coding capacity is strongly influenced by the anisotropic parameters and additional magnetic field:(1)the magnitude of the dense coding capacity is strongly depended on the value of B;(2)when the system of the external magnetic field B constant,the dense coding value of χis decreased quickly with decreasing the anisotropic parameter Δ for fixed B.The influence of the inhomogeneous magnetic field on χis very weakly,and the physical reason is also given.By studying the influences of different parameters on χ,it shows that by proper adjusting the value of B,b,J and the anisotropic parameters Δ one can obtain obtain the effective dense coding(χ is greater than 1).One point we must mention is that not only the dense coding capacity value is increased but also the range of effective dense coding is broadened.In chapter 7,we give a summary and prospects.