The Study Of The Thermal Entanglement And Intrinsic Decoherence Of Multi-qubit Heisenberg Spin Chain

Quantum entanglement is the most important feature in quantum mechanics,and also is a very useful resource in quantum information processing and quantum computation.Heisenberg model is one of the simplest models in quantum systems,and it has many rich entanglement characters which have been intensively studied.Temperature can obviously influence the entanglement,thus we have to control the temperature to manipulate the thermal entanglement when the entangled states are prepared.On the other hand,all real physical systems are not isolated,and they can inevitably be influenced by the surrounding environment,with the substantial and energy interaction,thus it leads to intrinsic decoherence.In the previous works,it is found that intrinsic decoherence has important effects on entanglement.In this thesis,the three main subjects studied are given as follows:1.We study the thermal entanglement of a five qubits anisotropic Heisenberg spin chain with the next nearest neighboring interaction.We calculate numerically the concurrence of the system for different parameters.It is found that the temperature can immensely suppress the entanglement,and the effects of frustration parameter on the entanglement between the nearest two qubits are different from the case of the next nearest two qubits.Additionally,the magnetic field,the Dzyaloshinskii-Moriya(DM)interaction and the anisotropic parameter also have important influence on the entanglement.Thus,the proper choosing temperature,frustration parameter,magnetic field,DM interaction parameter and anisotropic parameter are very important to control the entanglement of the system.2.We study the effect of intrinsic decoherence on the thermal entanglement of a four qubits XX Heisenberg spin chain model.We calculate the concurrence of the nearest two qubits and the next nearest two qubits,respectively.It is found that when the system is in the absence of intrinsic decoherence,the entanglement of the nearest neighboring qubits oscillates periodically with time involving.When the intrinsic decoherence parameter is considered,the intrinsic decoherence can damage the entanglement.But in the certain region of intrinsic decoherence parameter,it can enhance the entanglement of the nearest neighboring qubits.The DM interaction also can weaken the entanglement of the nearest neighboring qubits.While in the certain region of DM interaction parameter,it can enhance the entanglement.Both the effect of the intrinsic decoherence and DM interaction on entanglement of the next nearest neighboring qubits is similar to that of the nearest neighboring qubits,but their influence on the entanglement of the next nearest neighboring qubits are much stronger than that of the nearest neighboring qubits.It is shown that the intrinsic decoherence can have important influence on the entanglement,and thus we can choose the proper parameter to get the entangled states.3.We use the method of negativity to study the influence of intrinsic decoherence on the entanglement of a three qubits XX Heisenberg spin chain model.It is found that when the initial states are different,the intrinsic decoherence and the DM interaction have different influences on the negativity of the system.And the negativity of the stationary state is irrelevant to the decoherence factor,but it depends on both the initial state and the DM interaction parameter.