Quantum Phase Transition and Thermal Entanglement in the Isotropic XXX Model

We investigate the quantum phase transition (QPT) and the pairwise thermal entanglement in the three-qubit Heisenberg XXX chain with Dzyaloshinskii--Moriya (DM) interaction under a magnetic field. The ground states of the system exist crossing points, which shows that the system exhibits a QPT. At a given temperature, the entanglement undergoes two sudden changes (platform-like behavior) as the DM interaction or external magnetic field increases. This special property can be used as the entanglement switch, which is also influenced by the temperature. We can modulate the DM interaction or external magnetic field to control the entanglement switch.     

Thermal entanglement of a two-qutrit XX spin chain with Dzialoshinski-Moriya interaction

The effect of Dzialoshinski-Moriya (DM) interaction on thermal entanglement of a two-qutrit XX spin chain in a homogenous magnetic field is investigated. Our results imply that DM interaction and the magnetic field play competing roles in enhancing thermal entanglement.     

Entanglement in One-Dimensional Random XY Spin Chain with Dzyaloshinskii--Moriya Interaction

The impurities of exchange couplings, external magnetic fields and Dzyaloshinskii-Moriya （DM） interaction considered as Gaussian distribution, and the entanglement in one-dimensional random XY spin systems is investigated by the method of solving the different spin-spin correlation functions and the average magnetization per spin. The entanglement dynamics at central locations of ferromagnetic and antiferromagnetic chains have been studied by varying the three impurities and the strength of DM interaction. （i） For the ferromagnetic spin chain, the weak DM interaction can improve the amount of entanglement to a large value, and the impurities have the opposite effect on the entanglement below and above critical DM interaction. （ii） For the antiferromagnetic spin chain, DM interaction can enhance the entanglement to a steady value. Our results imply that DM interaction strength, the impurity and exchange couplings （or magnetic field） play competing roles in enhancing quantum entanglement.     

Thermal Entanglement in Two-Qutrit Heisenberg XX Chain with Different Dzyaloshinskii-Moriya Interaction and Nonuniform Magnetic Field

The effects of the different Dzyaloshinskii-Moriya (DM) interaction on thermal entanglement of a two-qutrit Heisenberg XX spin chain in a nonuniform magnetic field are investigated. Our results imply that the x-component DM interaction plays a central role in enhancing quantum entanglement and it has a higher critical temperature than the z-component DM interaction. The entanglement can be tunable controlled by changing the multiple of the magnetic fields B1 and B2 . Also we found that different DM interaction are competitive to each other in some conditions.     

Thermal Entanglement in Two-Qutrit Heisenberg XX Chain with Different Dzyaloshinskii-Moriya Interaction and Nonuniform Magnetic Field

The effects of the different Dzyaloshinskii-Moriya (DM) interaction on thermal entanglement of a two-qutrit Heisenberg XX spin chain in a nonuniform magnetic field are investigated. Our results imply that the x-component DM interaction plays a central role in enhancing quantum entanglement and it has a higher critical temperature than the z-component DM interaction. The entanglement can be tunable controlled by changing the multiple of the magnetic fields B₁ and B₂. Also we found that different DM interaction are competitive to each other in some conditions.     

具有次近邻相互作用的五量子比特XXZ海森伯自旋链的热纠缠

研究具有次近邻相互作用五量子比特XXZ海森伯自旋链在磁场作用下的热纠缠性质,利用数值计算求出最近邻两量子比特和次近邻两量子比特的共生纠缠度(concurrence),分别记为C_(12)和C_(13).研究结果表明,阻挫参数对配对热纠缠具有重要影响,而且阻挫参数的变化对C_(12)和C_(13)的影响也各不相同;温度、磁场、Dzyaloshinkii-Moriya相互作用以及各向异性参数对配对热纠缠有着不同程度的影响;通过选择适当的模型参数,可以有效地调节和提高五量子比特XXZ海森伯自旋链的配对热纠缠.     

海森堡XYZ自旋链系统的热纠缠与局域量子不确定性研究

研究了热平衡温度,自旋交换相互作用,Dzyaloshinskii-Moriya(DM)相互作用及外加非一致性磁场对两比特海森堡XYZ自旋链量子系统的热纠缠与局域量子不确定度的影响,对比分析了并发度量子纠缠与局域量子不确定度描述自旋链系统量子关联的差别.结果表明自旋链系统的量子纠缠在热平衡温度,DM相互作用及外加磁场的非一致性参数的变化情况下均会出现纠缠突然死亡的再生现象,而自旋链系统的局域量子不确定度随着这些参数呈连续变化现象.并且,自旋交换相互作用,DM相互作用及外加横向磁场作用强度较小时,他们的变化对自旋链系统的量子纠缠与局域量子不确定度的影响有着明显的差别.     

Thermal Entanglement of a Two-Qubit XXZ Heisenberg Chain with Dzialoshinski-Moriya Interaction

The effect of Dzialoshinski-Moriya (DM) interaction on thermal entanglement of a two-qubit XXZ spin chain in a homogenous magnetic field is investigated. It is found that the DM interaction can enhance thermal entanglement. When D is large enough, the entanglement can exist for larger temperatures and strong magnetic field.     

Effect of Dzialoshinski-Moriya interaction on thermal entanglement of a mixed-spin chain

The effect of Dzialoshinski-Moriya (DM) interaction on thermal entanglement of a mixed-spin chain in an external magnetic field is investigated. It is found that DM interaction may enhance quantum thermal entanglement to a maximal value even though the magnetic field plays a positive role in shrinking thermal entanglement in the mixed-spin chain. Furthermore, the effect of inhomogeneity of the magnetic field on quantum entanglement is analyzed. Our analysis will shed some light on the understanding of the effect of the DM interaction on thermal entanglement of a mixed-spin chain. Supported by the National Natural Science Foundation of China (Grant No. 60573008) and Anhui Foundation for Young Teachers (Grant No. 2008jq1025zd)     

Entanglement in Mixed-Spin (1/2, 3/2) Heisenberg XXZ Model with Dzyaloshinskii-Moriya Interaction

In this paper, the entanglement in a mixed-spin (1/2, 3/2) Heisenberg XXZ model with Dzyaloshinskii-Moriya (DM) interaction in an inhomogeneous external magnetic field is studied. We not only calculate the ground-state entanglement but also investigate the behaviors of quantum phase transition following the changes of DM interaction and nonuniform magnetic field. More importantly, we note that the DM interaction improves the critical magnetic field B _c , the critical temperature T _c and broadens the region of entanglement.     

Entanglement Dynamics of Ising Model with Dzialoshinskii-Moriya Interaction in an Inhomogeneous Magnetic Field

This paper investigates the entanglement dynamics of two-spin Ising model with the Dzialoshinskii-Moriya (DM) interaction in an inhomogeneous magnetic field. The system is initially prepared in the Werner state.It is found that the phenomenon of the entanglement sudden death (ESD) appears during the evolution process. We also analyze in detail the effects of the DM interaction, the degree of inhomogeneous magnetic field and the purity of the initial state on the time of ESD. These effects indicate that one can control the entanglement of the system. It is helpful for quantum information processing.     

Entanglement in a spin-one spin chain

The thermal entanglement in a two-spin-qutrit system with two spins coupled by exchange interaction is investigated in terms of the measure of entanglement called ‘negativity’. We strictly show that for any temperature the entanglement is symmetric with respect to zero magnetic field. The behavior of negativity is presented for four different cases. We find that the entanglement may be enhanced under a nonuniform magnetic field. Because there is not any necessary and sufficient condition for quantum separability in systems of dimension 3⊗3, our results are qualitative, not quantitative.     

Entanglement dynamics and Bell violation of three-qubit states coupled to an XY spin chain with Dzyaloshinski–Moriya interaction

We investigate the entanglement dynamics and Bell violation of three-qubit quantum states under an environment consisting of an XY spin chain with Dzyaloshinski–Moriya (DM) interaction. From the results, we find that the entanglement dynamics depends not only on the DM interaction, the magnetic field, and the anisotropy parameter but also on the number of the freedom degrees of the environment. The decoherence-free subspaces of our model have been identified and the Bell violation of quantum states is also discussed.     

Thermal Entanglement in the Anisotropic Heisenberg XYZ Model with Different Dzyaloshinskii-Moriya Couplings

Thermal entanglement is investigated in a two-qubit Heisenberg XYZ system with different Dzyaloshinskii Moriya （DM） couplings. It is shown that different DM interaction parameters have different influences on the entanglement and the critical temperature. In addition, according to the relation between the real coupling coefficients Jx and Jz, a more efficient DM control parameter can be obtained by adjusting the direction of DM interaction.     

The entanglement dynamics of interacting qubits embedded in a spin environment with Dzyaloshinsky-Moriya term

We investigate the entanglement dynamics of two interacting qubits in a spin environment, which is described by an XY model with Dzyaloshinsky-Moriya (DM) interaction. The competing effects of environmental noise and interqubit coupling on entanglement generation for various system parameters are studied. We find that the entanglement generation is suppressed remarkably in weak-coupling region at quantum critical point (QCP). However, the suppression of the entanglement generation at QCP can be compensated both by increasing the DM interaction and by decreasing the anisotropy of the spin chain. Beyond the weak-coupling region, there exist resonance peaks of concurrence when the system-bath coupling equals to external magnetic field. We attribute the presence of resonance peaks to the flat band of the self-Hamiltonian. These peaks are highly sensitive to anisotropy parameter and DM interaction.     

Berry Phases and Entanglement of a Two Spin-1/2 Model with Dzyaloshinski-Moriya Interaction in Magnetic Fields

We show that the entanglement (as quantified by the concurrence) and Berry phases of the adiabatic quantum states vanish for a two spin-1/2 system with Dzyaloshinski-Moriya (DM) interaction, while one of the spins is driven by a time-varing rotating magnetic field and the other one is coupled with a strong static magnetic field. The system is described by the Heisenberg XX model and the static field is in the direction of the rotation axis. We also investigate that how the concurrence and Berry phases depend on the DM interaction, coupling coefficient and the static magnetic field. In addition, we show that reversing the sign of the static magnetic field can cause exchange of the Berry phases and entanglement between the adiabatic states. Finally it is shown that each energy level approach causes jumps or cusp-like behaviour in the Berry phases and the concurrences.     

Effect of Dzialoshinski-Moriya interaction on the quantum discord of a spin-star model

The effect of Dzialoshinski-Moriya (DM) interaction on the quantum discord and the thermal entanglement of the density matrix of a spin star model is investigated. Our results imply that the quantum correlation measured by quantum discord and thermal entanglement can be established between two surrounding parties both of which have no interaction with each other but interact with the central party independently. From the analysis, we find that the strong DM interaction can enhance the quantum discord and thermal entanglement while the external magnetic field with a large value and the high temperature can shrink them. Specially, the quantum discord is more robust than the thermal entanglement in the sense that the range of parameters in which the quantum discord takes a zero value is much smaller than that of the thermal entanglement.     

Quantum Correlation in Three-Qubit Heisenberg Model with Dzyaloshinskii-Moriya Interaction

We investigate the pairwise thermal quantum discord in a three-qubit XXZ model with Dzyaloshinskii-Moriya (DM) interaction. We find that the effects of DM interaction on antiferromagnetic system is distinct from that of ferromagnetic system. The magnetic field supplemented with DM term contribute to enhance the range of quantum discord. It is revealed that the situations where quantum discord fails to indicate a sudden change of groundstate at finite temperature though indicating such a sudden change of groundstate at zero temperature. Dynamics of pairwise thermal quantum discord is considered as well. Thermal quantum discord vanishes in asymptotic limit regardless of its initial values, however, thermal entanglement suddenly disappears in finite time.     

Quantum Correlation in Three-Qubit Heisenberg Model with Dzyaloshinskii-Moriya Interaction

We investigate the pairwise thermal quantum discord in a three-qubit XXZ model with Dzyaloshinskii-Moriya (DM) interaction. We find that the effects of DM interaction on antiferromagnetic system is distinct from that of ferromagnetic system. The magnetic field supplemented with DM term contribute to enhance the range of quantum discord. It is revealed that the situations where quantum discord fails to indicate a sudden change of groundstate at finite temperature though indicating such a sudden change of groundstate at zero temperature. Dynamics of pairwise thermal quantum discord is considered as well. Thermal quantum discord vanishes in asymptotic limit regardless of its initial values, however, thermal entanglement suddenly disappears in finite time.     

The effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system

We investigate the effects of nonlinear couplings and external magnetic field on the thermal entanglement in a two-spin-qutrit system by applying the concept of negativity. It is found that the nonlinear couplings favor the thermal entanglement creating. Only when the nonlinear couplings ∣K∣ are larger than a certain critical value does the entanglement exist. The dependence of the thermal entanglement in this system on the magnetic field and temperature is also presented. The critical magnetic field increases with the increasing nonlinear couplings constant ∣K∣. And for a fixed nonlinear couplings constant, the critical temperature is independent of the magnetic field B.