Enhancement of next-nearest-neighbouring entanglement in quantum Ising spin chain

Using the method of the Jordan--Wigner transformation for solving different spin--spin correlation functions, we have investigated the generation of next-nearest-neighbouring entanglement in a one-dimensional quantum Ising spin chain with the Gaussian distribution impurities of exchange couplings and external magnetic fields taken into account. The maximal value of entanglement between the next-nearest-neighbouring qubits in the transverse Ising model was analysed in detail by varying the effectively controlled parameters such as interchange coupling, magnetic field and the system impurity. For such systems, where both exchange couplings and external magnetic field disorder appear, we show that it is possible to achieve next-nearest-neighbouring entanglement better than the previously discussed pure Ising spin chain case. We also show that the Gaussian distribution impurity can induce next-nearest-neighbouring entanglement, which can be used as a means to characterize quantum phase transition.     

Ground-state entanglement in a three-spin transverse Ising model with energy current

The ground-state entanglement associated with a three-spin transverse Ising model is studied. By introducing an energy current into the system, a quantum phase transition to energy-current phase may be presented with the variation of external magnetic field; and the ground-state entanglement varies suddenly at the critical point of quantum phase transition. In our model, the introduction of energy current makes the entanglement between any two qubits become maximally robust.     

Quantum entanglement transition in vertically coupled two single-electron quantum dots with charged impurity

Effects of a charged impurity on the ground state of two vertically coupled identical single-electron quantum dots with and without applied magnetic field are investigated. In the absence of the magnetic field, the investigations of the charged impurity effect on the quantum entanglement (QE) in some low-lying states are carried out. It is found that, both the positive charged impurity (PCI) and the negative charged impurity (NCI)reduce the QE in the low-lying states under consideration except that the QE in the ground state is enhanced by the NCI. Additionally, in the domain of B from 0 Tesla to 15 Tesla, the ground state energy E, the ground state angular momentum L and the ground state QE entropy S are worked out. As far as the ground state are concerned, the PCI (NCI) blocks (induces) the angular momentum phase transition and the QE phase transition besides the known fact (i. e., the PCI/NCI decreases/increases the energy) in the magnetic field.     

The ground-state entanglement in the spin-XX chain with a magnetization current

The ground-state entanglement in a transverse spin-1/2 XX chain with a magnetization current is studied.By introducing a magnetization current to the system,a quantum phase transition to current-carrying phase may be presented with the variation of the driving field λ for the magnetic field h > 1;and the ground-state entanglement arises simultaneously at the critical point of quantum phase transition.In our model,the introduction of magnetization current may result in more entanglement between any two nearest-neighbour spins.     

Quantum Phase Transition and Thermal Entanglement in the Ising Model with Dzyaloshinskii-Moriya Interaction in an Inhomogeneous Magnetic Field

We investigate the quantum phase transition (QPT) and thermal entanglement in the two-qubit Ising model in an inhomogeneous magnetic field, and the DM interaction between the two lattices is considered. The results show the QPT highly relates to the magnetic intensity B and DM parameter D, by controlling the DM interaction D and external magnetic B we can change the positions of QPT points and the level spacing. Moreover, the QPT is closely related to thermal entanglement, when the QPT happens the ground state always changes between entanglement state and disentanglement state. The thermal entanglement highly depends on the system’s temperature T, DM intensity D and external magnetic field B. When T is lower, the entanglement can exhibit a platform-like region. By modulating the parameters D and B, the entanglement can be controlled and the entanglement switch can be realized.     

量子位Heisenberg XX开链中边界量子位之间的纠缠

Heisenberg 开链对研究量子态在自旋链上的传递有重要的意义.本文研究了五量子位Heisenberg XX 开链中边界量子位之间的纠缠，在该系统中引入了磁杂质和系统杂质且它们之间满足线性关系时，该系统可以精确求解，此时边界量子位之间存在纠缠.选择合适的杂质参数和磁场参数，该边界纠缠的最大值可达到0.5. 关键词： Heisenberg XX开链 边界纠缠 杂质     

Universality in the entanglement structure of ferromagnets

Systems of exchange-coupled spins are commonly used to model ferromagnets. The quantum correlations in such magnets are studied using tools from quantum information theory. Isotropic ferromagnets are shown to possess a universal low-temperature density matrix which precludes entanglement between spins, and the mechanism of entanglement cancellation is investigated, revealing a core of states resistant to pairwise entanglement cancellation. Numerical studies of one-, two-, and three-dimensional lattices as well as irregular geometries showed no entanglement in ferromagnets at any temperature or magnetic field strength.     

Cumulative quantum work-deficit versus entanglement in the dynamics of an infinite spin chain

We find that the dynamical phase transition (DPT) in nearest-neighbor bipartite entanglement of time-evolved states of the anisotropic infinite quantum XY spin chain, in a transverse time-dependent magnetic field, can be quantitatively characterized by the dynamics of an information-theoretic quantum correlation measure, namely, quantum work-deficit (QWD). We show that only those nonequilibrium states exhibit entanglement resurrection after death, on changing the field parameter during the DPT, for which the cumulative bipartite QWD is above a threshold. The results point to an interesting inter-relation between two quantum correlation measures that are conceptualized from different perspectives.     

Entanglement dynamics of three-qubit coupled to an XY spin chain at finite temperature with three-site interaction

The entanglement dynamics of three-qubit states under an XY spin chain at finite temperature with three-site interaction is investigated. It is shown that the three-site interaction does not affect the behavior of quantum phase transition (QPT) induced by the external magnetic field and does not induce new critical regions in the XY model. In addition, concerned with the effect of the three-site interaction on the entanglement evolution, we find the three-site interaction can not only enhance but also suppress the decay of the entanglement between the three-qubit system, which depends on the initial states of the system and the parameters related with the environmental spin chain.     

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.     

Entanglement Dynamics of Quantum States Undergoing Decoherence from a Driven Critical Environment

In this paper, we have investigated the quantum entanglement of quantum states undergoing decoherence from a spin environment which drives a quantum phase transition. From our analysis, we find that the entanglement dynamics depends not only on the coupling strength but also on the external magnetic field and the number of the freedom degrees of the environment. Specially, our results imply that the decay of the entanglement can be enhanced by the quantum phase transition of the environment when the system is coupled to the environment weakly. Additionally, the discussion of the case of the multipartite states with high dimensions is made.     

Entanglement Dynamics of Quantum States Undergoing Decoherence from a Driven Critical Environment

In this paper,we have investigated the quantum entanglement of quantum states undergoing decoherence from a spin environment which drives a quantum phase transition.From our analysis,we find that the entanglement dynamics depends not only on the coupling strength but also on the external magnetic field and the number of the freedom degrees of the environment.Specially,our results imply that the decay of the entanglement can be enhanced by the quantum phase transition of the environment when the system is coupled to the environment weakly.Additionally,the discussion of the case of the multipartite states with high dimensions is made.     

Entanglement and criticality in quantum impurity systems

We investigate the entanglement between a spin and its environment in impurity systems which exhibit a second-order quantum phase transition separating a delocalized and a localized phase for the spin. As an application, we employ the spin-boson model, describing a two-level system (spin) coupled to a sub-Ohmic bosonic bath with power-law spectral density, J(omega) proportional to omega(s) and 0 < s < 1. Combining Wilson's numerical renormalization group method and hyperscaling relations, we demonstrate that the entanglement between the spin and its environment is always enhanced at the quantum phase transition resulting in a visible cusp (maximum) in the entropy of entanglement. We formulate a correspondence between criticality and impurity entanglement entropy, and the relevance of these ideas to nanosystems is outlined.     

Anisotropy and Magnetic Field Effects on Entanglement of a Two-Spin (1/2,1) Mixed-Spin Heisenberg XY Chain

We study the entanglement in anisotropic (1/2,1) mixed-spin Heisenberg XY model under the presence of an external magnetic field at thermal equilibrium. By adjusting the anisotropic parameter and the magnetic field, one is able to obtain entanglement at higher temperature. We find the evidence of the quantum phase transition in the model and observe that the quantum phase transition point at low temperature moves toward weak magnetic field with the increase of the anisotropic parameter.     

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.     

Critical entanglement and geometric phase of a two-qubitmodel with Dzyaloshinski--Moriya anisotropic interaction

We consider a two-qubit system described by the Heisenberg XY model with Dzyaloshinski--Moriya (DM) anisotropic interaction in a perpendicular magnetic field to investigate the relation between entanglement, geometric phase and quantum phase transition (QPT). It is shown that the DM interaction has an effect on the critical boundary. The combination of entanglement and geometric phase may characterize QPT completely. Their jumps mean that the occurrence of QPT and inversely the QPT at the critical point at least corresponds to a jump of one of them.     

非均匀磁场和杂质磁场对自旋1系统量子关联的影响

通过负值度和测量诱导的扰动, 研究了非均匀磁场和杂质磁场对自旋为1的Heisenberg系统量子关联的影响. 研究发现非均匀磁场的增加会降低纠缠, 但也可用来产生纠缠, 并且会提高临界非线性作用K_c的值, 测量诱导的扰动的临界磁场要高于负值度的临界磁场, 而且测量诱导的扰动不会随着非线性作用|K| 的减小而消失, 它能全面反映量子关联的存在. 研究还发现, 不同杂质磁场对测量诱导的扰动的影响彼此间无交叉. 杂质磁场下, 相互作用|J| 必须小于非线性作用|K| 才会有纠缠存在, 但是测量诱导的扰动却可以在相互作用|J| 大于非线性作用|K| 时依然存在, |J| 与|K| 相同时只是测量诱导的扰动的最小取值点. 此外, 系统粒子数目对量子关联也具有重要影响.     

具有三角自旋环的伊辛-海森伯链的热纠缠

研究了具有三角自旋环的伊辛-海森伯链在磁场作用下的热纠缠性质.分别讨论了三角自旋环中自旋1/2粒子间相互作用的三种情形,即XXX,XXZ和XY Z海森伯模型.利用转移矩阵方法,数值计算了具有三角自旋环的伊辛-海森伯链的配对纠缠度.计算结果表明,外加磁场强度和温度对系统处于上述三种海森伯模型的热纠缠性质均有重要影响.给出了系统在不同的海森伯模型下,纠缠消失对应的临界温度随磁场强度的变化图,由此可以得到系统存在配对纠缠的参数区域,同时发现在特定的参数区域存在纠缠恢复现象.因此适当调节温度和磁场强度,可以有效调控具有三角自旋环的伊辛-海森伯链热纠缠性质.     

{Cu_3}单分子磁体在热平衡和磁场作用下的三体纠缠

本文研究了Na_9[Cu_3Na_3(H_2O)_9(α-As W_9O_(33))_2]·26H_2O(简记为{Cu_3})单分子磁体在热平衡和外加磁场作用下的三体纠缠性质,利用等效自旋模型和实验拟合参数,数值计算了{Cu_3}型三角自旋环中三体负性纠缠度(tripartite negativity).分别考虑沿垂直于三角自旋环方向的磁场、平行于三角自旋环方向的磁场,以及倾斜磁场的情形.结果表明,磁场的方向、大小以及温度对系统三体负性纠缠度有着重要影响.文中给出了在不同磁场方向下,临界温度随磁场强度的变化图,由此可以得到三体纠缠存在的参数区域.同时发现在特定的参数区域,该系统存在纠缠恢复现象.因此适当调节温度、磁场强度大小和磁场方向可以有效调控{Cu_3}型三角自旋环中的三体纠缠性质.     

Entanglement dynamics of a three-qubit system interacting with a quantum spin environment

We investigate the entanglement dynamics and decoherence of a three-qubit system under a quantum spin environment at a finite temperature in the thermodynamics limit. For the case under study, we find the evolution of pairwise entanglement depends not only on the initial states but also on the parameters related to the system and the spin environment. In addition, an undesirable entanglement sudden death occurs in the process of entanglement evolution, and this effect can be controlled by the coupling constant between two qubits, external magnetic field, and the interaction between the system and the environment.