Thermal Entanglement Properties in Two Qubits One-Axis Spin Squeezing Model with an External Magnetic Field

In a recent article in this journal Guo-Hui (Int. J. Theor. Phys. 56, 600, 2017) has investigated the quantum correlation properties in the two qubits one-axis spin squeezing model with an external magnetic field by means of concurrence (C) and quantum discord (QD) criteria. We are to reveal numerically and analytically that in the limiting case T→ 0 (ground state) the sudden disappearance phenomenon (SDP) does not occur in the behavior of C and the values obtained for C are incorrect.

     

Spin Squeezing and Quantum Fisher Information in Generalized Two-Axis Twisting Model

We investigated spin squeezing and quantum Fisher information in generalized two-axis twisting model; which generalizes the two-axis twisting model including a linear interaction controlled by an external field. In particular, we are interested in the dependence of spin squeezing and quantum Fisher information on the external field. By adopting frozen-spin approximation, we derive the theoretical and numerical results for spin squeezing and quantum Fisher information. Except certain special conditions, the stronger external field induces to stronger squeezing. Spin squeezing parameter and the reciprocal of the mean quantum Fisher information per particle are periodic function; but the external field has not important effect on the period.     

Various Correlations in a Two-Qubit Heisenberg XXZ Spin System Both in Thermal Equilibrium and Under the Intrinsic Decoherence

In this paper we discuss various correlations measured by the concurrence (C), classical correlation (CC), quantum discord (QD), and geometric measure of discord (GMD) in a two-qubit Heisenberg XXZ spin system in the presence of external magnetic field and Dzyaloshinskii-Moriya (DM) anisotropic antisymmetric interaction. Based on the analytically derived expressions for the correlations for the cases of thermal equilibrium and the inclusion of intrinsic decoherence, we discuss and compare the effects of various system parameters on the correlations in both cases. We also found that there is not a definite ordering of these quantities in thermal equilibrium, whereas there is a descending order of the CC, C, GMD and QD under the intrinsic decoherence with a nonnull B when the initial state is $|\varPsi_{2}(0) \rangle = \frac{1}{\sqrt{2}} (|00 \rangle + |11 \rangle)$ .     

Entanglement Teleportation via Two-Qubit Spin Squeezing Model

Entanglement teleportation via two spins coupled to each other by one axis twisting spin squeezing interaction is investigated. We mainly concentrate on the properties of the channel entanglement, the output entanglement and the teleportation fidelity. It shows that the output entanglement increases linearly with increasing the value of the input entanglement. With the increasing of T, the teleportated entanglement increases sharply from zero to a maximum value and then decreases slowly to zero when the temperature is improved to one threshold value of T _c , and the threshold value of T _c increases with the increasing of the input entanglement. When enlarging the external magnetic, F _a firstly decreases quickly to a minimum value as the critical magnetic field Ω _c is reached, then it increases abruptly to a maximum value and finally it will be a certain value. Besides, the critical external magnetic Ω _c increases when μ is larger. For Ω<Ω _c the value of the average fidelity increases with the increasing of μ, but for Ω>Ω _c the value of it decreases from a maximum value. The influence of T on the average fidelity is similar with μ.     

Spin Squeezing and Entanglement of Two-Axis Twisting Model

We investigate spin squeezing and entanglement of the two-axis twisting model with the bosonization method, The smaller linear interaction can produce a stronger spin squeezing, the better entanglement and the squeezing and entanglement can maintain a longer time interval. The stronger spin squeezing and the better entanglement can also be achieved by increasing the number of particles.     

Quantum Correlations in a Two-Qubit Heisenberg XX Model under Intrinsic Decoherence

Taking into account the intrinsic decoherence,we have investigated quantum correlations in a two-qubit Heisenberg XX model when a nonuniform magnetic field is included.We compare entanglement measured by entanglement of formation,quantum discord and measurement-induced measurement(MID)and illustrate their diferent characteristics.Quantum discord and MID show the same features and always exist even though there is no entanglement in the long time limit.In the time evolution,quantum discord could be generated or enhanced to the stable value,while MID just decreases to the stable value.     

Quantum Correlations in a Two-Qubit Heisenberg XX Model under Intrinsic Decoherence

Taking into account the intrinsic decoherence, we have investigated quantum correlations in a two-qubit Heisenberg XX model when a nonuniform magnetic field is included. We compare entanglement measured by entanglement of formation, quantum discord and measurement-induced measurement (MID) and illustrate their different characteristics. Quantum discord and MID show the same features and always exist even though there is no entanglement in the long time limit. In the time evolution, quantum discord could be generated or enhanced to the stable value, while MID just decreases to the stable value.     

Controlling Spin Squeezing and Quantum Fisher Information with a Strong External Field

We investigate the mean spin direction, length of mean spin and spin squeezing in a general superposition of a GHZ state and a W state. It is shown that the mean spin direction and spin squeezing parameter are determined by the coefficients of superposition and the relative phase between the GHZ state and the W state; while the length of mean spin is only determined by coefficients of superposition.     

Dense Coding in Two Kinds of Two-Qubit Spin Squeezing Model

International Journal of Theoretical Physics - We investigate the effects of spin squeezing interaction, external magnetic field and temperature on dense coding capacity properties in two kinds of...     

Thermal Entanglement in a Two-Qubit Heisenberg XXZ Spin Chain Under a Single Pulse Magnetic Field

The thermal entanglement in a two-qubit Heisenberg XXZ spin chain is investigated under a single pulse magnetic field Bsinθ. The paper shows that the greater the contribution of the inhomogeneity on the exchange interaction, the higher thermal entanglement will be attained at the fixed temperature except the case that $\theta=2k\pi+\frac{3}{2}\pi$ . J _z  significantly disturb the thermal entanglement in uniform case. When the structure of the spin system are given, changing the external magnetic field B can induce controllable entanglement. Our study may provide a useful tool to change the entanglement of spin chain system.     

非均匀外磁场下海森堡自旋链的热纠缠

研究了两量子比特海森堡XXX自旋链处于x方向的非均匀磁场时系统的纠缠特性,并用负度N来度量.得到N的解析表达式,并在此基础上进行数值计算.仔细讨论了均匀磁场B、非均匀磁场b、温度T和自旋耦合系数J对纠缠度N的影响.结果表明:N会随着■和T的增大而减小,但会随着J的增大而增大.同时,增大的J和b还会使临界磁场■和临界温度T_th变大,从而使系统中热纠缠存在的磁场范围和温度范围都变大.这一点在较大磁场和较高温度下需要纠缠具有实际意义.由此,我们可以通过调节B、b、T和J来控制热纠缠,这对固态系统中通过构建和选择参数调整系统的纠缠度具有一定的作用和意义.     

Spin Squeezing in Mixed Model of the One-Axis Twisting and Two-Axis Counter-Twisting with Decay

We investigate spin squeezing of a collective spin system governed by mixed Hamiltonian of the one-axis twisting and two-axis counter-twisting with decay. The analytic solutions of the optimally squeezed angle and the spin squeezing parameter are derived. It is shown that the two-axis counter-twisting Hamiltonian induces stronger spin squeezing than the one-axis twisting one. It is also shown that the decay rate has no effect on the size of the squeezing parameter, it only affects the period of squeezing parameter.     

Quantum Monte Carlo Study of Frustrated Spin Chain under External Field

In this paper a stochastic series expansion quantum Monte Carlo algorithm is used to study a frustrated spin chain with diagonal next-nearest-neighbor interactions. The detailed balance conditions are carefully analyzed to improve the efficiency of simulation process. As an application of this algothrim, the total magnetization, the static structure factor and spin-stiffness are calculated for a certain set of system parameters as a function of external field strength.     

Quantum Teleportation via a Two-Qubit Ising Heisenberg Chain with an Arbitrary Magnetic Field

The quantum teleportation via a two-qubit Ising Heisenberg chain in the presence of an external magnetic field with an arbitrary direction are investigated. The effect of the orientation of an external magnetic field on the entanglement teleportation has been analyzed numerically. It is found that the teleported thermal concurrence and average fidelity can be maximized by rotating the magnetic field (with fixed magnitude) to an optimal direction. The ferromagnetic channel is not suitable to teleportation. A minimal entanglement of the thermal state is needed to realize the entanglement teleportation for antiferromagnetic channel. It is also found that the entanglement of the channel cannot completely reflect the teleported concurrence and average fidelity. There exist double-value phenomena between them.     

Spin Squeezing and Quantum Fisher Information in the Lipkin-Meshkov-Glick Model

We investigate spin squeezing and quantum Fisher information in the Lipkin-Meshkov-Glick model. Approximate analytical expressions and the numerical analysis of spin squeezing and quantum Fisher information are derived. Spin squeezing and quantum Fisher information depend on the strength of the external transverse magnetic field and the anisotropic parameter     

Thermal Quantum Discord in Pure Dzyaloshinskii-Moriya Model with Magnetic Field

We investigate the effects of the directions of Dzyaloshinskii-Moriya (DM) interaction vector and magnetic field on the quantum discord in the pure DM model. For different directions of DM vector, we find that there are different optimal parameter components of magnetic field. Moreover, we find that the optimal parameter components rules are the same for the Hamiltonian H₁ and H₂. According to the rules, for a certain axial DM vector, we can get the maximal quantum discord by adjusting the direction of the external magnetic field, which is feasible under the current experimental technology.     

Spin Squeezing and Quantum Fisher Information for Mixed Hamiltonian Model

We investigate spin squeezing and the mean quantum Fisher information per particle χ ² for mixed Hamiltonian model. By adopting frozen-spin approximation, we first derive a analytical expression of spin squeezing parameter ξ ² and then numerically calculate ξ ² and χ ². It is shown that most of the time the spin squeezing appears alternatively in the x and y directions, but it cannot appear simultaneously in the two directions. It is also shown that the smaller external field strength induces better entanglement and larger period.     

Global and Local Spin Squeezing in Coupled Quantum Kicked Tops Model

We investigate the global and the local spin squeezing in a weakly coupled quantum kicked tops system. Two different situations are considered: (i) N=1 and (ii) N=30 for each subsystem, corresponding to quantum and classical cases, respectively. In the first case, since the two subsystems have no spin squeezing, the global squeezing completely originates from quantum correlations. For the second one, the global spin squeezing is enhanced over the local one. Due to the chaotic nature of the system, the spin squeezing is sensitive to the initial state. In chaotic region, the squeezing vanished time is much shorter than that in the regular region.     

Quantum Correlations in a Family of Two-Qubit Separable States

Quantum correlations in a family of two-qubit separable classical-quantum correlated states are intensively studied with four different approaches, namely, quantum discord [Phys. Rev. Lett. 88 (2002) 017901], measurement- induced disturbance (MID) [Phys. Rev. A 77 (2008) 022301], ameliorated MID [J. Phys. A: Math. Theor. 44 (2011) 352002] and quantum dissonance [Phys. Rev. Lett. 104 (2010) 080501]. Quantum correlations captured with different approaches are compared and discussed so that their three distinct features are exposed.     

Negativity in the Extended Hubbard Model under External Magnetic Field

We exactly calculate the negativity, a measurement of entanglement, in the two-site extended Hubbard model with external magnetic field. Its behaviour at different temperatures is presented. The negativity reduces with the increasing temperature or with the increasing uniform external magnetic field. It is also found that a non-uniform external magnetic field can be used to modulate or to increase the negativity.