Dense coding with a two-qubit Heisenberg XYZ chain under the influence of phase decoherence

We investigate the joint effects of phase decoherence,Dzyaloshinskii-Moriya(DM) interaction and inhomogeneity of the external magnetic field(b) on dense coding in a two-qubit anisotropic Heisenberg XY Z spin chain.Analytical expressions are obtained for the dense coding capacity.It is found that valid dense coding is always possible with this model when the system is initially prepared in the maximum entangled state.Moreover,optimal dense coding can be implemented for this initial state as long as the mean spin-spin coupling constant J + of the XY plane is larger than b and the DM interaction despite the intrinsic decoherence.Non-maximal entangled initial states are found to be undesirable for dense coding with this model.     

Dzyaloshinskii-Moriya相互作用和内禀消相干对基于两量子比特Heisenberg自旋系统的量子密集编码的影响

通过求解Milburn方程, 研究了内禀消相干条件下包含Dzyaloshinskii-Moriya (DM) 相互作用的两量子比特Heisenberg自旋系统实现的量子密集编码最佳传输容量的演化特性, 分析了不同方向DM相互作用、不同初态、各向异性以及内禀消相干因子等参数对最佳编码容量的影响. 研究表明: 初态的选择对系统密集编码最佳传输容量的影响很大, 不同类型初态下密集编码容量的依赖参数不完全相同; 当系统初态处于c|01>+ d|10> 形式的非最大纠缠时, 引入较弱的DM相互作用z分量可提高最佳编码容量; 相位消相干可抑制最佳编码容量的涨落并使其在长时间演化下趋于稳定. 研究还发现: 内禀消相干下, 通过选取合适的最大纠缠初态, 系统密集编码的最佳传输容量能够保持理想极大值2; 而且无论引入哪个方向的DM相互作用, 基于两量子比特Heisenberg自旋系统的最佳编码容量总可优于经典通信的传输容量.     

Entanglement and Optimal Dense Coding in Spin Chain with an Arbitrary Magnetic Field

The entanglement and optimal dense coding at entangled states of a 1D Ising chain in the presence of an external magnetic field with an arbitrary direction, are investigated. The entanglement concurrence and the optimal dense coding capacity are calculated for different orientations of the magnetic field. It has been found that the direction of external magnetic field has effects on the entanglement concurrence and optimal dense coding capacity. In the case of antiferromagnet, the quantum phase transition occurs when an external magnetic field is parallel to Ising orientation. The concurrence increases when the angle between the direction of magnetic field and Ising orientation become smaller at ground state in certain parameter regimes, so does the optimal dense coding. The maximum moves toward the direction perpendicular to the Ising orientation in higher temperature. In contrast, the more concurrence and optimal dense coding can be produced only in the case of an external magnetic field perpendicular to Ising orientation at zero and low temperature for ferromagnetic case.     

Entanglement in Ising Chain with Inhomogeneous Magnetic Field

We have numerically calculated the thermal entanglement of a two-qubit system at low temperatures in a isotropic Ising chain under an inhomogeneous magnetic field. It is shown that in the homogeneous magnetic field, the two- qubit system has entangled states. It is concluded that the presence of the inhomogeneity in the magnetic field plays an effective role on the entangled states. Finally, it is suggested that the inhomogeneity in the magnetic field can be used to create two separated entangled formations in a two-qubit system.     

不同方向外磁场下海森堡自旋链的热纠缠

研究了两量子比特的海森堡XXX自旋链分别处于x方向和y方向均匀外磁场时系统的纠缠特性,并用负度N来度量。得到纠缠度N的解析表达式,并在此基础上进行数值计算。仔细讨论了磁场B、温度T和自旋耦合系数J对纠缠度N的影响。结果表明：纠缠度N会随着磁场|B|和温度T的增大而减小,但会随着自旋耦合系数J的增大而增大。另外,增大的J还会使临界磁场|Bc|和临界温度Tth变大。所以,我们可以通过调节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。研究还发现,加在x方向均匀外磁场和加在y方向均匀外磁场对两量子比特的海森堡XXX自旋链的作用效果是一样的。     

Entangled quantum heat engine based on two-qubit Heisenberg XY model

Based on a two-qubit isotropic Heisenberg XY model under a constant external magnetic field,we construct a four-level entangled quantum heat engine(QHE).The expressions for the heat transferred,the work,and the efficiency are derived.Moreover,the influence of the entanglement on the thermodynamic quantities is investigated analytically and numerically.Several interesting features of the variations of the heat transferred,the work,and the efficiency with the concurrences of the thermal entanglement of two different thermal equilibrium states in zero and nonzero magnetic fields are obtained.     

Quantum Dense Coding About a Two-Qubit Heisenberg XYZ Model

By taking into account the nonuniform magnetic field, the quantum dense coding with thermal entangled states of a two-qubit anisotropic Heisenberg XYZ chain are investigated in detail. We mainly show the different properties about the dense coding capacity (χ) with the changes of different parameters. It is found that dense coding capacity χ can be enhanced by decreasing the magnetic field B, the degree of inhomogeneity b and temperature T, or increasing the coupling constant along z-axis J _z . In addition, we also find χ remains the stable value as the change of the anisotropy of the XY plane Δ in a certain temperature condition. Through studying different parameters effect on χ, it presents that we can properly turn the values of B, b, J _z , Δ or adjust the temperature T to obtain a valid dense coding capacity (χ satisfies χ > 1). Moreover, the temperature plays a key role in adjusting the value of dense coding capacity χ. The valid dense coding capacity could be always obtained in the lower temperature-limit case.     

Concurrence of Quantum States： Algebraic Dynamical Method Study XXX Models in a Time-Depending Random External Field

Based on algebraic dynamics and the concept of the concurrence of the entanglement, we investigate the evolutive properties of the two-qubit entanglement that formed by Heisenberg XXX models under a time-depending external field. For this system, the property of the concurrence that is only dependent on the coupling constant J and total values of the external field is proved. Furthermore, we found that the thermal concurrence of the system under a static random external field is a function of the coupling constant J, temperature T, and the magnitude of external field.     

两个海森堡热纠缠信道中纠缠态的隐形传送

利用两个海森堡热纠缠信道,实现了一对量子纠缠态的隐形传送,求得输出的密度矩阵,进而算出隐形传送的保真度.分析外界环境（温度T和磁场B）对热纠缠及量子隐形传送的影响.发现在磁场较弱,温度很低的时候,以海森堡热纠缠混合态作为量子信道进行隐形传送优于经典信道.     

Thermal Entangled Quantum Heat Engine Working with a Three-Qubit Heisenberg XX Model

Based on a three-qubit isotropic Heisenberg XX model with a constant external magnetic field, we construct an entangled quantum heat engine (QHE). The expressions for several thermodynamic quantities such as the heat transferred, the work and the efficiency are derived. Moreover, the influence of the entanglement on the thermodynamic quantities is investigated analytically and numerically. Several interesting features of the variation of the heat transferred, the work and the efficiency with the concurrences of the thermal entanglement of two different thermal equilibrium states in zero and nonzero magnetic fields are obtained. At last, we discussed the maximum efficiency of the QHE.     

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.     

Thermal Entangled Quantum Refrigerator Working with Three-Qubit Heisenberg XX Model

An entangled quantum refrigerator working with a three-qubit one-dimensional isotropic Heisenberg XX model in a constant external magnetic field is constructed in this paper. Based on the quantum first law of thermodynamics, the expressions for several basic thermodynamic quantities such as the heat transferred, the net work and the coefficient of performance are derived. Moreover, the influence of the thermal entanglement on the basic thermodynamic
quantities is investigated. Several interesting features of the variation of the basic thermodynamic quantities with the thermal entanglement in zero and nonzero magnetic field are obtained.     

Entanglement Purification of Noisy Two-Qutrit States Via Environment Measurement

Entanglement swapping combined with environment measurement is proposed to purify entanglement of two-qutrit entangled states subjected to the local individual amplitude damping channels. The resultant states of our scheme have much more entanglement even though entanglement swapping itself cannot purify entanglement. When the scheme is applied to dense coding, the dense coding capacity can be significantly improved.     

Device-Independent Certification of Maximal Randomness from Pure Entangled Two-Qutrit States Using Non-Projective Measurements

While it has recently been demonstrated how to certify the maximal amount of randomness from any pure two-qubit entangled state in a device-independent way, the problem of optimal randomness certification from entangled states of higher local dimension remains open. Here we introduce a method for device-independent certification of the maximal possible amount of 2log23 random bits using pure bipartite entangled two-qutrit states and extremal nine-outcome general non-projective measurements. To this aim, we exploit a device-independent method for certification of the full Weyl–Heisenberg basis in three-dimensional Hilbert spaces together with a one-sided device-independent method for certification of two-qutrit partially entangled states.     

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.     

非匀强磁场中双量子位Heisenberg XY链的基态纠缠和热纠缠(英文)

研究了非匀强磁场中各向异性Heisenberg XY链的基态纠缠和热纠缠.结果表明对双量子位情形,纠缠与格点间耦合常数J、外部磁场B、各向异性参数γ和b的正负无关.对绝对零度情形,我们给出了纠缠C的解析表达式,并指出临界磁场Bc随磁场各向异性参数b的增大而增大.对有限温度情形,我们给出了γ=0时C的解析表达式和γ≠0时的数值模拟结果,结果发现引进非匀强磁场可以使纠缠在某些区域明显增大;同时我们还指出当γ=0时,纠缠存在的临界温度Tc仅是b的增函数,而当γ≠0时,它却由B和b共同决定.     

Thermal entanglement of two interacting qubits in a static magnetic field

We study systematically the entanglement of a two-qubit Heisenberg XY model in thermal equilibrium in the presence of an external arbitrarily-directed static magnetic field, thereby generalizing our prior work [G. Lagmago Kamta, A.F. Starace, Phys. Rev. Lett. 88, 107901 (2002)]. We show that a magnetic field having a component in the xy-plane containing the spin-spin interaction components produces different entanglement for ferromagnetic (FM) and antiferromagnetic (AFM) couplings. In particular, quantum phase transitions induced by the magnetic field-driven level crossings always occur for the AFM-coupled qubits, but only occur in FM-coupled qubits when the coupling is of Ising type or when the magnetic field has a component perpendicular to the xy-plane. When the magnetic field has a component in the xy-plane, the cut-off temperature above which the entanglement of both the FM- and AFM-coupled qubits vanishes can always be controlled using the magnetic field for any value of the XY coupling anisotropy parameter. Thus, by adjusting the magnetic field, an entangled state of two spins can be produced at any finite temperature. Finally, we find that a higher level of entanglement is achieved when the in-plane component of the magnetic field is parallel to the direction in which the XY exchange coupling is smaller.     

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.     

Almost perfect transport of an entangled two-qubit state through a spin chain

We show that using a slightly modified XX model for a spin-1/2 chain, one can transmit almost perfectly a maximally entangled two-qubit state from one end of the chain to the other one. This is accomplished without external fields or modulation of the coupling constants among the qubits. We also show that this strategy works for any size of the chain and is relatively robust to imperfections in the coupling constants among the qubits belonging to the chain. Actually, under certain scenarios of small disorder, we obtain better results than those predicted by the optimal ordered and noiseless case.     

Thermal entanglement in the anisotropic Heisenberg XYZ model with different inhomogeneous magnetic fields

Thermal entanglement is investigated in a two-qubit Heisenberg XYZ system with different inhomogeneous magnetic fields. It is found that different magnetic fields have different entanglement and critical values. In addition, according to the relation of spin-spin coupling coefficients, a more efficient control parameter of magnetic field can be obtained by adjusting the direction of external magnetic field.