内禀退相干下薛定谔猫态光场与原子相互作用系统的场熵演化

通过求解系统的Milburn方程,研究了二能级原子与薛定谔猫态光场相互作用系统中的场熵演化特性.讨论了内禀退相干,光场强度,相干态间的相位角对场熵演化的影响.结果表明: 内禀退相干下,随着时间的演化,场熵振荡逐渐减弱,光场与原子的纠缠度逐渐趋于恒值.并且光场与原子的最大纠缠度值只取决光场强度和相干态间的相位角,与内禀退相干因子无关.光场强度较小时,奇相干态光场与原子的纠缠度最大;偶相干态光场与原子的纠缠度值为最小;Yurke-Stoler相干态光场与原子的纠缠度值介于两者之间. 当内禀退相干因子不变、光场强度较大时,分别处于Yurke-Stoler相干态、偶相干态和奇相干态的光场与原子的纠缠度值趋近于相同.     

内禀退相干下薛定谔猫态光场与原子相互作用系统的场熵演化

通过求解系统的Milburn方程,研究了二能级原子与薛定谔猫态光场相互作用系统中的场熵演化特性.讨论了内禀退相干,光场强度,相干态间的相位角对场熵演化的影响.结果表明: 内禀退相干下,随着时间的演化,场熵振荡逐渐减弱,光场与原子的纠缠度逐渐趋于恒值.并且光场与原子的最大纠缠度值只取决光场强度和相干态间的相位角,与内禀退相干因子无关.光场强度较小时,奇相干态光场与原子的纠缠度最大;偶相干态光场与原子的纠缠度值为最小;Yurke-Stoler相干态光场与原子的纠缠度值介于两者之间. 当内禀退相干因子不变、光场强度较大时,分别处于Yurke-Stoler相干态、偶相干态和奇相干态的光场与原子的纠缠度值趋近于相同.     

双模场与原子相互作用中的量子纠缠和内禀退相干

通过求解系统的Milburn方程，研究了两能级原子与双模SU(1,1)相干态光场发生相互作用系统中，原子与场的纠缠及双模SU(1,1)相干态场的模间纠缠随时间的演化问题，讨论了内禀退相干、双模光子数差等对纠缠度的影响.结果表明，存在内禀退相干时，随着时间的演化，场-原子纠缠逐渐减小到一个确定值，而模间纠缠逐渐增大到一个确定值，两者演化的最终值只取决于双模光子数差和平均光子数，而与内禀退相干因子无关. 关键词： Milburn理论 SU(1 1)相干态 量子约化熵 量子相对熵     

两粒子XXZ海森堡系统中的量子纠缠动力学

研究了考虑内禀退相干情况下,两粒子XXZ海森堡系统中量子纠缠在各种外界因素影响下的动态演化规律。研究发现无论系统的初态是否处于最大纠缠态,内禀退相干都会对系统纠缠的含时演化有明显的抑制作用。如果系统的初态为纠缠态|ψ(0)〉=c|01〉+d|10〉,系统的纠缠与各向异性参数和外加磁场没有关系,却受到Dzyaloshinskii-Moriya(DM)相互作用程度和内禀退相干因素的明显影响;如果系统的初态为纠缠态|ψ(0)〉=a|00〉+b|11〉,系统的纠缠与DM相互作用大小和各向异性参数无关,纠缠程度不仅要受到内禀退相干因素的影响,还会被外加磁场进一步削弱。     

三量子位系统的消相干和退纠缠

基于多谐振子热库环境模型,采用求解非马尔可夫近似主方程方法,研究了三量子位系统由于相对位相变化引起的消相干和退纠缠.计算得到了初始态为GHZ-class态、W-class态和WGHZ-class态在演化过程中的消相干和退纠缠时间尺度.结果表明,对初始GHZ-class态,相干性全部丧失,退纠缠完全发生,纠缠度变为0；对初始W-class态,相干性和纠缠度被保持；对初始WGHZ-class态,消相干部分发生,纠缠首先随时间变化最后达到一个渐近值.     

在不同初态下Dzyaloshinskii-Moriya相互作用及内禀退相干对海森伯系统的量子纠缠的影响

研究了存在内禀退相干时,对于不同的系统初态,具有DM相互作用和各向异性的三粒子XXZ海森伯模型的对纠缠动力学特性.得出了一些结论:系统的对纠缠度与各向异性参数?无关,但内禀退相干对系统的纠缠有明显的抑制作用;在内禀退相干存在时,若系统初态为纠缠态,选择合适的DM相互作用的参数,系统的对纠缠有一个非零的稳定值;系统初态为分离态时,系统的对纠缠会随时间震荡衰减,并且每次震荡会出现纠缠突然死亡现象,系统的对纠缠最终达到解纠缠状态.因此,选择合适的系统初态和DM相互作用参数可以有效地控制系统的对纠缠.     

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.     

不同方向Dzyaloshinskii-Moriya相互作用和磁场对自旋系统纠缠和保真度退相干的影响

通过求解系统的Milburn方程,研究了包含Dzyaloshinskii-Moriya相互作用的自旋链系统中纠缠和保真度的动力学演化特性,讨论了不同方向Dzyaloshinskii-Moriya相互作用、不同方向均匀和非均匀磁场、不同初始态对纠缠以及保真度退相干的影响.研究发现,非均匀磁场的引入能够抑制纠缠退相干的发生,初始态的选择对系统纠缠状态的影响很大,可以通过调制Dzyaloshinskii-Moriya相互作用的方向来获得所需纠缠和较高的保真度.研究还发现,退相干条件下,无论是均匀还是非均匀磁场对于保真度的提高并不明显.纠缠和保真度随初始态角度的变化具有周期性,可以根据需要来选取不同系统中的最优初始态.     

Dzyaloshinskii-Moriya相互作用和內禀退相干对三粒子XXZ海森堡系统的量子纠缠的影响

根据Milburn理论,研究了考虑内禀退相干情况下,三粒子XXZ海森堡系统在DzyaloshinskiiMoriya(DM)相互作用和各向异性参数的影响下的量子纠缠演化特性.分析了不同DM相互作用和內禀退相干因子等参数对量子对纠缠度的影响.研究表明:系统的对纠缠度与各向异性参数Δ无关,但DM相互作用和內禀退相干都对系统的对纠缠都有明显的影响.当无DM相互作用只存在內禀退相干时,系统的三对对纠缠度各不相同;引入DM相互作用后,系统的对纠缠度的稳定值将改变;选择合适的DM相互作用参数时,系统的对纠缠度随时间做震荡,随着时间的延长,震荡减弱至一个非零的稳定值;并且系统的对纠缠随內禀退相干增大,震荡幅度变小,震荡时间变短.因此,在內禀退相干存在时,合适的DM相互作用参数可以有效的控制对纠缠.     

内禀退相干下二项式光场与原子互作用的场熵演化

通过求解系统的Milburn 方程,研究了二项式光场与二能级原子相互作用系统中的场熵演化特性,讨论了内禀退相干、二项式光场系数和场的最大光子数对场熵演化的影响.结果表明二项式光场处于中间态时,由于内禀退相干,场熵振荡准周期性减弱,并随着时间的演化,场与原子纠缠逐渐趋近于一个定值,说明存在內禀退相干时,场与原子仍能纠缠;当內禀退相干因子不变时,二项式光场从相干态过渡到中间态,直至数态过程中,场熵值减小,场与原子的纠缠减弱.     

Coherence-controlled entanglement and nonlocality of two qubits interacting with a thermal reservoir

We investigate the entanglement and the nonlocality of two qubits interacting with a thermal reservoir. It is shown that the time behavior of these quantities exhibits a strong dependence on the initial state of two qubits, and that the entanglement and the nonlocality of two qubits can be manipulated by changing the relative phases and the amplitudes of the polarized qubits.     

Sudden birth versus sudden death of entanglement for the extended Werner-like state in a dissipative environment

In this paper,we investigate the dynamical behaviour of entanglement in terms of concurrence in a bipartite system subjected to an external magnetic field under the action of dissipative environments in the extended Werner-like initial state.The interesting phenomenon of entanglement sudden death as well as sudden birth appears during the evolution process.We analyse in detail the effect of the purity of the initial entangled state of two qubits via Heisenberg XY interaction on the apparition time of entanglement sudden death and entanglement sudden birth.Furthermore,the conditions on the conversion of entanglement sudden death and entanglement sudden birth can be generalized when the initial entangled state is not pure.In particular,a critical purity of the initial mixed entangled state exists,above which entanglement sudden birth vanishes while entanglement sudden death appears.It is also noticed that stable entanglement,which is independent of different initial states of the qubits (pure or mixed state),occurs even in the presence of decoherence.These results arising from the combination of the extended Werner-like initial state and dissipative environments suggest an approach to control and enhance the entanglement even after purity induced sudden birth,death and revival.     

Sudden Death, Birth and Stable Entanglement in a Two-Qubit Heisenberg XY Spin Chain

Taking the decoherence effect due to population relaxation into account, we investigate the entanglement properties for two qubits in the Heisenberg XY interaction and subject to an external magnetic field. It is found that the phenomenon of entanglement sudden death (ESD) as well as sudden birth (ESB) appear during the evolution process for particular initial states. The influence of the external magnetic field and the spin environment on ESD and ESB are addressed in detail. It is shown that the concurrence, a measure of entanglement, can be controlled by tuning the parameters of the spin chain, such as the anisotropic parameter, external magnetic field, and the coupling strength with their environment. In particular, we find that a critical anisotropy constant exists, above which ESB vanishes while ESD appears. It is also notable that stable entanglement, which is independent of different initial states of the qubits, occurs even in the presence of decoherence.     

Delayed creation of entanglement in superconducting qubits interacting with a microwave field

We explore the role played by the intrinsic decoherence in superconducting charge qubits in the presence of a microwave field applied as a magnetic flux. We study how the delayed creation of entanglement, which is opposite to the sudden death of entanglement, can be induced. We compute the time evolution of the population inversion, total correlation and entanglement, taking into account the junction mixed state and dissipation of the cavity field. We show that although decoherence destroys the correlation of the junction and field, information of the initial state may be obtained via quasi-probability distribution functions.     

Pairwise entanglement of a three-qubit Heisenberg XY chain in a nonuniform magnetic field with intrinsic decoherence

Taking into account the intrinsic decoherence, the concurrence of the nearest and the next-to-nearest neighbor qubits in a three-qubit Heisenberg XY chain are investigated when a nonuniform magnetic field is included. We show that the effects of the external magnetic field, including the uniform and inhomogeneous magnetic fields, on the time evolution of entanglement between the nearest and the next-to-nearest neighbor qubits rely deeply on the initial states. We can moderate the destructive effect of intrinsic decoherence by controlling the uniform and inhomogeneous magnetic fields, so that a proper value of uniform and inhomogeneous magnetic fields can, to a great extent enhance the stationary entanglement.     

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.     

Quantum Correlation and Coherence in Dissipative Two SC-Qubit Systems Interacting with a Coherent SC-Cavity

An analytical solution of the master equation is obtained for a coherent SC-cavity interacts with two SC-qubits. With this solution, the effect of an intrinsic decoherence on the growth of coherence loss (mixture) and the entanglement degradation is shown. Due to the intrinsic decoherence, the oscillatory behavior of the qubit-inversion is deteriorated, and its amplitude decreases. Where its stationary state refers to that the energy is stored more in the SC-qubits. The growth of the mixture is investigated for the entire system and its sub-systems of the SC-cavity and two qubits. The entanglement is studied for two different partitions, between the SC-cavity and the two qubits as well as between the two qubits. It is found that the generation of the entanglement and mixture and their stationary values depend on the initial coherence intensity and the intrinsic decoherence rate. The stationary entanglement and mixture, and the phenomena of sudden appearance and disappearance of the entanglement could be controlled by adjusting the intrinsic noise rate and the initial coherence intensity.

     

Bipartite and Tripartite Entanglement in a Three-Qubit Heisenberg Model

The bipartite and tripartite entanglement in a three-qubit Heisenberg XY model with a nonuniform magnetic field is studied. There are two or four peaks in the concurrence of the bipartite entanglement when the amplitudes of the magnetic fields are. differently distributed between the three qubits. It is very interesting to note that there is no tangle of tripartite entanglement between the three qubits when the amplitudes of the magnetic fieMs are varied. However, the variation of the magnetic field direction can induce the tangle. The tangle is periodic about the angle between the magnetic field and the z axis of the spin.     

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.     

Controlling Sudden Birth and Sudden Death of Entanglement at Finite Temperature

The decoherence and the decay of quantum entanglement due to both population relaxation and thermal effects are investigated for the two qubits initially prepared in the extended Werner-like state by solving the Lindblad form of the master equation, where each qubit is interacting with an independent reservoir at finite temperature T. Entanglement sudden death (ESD) and entanglement sudden birth (ESB) are observed during the evolution process. We analyze in detail the effects of the mixedness, the degree of entanglement of the initial states and finite temperature on the time of entanglement sudden death and entanglement sudden birth. We also obtain an analytic formula for the steady state concurrence that shows its dependence on both the system parameters, the decoherence rate and finite temperature. These results arising from the combination of extended Werner-like initial state and independent thermal reservoirs suggest an approach to control the maximum possible concurrence even after the purity and finite temperature induce sudden birth, death and revival.