具有三体相互作用海森堡自旋链模型的纠缠与几何失协

近年来有研究发现量子失协可以度量非纠缠的量子关联,而且非纠缠的量子关联在量子通信和量子计算中起到很重要的作用.本文研究了磁场,两种三体相互作用,各向异性参数,耦合常数,温度等参数对同时具有两种三体相互作用海森堡XXZ自旋链系统的量子纠缠,几何失协的影响以及与量子相变的关系.研究表明:量子纠缠和几何失协都可以清晰的表征本模型系统的量子相变现象;随着XZX+YZY型三体相互作用的增加量子纠缠和几何失协即使在高温时也可达到最大值;几何失协比量子纠缠更全面地描述了量子关联;XZY-YZX型三体相互作用的增加对量子纠缠有抑制作用;XZY-YZX型三体相互作用在一定区域内的增加对几何失协有抑制作用,在另一区域的增加可使几何失协增大到一个稳定的非零值.增大磁场和自旋耦合常数,减小各向异性参数会使纠缠的临界温度变大;调节自旋耦合常数可更有效的使量子纠缠和几何失协在高温时仍有一个较大的值.同时发现,在磁场0B5和各向异性参数-1J_Z10的区域两种量子关联都可以维持在最大值.     

具有三体相互作用海森堡自旋链模型的纠缠与几何失协

近年来有研究发现量子失协可以度量非纠缠的量子关联,而且非纠缠的量子关联在量子通信和量子计算中起到很重要的作用.研究了磁场,两种三体相互作用,各向异性参数,耦合常数,温度等参数对同时具有两种三体相互作用海森堡XXZ自旋链系统的量子纠缠,几何失协的影响以及与量子相变的关系.研究表明:量子纠缠和几何失协都可以清晰的表征本模型系统的量子相变现象; 随着XZX+YZY型三体相互作用的增加量子纠缠和几何失协即使在高温时也可达到最大值;几何失协比量子纠缠更全面地描述了量子关联; XZY-YZX型三体相互作用的增加对量子纠缠有抑制作用;XZY-YZX型三体相互作用在一定区域内的增加对几何失协有抑制作用,在另一区域的增加可使几何失协增大到一个稳定的非零值。增大磁场和自旋耦合常数,减小各向异性参数会使纠缠的临界温度变大; 调节自旋耦合常数可更有效的使量子纠缠和几何失协在高温时仍有一个较大的值.同时发现,在磁场 和各向异性参数 的区域两种量子关联都可以维持在最大值.     

Long-range quantum discord in critical spin systems

We show that quantum correlations as quantified by quantum discord can characterize quantum phase transitions by exhibiting nontrivial long-range decay as a function of distance in spin systems. This is rather different from the behavior of pairwise entanglement, which is typically short-ranged even in critical systems. In particular, we find a clear change in the decay rate of quantum discord as the system crosses a quantum critical point. We illustrate this phenomenon for first-order, second-order, and infinite-order quantum phase transitions, indicating that pairwise quantum discord is an appealing quantum correlation function for condensed matter systems.     

Global quantum discord and quantum phase transition in XY model

We study the relationship between the behavior of global quantum correlations and quantum phase transitions in XY model. We find that the two kinds of phase transitions in the studied model can be characterized by the features of global quantum discord (GQD) and the corresponding quantum correlations. We demonstrate that the maximum of the sum of all the nearest neighbor bipartite GQDs is effective and accurate for signaling the Ising quantum phase transition, in contrast, the sudden change of GQD is very suitable for characterizing another phase transition in the XY model. This may shed lights on the study of properties of quantum correlations in different quantum phases.     

Classical correlation, quantum discord and entanglement for two-qubit system subject to heat bath

Classical correlation (CC), quantum discord (QD) and entanglement (QE) of two qubits in one-side and two-side decoherence models are investigated. The sudden change of quantum discord (DSC) as well as classical correlation and sudden death of entanglement (ESD) are found. It is proved that QE (QD) presents no sudden change (sudden death). We prove that, for nonzero occupation number of the reservoir, QE must suffer sudden death; For zero occupation number and X-form initial states, we obtain the states which are robust and the states which experience sudden death. It is verified that if DSC and ESD occur under one-side decoherence, then it must appear in the two-side decoherence, while the reverse does not hold. We obtain the boundaries of CC-QE plane and QD-QE plane, and give the state possessing maximal amount of CC (QD) for a given amount of QE.     

Collapse–revival of quantum discord and entanglement

In this paper the correlations dynamics of two atoms in the case of a micromaser-type system is investigated. Our results predict certain quasi-periodic collapse and revival phenomena for quantum discord and entanglement when the field is in Fock state and the two atoms are initially in maximally mixed state, which is a special separable state. Our calculations also show that the oscillations of the time evolution of both quantum discord and entanglement are almost in phase and they both have similar evolution behavior in some time range. The fact reveals the consistency of quantum discord and entanglement in some dynamical aspects.     

Sudden change of geometric quantum discord in finite temperature reservoirs

We investigate sudden change (SC) behaviors of the distance-based measures of geometric quantum discords (GQDs) for two non-interacting qubits subject to the two-sided and the one-sided thermal reservoirs. We found that the GQDs defined by different distances exhibit different SCs, and thus the SCs are the combined result of the chosen discord measure and the property of a state. We also found that the thermal reservoir may generate states having different orderings related to different GQDs. These inherent differences of the GQDs reveal that they are incompatible in characterizing quantum correlations both quantitatively and qualitatively.     

General monogamy property of global quantum discord and the application

We provide a family of general monogamy inequalities for global quantum discord (GQD), which can be considered as an extension of the usual discord monogamy inequality. It can be shown that those inequalities are satisfied under the similar condition for the holding of usual monogamy relation. We find that there is an intrinsic connection among them. Furthermore, we present a different type of monogamy inequality and prove that it holds under the condition that the bipartite GQDs do not increase when tracing out some subsystems. We also study the residual GQD based on the second type of monogamy inequality. As applications of those quantities, we investigate the GQDs and residual GQD in characterizing the quantum phase transition in the transverse field Ising model.     

双原子Jaynes-Cummings模型下的量子失协和超量子失协

在量子信息的研究中,量子关联起着至关重要的作用,而弱测量对揭示量子关联有重要作用.在双原子Jaynes-Cummings(JC)模型下,分析了量子失协和超量子失协两种量子关联随时间演化过程中的差异性,以及测量强度参数、平均光子数在决定量子关联和影响其动态演化的一般作用.结果表明,弱测量可以更好地揭示量子关联,不仅可以更好地揭示量子关联的复活,还可以获得更大的关联最大值;平均光子数会有效影响两种关联的消失与复活,平均光子数越大,量子关联的消失与复活越快;平均光子数较大时,使用旋波近似得到的弱测量下的超量子失协比强测量下的量子失协与实际误差更小.     

Dynamics control of geometric quantum discord for two coupling qubits in a squeezed vacuum reservoir

We investigate the dynamics of geometric quantum discord of coupled qubits in a squeezed vacuum reservoir. The results show that there is distinct difference between the dynamics of geometric quantum discord and that of quantum entanglement near (or away from) the decoherence free subspace. We also find that the squeezed vacuum reservoir with high squeezed amplitude is more suitable for geometric quantum discord to survive. The robustness of geometric quantum discord is stronger than that of quantum entanglement.     

A symmetric geometric measure and the dynamics of quantum discord

A symmetric measure of quantum correlation based on the Hilbert-Schmidt distance is presented in this paper. For two-qubit states, we considerably simplify the optimization procedure so that numerical evaluation can be performed efficiently. Analytical expressions for the quantum correlation are attained for some special states. We further investigate the dynamics of quantum correlation of the system qubits in the presence of independent dissipative environments. Several nontrivial aspects are demonstrated. We find that the quantum correlation can increase even if the system state is suffering from dissipative noise. Sudden changes occur, even twice, in the time evolution of quantum correlation. There exists a certain correspondence between the evolution of quantum correlation in the systems and that in the environments, and the quantum correlation in the systems will be transferred into the environments completely and asymptotically.     

Induced modification of the geometric phase of a qubit coupled to an XY spin chain by Dzyaloshinsky-Moriya interaction

We consider a qubit symmetrically and transversely coupled to an XY spin chain with Dzyaloshinsky-Moriya(DM) interaction in the presence of a transverse magnetic field.An analytical expression for the geometric phase of the qubit is obtained in the weak coupling limit.We find that the modification of the geometrical phase induced by the spin chain environment is greatly enhanced by DM interaction in the weak coupling limit around the quantum phase transition point of the spin chain.     

Critical point estimation and long-range behavior in the one-dimensional XY model using thermal quantum and total correlations

We investigate the thermal quantum and total correlations in the anisotropic XY spin chain in transverse field. While we adopt concurrence and geometric quantum discord to measure quantum correlations, we use measurement-induced non-locality and an alternative quantity defined in terms of Wigner–Yanase information to quantify total correlations. We show that the ability of these measures to estimate the critical point at finite temperature strongly depend on the anisotropy parameter of the Hamiltonian. We also identify a correlation measure which detects the factorized ground state in this model. Furthermore, we study the effect of temperature on long-range correlations.     

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.     

Tavis-Cummings模型中的几何量子失协特性

采用几何量子失协的计算方法,通过改变两原子初始状态、腔内光子数和偶极-偶极相互作用强度,研究了Tavis-Cummings模型中的几何量子失协特性.结果表明:几何量子失协都是随时间周期性振荡的,选取适当的初态可以使两原子一直保持失协状态,增加腔内光子数和偶极相互作用对几何量子失协有积极的影响.     

Geometric measure of quantum discord in non-Markovian environments

We study the dynamics of geometric measure of quantum discord between two non-interacting qubits each immersed in its own non-Markovian environment with a spectal distribution representing the electromagnetic field inside off-resonant high-Q cavity. We compared the dynamics of geometric measure of quantum discord with quantum discord for an initial Werner-like state and conclude three important findings. First, when there is an instantaneous disappearance in the dynamics of quantum discord at some timepoints, there is a disappearance in geometric measure of quantum discord, but not instantly. Second, the sudden change in the decay rate of geometric measure of quantum discord might not imply the sudden change in the decay rate of the dynamics of quantum discord. Third, there is a preservation for a long time in both quantum discord and geometric measure of quantum discord when the detuning and non-Markovian conditions are simultaneously satisfied.     

Steady state quantum discord for circularly accelerated atoms

We study, in the framework of open quantum systems, the dynamics of quantum entanglement and quantum discord of two mutually independent circularly accelerated two-level atoms in interaction with a bath of fluctuating massless scalar fields in the Minkowski vacuum. We assume that the two atoms rotate synchronically with their separation perpendicular to the rotating plane. The time evolution of the quantum entanglement and quantum discord of the two-atom system is investigated. For a maximally entangled initial state, the entanglement measured by concurrence diminishes to zero within a finite time, while the quantum discord can either decrease monotonically to an asymptotic value or diminish to zero at first and then followed by a revival depending on whether the initial state is antisymmetric or symmetric. When both of the two atoms are initially excited, the generation of quantum entanglement shows a delayed feature, while quantum discord is created immediately. Remarkably, the quantum discord for such a circularly accelerated two-atom system takes a nonvanishing value in the steady state, and this is distinct from what happens in both the linear acceleration case and the case of static atoms immersed in a thermal bath.     

Generation of non-equilibrium thermal quantum discord and entanglement in a three-spin XX chain by multi-spin interaction and an external magnetic field

The generation of non-equilibrium thermal quantum discord and entanglement is investigated in a three-spin chain whose two end spins are respectively coupled to two thermal reservoirs at different temperatures. We show that the spin chain can be decoupled from the thermal reservoirs by homogeneously applying a magnetic field and including a strong three-spin interaction, and then the maximal steady-state quantum discord and entanglement in the two end spins can always be created. In addition, the present investigation may provide a useful approach to control coupling between a quantum system and its environment.