Quantum Discord for Two-Qubit System in a Symmetry-Broken Environment

In contrast with the entanglement, we study the quantum discord dynamics of the two-qubit system in a symmetry-broken environment consisting of a fermionic bath. The quantum discord decay induced by the bath is analysed. By considering the two qubits that are initially prepared in the different X-states, we find that the behaviors of quantum discord and entanglement are different, the robustness of quantum discord depends on the initial state prepared in.     

Quantum Measurement of Two-Qubit System in Damping Noise Environment

It is known that the inevitable interaction of the entangled qubits with their environments may result in the degradation of quantum correlation.We study the decoherence of two remote qubits under general local single-and two-sided amplitude-damping channel(ADC).By using concurrence,quantum discord and Clauser-Horne-ShimonyHolt(CHSH)inequality,we find that the relation between the residual quantum correlations and the initial ones are different.Recently,Wang et al.[Int.J.Theor.Phys.54(2015)5]showed that there exist a set of partially entangled states that are more robust than maximally entangled states in terms of the residual quantum correlation measured by concurrence,fully entangled fraction and quantum discord,respectively.Here we find that both in single-and two-sided ADC,only the evolution of CHSH inequality with the initial parameter is proportional to that of the initial nonlocality.That means the initial state with maximally nonlocality will retain its role in the evolution.It implies that the evolution of nonlocality may reveal the characteristics of quantum state better.Furthermore,we discuss the evolutions of the three different quantum measurements with the initial parameter under generalized amplitude damping channel(GADC)and find that they are all proportional to that of the initial state.     

Quantum Discord for a Central Two-Qubit System Coupled to an XY Spin Chain with Energy Current

We investigate the quantum discord dynamics of a two-qubit system coupled to an XY spin-chain environment with energy current. We compared dynamical behaviors of quantum discord under different system-environment couplings, the size of the degrees of freedom of the environment, the anisotropy parameter, and the energy current. The results indicate that the energy current can strongly suppress the quantum discord in the weak-coupling region, while it has no obvious effect on the quantum discord in the strong-coupling regime.     

Quantum Entanglement and Quantum Discord of a Two-Qubit System in a Dissipative Cavity

The goal of this work is to investigate quantum entanglement and quantum discord of a pair of two-level atoms which is driven by an external classical field and interacts with a cavity field. After extracting density matrix of the atom-atom subsystem, it is shown that we have stronger quantum discord by increasing atom-field coupling constant for the case in which there is no entanglement. Moreover, for the atom-atom subsystem it is realized that quantum entanglement and quantum discord cannot increase, they decrease after passing some times due to cavity dissipation. Also quantum entanglement and quantum discord decrease faster by increasing atom-field coupling constant.     

Discontinuity and Protection of Quantum Fisher Information for a Two-Qubit System

We analytically investigate the dynamics of quantum Fisher information of two qubits in independent environments for both uncorrelated and entangled input states.Especially,we observe that,in the non-Markovian regime and resonant case,quantum Fisher information of uncorrelated input state vanishes only at discrete time point whereas that of entangled input state can disappear during finite time interval.It shows that quantum Fisher information,which determines the parameter estimation precision,is strongly affected by the environment memory effects,and the advantage of entanglement-enhance metrology no longer exists even for a very short time.We also note that quantum Fisher information for two kinds of input states can be preserved under appropriate qubit-reservoir detuning.     

Discontinuity and Protection of Quantum Fisher Information for a Two-Qubit System

We analytically investigate the dynamics of quantum Fisher information of two qubits in independent environments for both uncorrelated and entangled input states. Especially, we observe that, in the non-Markovian regime and resonant case, quantum Fisher information of uncorrelated input state vanishes only at discrete time point whereas that of entangled input state can disappear during finite time interval. It shows that quantum Fisher information, which determines the parameter estimation precision, is strongly affected by the environment memory effects, and the advantage of entanglement-enhance metrology no longer exists even for a very short time. We also note that quantum Fisher information for two kinds of input states can be preserved under appropriate qubit-reservoir detuning.     

光纤耦合双光学腔系统的相干动力学

为了研究量子相干性在腔量子电动力学系统中的动力学和分布特性,基于两个各自捕获原子系综的光学腔建立了双光学腔系统,腔与腔之间由光纤耦合.利用相对熵度量的量子相干性,引入量子相干非平衡性的概念,分析了系统中相干动力学和光纤-腔耦合强度对相干性分布的影响.结果表明:在强耦合极限下,光纤-腔耦合强度的增加有利于保持两腔中的原子的整体相干性;光纤-腔耦合强度、原子-腔耦合强度以及原子数三个参数之间满足特定条件时,腔内的原子相干性可以传输至另一个腔.考虑腔、光纤及原子都存在耗散的情形,对比了不同耗散速率和非耗散情形下的相干性演化,发现耗散使得耦合双腔系统的相干性以及各个腔中的原子相干性发生衰减.     

Markov Quantum Feedback Control Based on Homodyne Measurement of a Two-Qubit System

We consider the system consisting of two qubits collectively damped,with the output being unit-efficiency measured and subsequently fedback to control the system state. Our primary goal in this paper is (i) to solve the feedback-modified master equation, (ii) to demonstrate the ability of feedback control based on the solutions, and (iii) to pick out different steady states by choosing different driving strengths and feedback strengths tocounteract the effects of both damping and the measurement back-action on the system. We further investigate some properties of the equilibrium steady state, its distribution probability and entanglement vs. the driving and feedback amplitudes. We find that in our feedback model feedback plays a negative role in producing entanglement.     

Quantum Discord in Two-Qubit System Constructed from the Yang–Baxter Equation

Quantum correlations among parts of a composite quantum system are a fundamental resource for several applications in quantum information. In general, quantum discord can measure quantum correlations. In that way, we investigate the quantum discord of the two-qubit system constructed from the Yang-Baxter Equation. The density matrix of this system is generated through the unitary Yang-Baxter matrix R. The analytical expression and numerical result of quantum discord and geometric measure of quantum discord are obtained for the Yang-Baxter system. These results show that quantum discord and geometric measure of quantum discord are only connect with the parameter θ, which is the important spectral parameter in Yang Baxter equation.     

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.     

Enhancement of the Quantum Coherence in a Hierarchical Environment System by Weak Measurement and Weak Measurement Reversal

The quantum coherence of a two-qubit system in which each qubit is coupled to its own hierarchical environment is studied. The effect of the hierarchical environment is explored in order to improve the coherence effectively. It is discovered that the dynamics of coherence can be manipulated by the number N of the second-layer cavities ($m_n$), the coupling strengths Ω₀, Ω, κ, respectively for the qubit-cavity (m₀), nearest-neighbor, and m₀-$m_n$ cavities. It depends also on the decay rate Γ of second-layer cavities. Furthermore, an effective scheme for enhancing coherence is proposed by using weak measurement and weak measurement reversal. The explicit conditions of the optimal measurement strengths to improve the coherence comparatively are derived. It is seen that the final stable value of the coherence is independent of the number N of second-layer cavities but is related to the weak measurement strength m when the coupling strength between two nearest-neighbor cavities is taken into account.     

简并能级系统中的量子相干效应与暗态

文章分析了简并二能级原子系统Fg=1→Fe=0中的吸收、色散特性与暗态的对应关系,比较了双Λ型和Λ型能级结构下的量子相干效应.结果表明,双Λ型系统中多个基态相干性会增强原子相干特性.     

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 diferent approaches,namely,quantum discord[Phys.Rev.Lett.88(2002)017901],measurementinduced 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 diferent approaches are compared and discussed so that their three distinct features are exposed.     

Energy Spectrum of Bound-Spinons in the Quantum Ising Spin-Chain Ferromagnet

We study the excitation energy spectrum in the S=1/2 ferromagnetic Ising spin chain with the easy axis z in a magnetic field h={h _x ,0,h _z }. According to Wu and McCoy’s scenario of weak confinement, the fermionic spinon excitations (kinks), being free at h _z =0 in the ordered phase, are coupled into bosonic bound states at arbitrary small h _z >0. We calculate the energy spectrum of such excitations in the leading order in small h _z , using different perturbative methods developed for the similar problem in the Ising Field Theory.     

Quantum Teleportation Through a Two-Qubit Heisenberg XXZ Chain

We consider a two-qubit Heisenberg XXZ chain as a resource for quantum teleportation via the standard teleportation protocol To. The effects of anisotropic on teleportation fidelity and entanglement are studied in detail. We find anisotropic not only improves the criticM temperature Tc and criticM magnetic field Bc, beyond which quantum teleportation is inferior to classicM communication protocol, but also enhances the fidelity for fixed magnetic field B and temperature T. For entanglement teleportation, the effects of magnetic field on average fidelity and output entanglement are studied.     

Quantum Discord of Two-Qubit in Dephasing Model

We evaluate the dynamics of two-qubit quantum discord(QD) under the classical phase noise. We compare the dynamics of QD with that of entanglement as measured with concurrence. The influence of mixture degree on the dynamics is also discussed. The results show that there is no simple relation between the quantum correlation and entanglement as seen that QD may be smaller or larger than entanglement, and QD is more robust than the entanglement.     

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.     

Quantum Coherence and Spin Squeezing in Optomechanical System

We investigate quantum coherence and spin squeezing in optomechanical system. We first determine the mean spin direction, the optimally squeezed angle and then calculate the first-order temporal correlation function, the squeezing parameter, which are independent of the frequency of the cavity field. The lager coupling strength more rapidly generates spin squeezed state, but the corresponding spin squeezed state maintains shorter time interval.