Entanglement and Classical Correlations in the Quantum Frame

The frame of classical probability theory can be generalized by enlarging the usual family of random variables in order to encompass nondeterministic ones. This leads to a frame in which two kinds of correlations emerge: the classical correlation that is coded in the mixed state of the physical system and a new correlation, to be called probabilistic entanglement, which may occur also at pure states. We examine to what extent this characterization of correlations can be applied to quantum mechanics. Explicit calculations on simple examples outline that a same quantum state can show only classical correlations or only entanglement depending on its statistical content; situations may also arise in which the two kinds of correlations compensate each other.     

Thermal Entanglement and Correlated Coherence in Two Coupled Double Quantum Dots Systems

In this work, the thermal quantum correlations in two coupled double semiconductor charge qubits are investigated. This is carried out by deriving analytical expressions for both the thermal concurrence and the correlated coherence. The effects of the tunneling parameters, the Coulomb interaction, and the temperature on the thermal entanglement and on the correlated coherence are studied in detail. It is found that the Coulomb potential plays an important role in the thermal entanglement and in the correlated coherence of the system. The results also indicate that the Coulomb potential can be used for significant enhancement of the thermal entanglement and quantum coherence. One interesting aspect is that the correlated coherence capture all the thermal entanglement at low temperatures, that is, the local coherences are totally transferred to the thermal entanglement. Finally, the role played by thermal entanglement and the correlated coherence responsible for quantum correlations are focused on. It is shown that in all cases, the correlated coherence is more robust than the thermal entanglement so that quantum algorithms based only on correlated coherence may be more robust than those based on entanglement.     

Generation of Entanglement for Coherent Excitonic States in Coupled Quantum Dots in a Microcavity

We investigate the generation of entanglement of coherent excitonic states in coupled quantum dots placed in a cavity by meaning of the state preparation fidelity [Nature （London） 404 （2002） 256; Phys. Rev. A 65 （2002） 012107; J. Uffink, Phys. Rev. Lett. 88 （2002） 230406.] The effect of the number of excitons and the coherent intensity ｜α｜ of the cavity field on the entanglement is also studied.     

Effects of Spatial Nonlocality versus Nonlocal Causality for Bound Electrons in External Fields

Using numerically exact solution of the time-dependent Schrödinger equation together with time-dependent quantum Monte Carlo (TDQMC) calculations, here we compare the effects of spatial nonlocality versus nonlocal causality for the ground state and for real-time evolution of two entangled electrons in parabolic potential in one spatial dimension. It was found that the spatial entanglement quantified by the linear quantum entropy is predicted with good accuracy using the spatial nonlocality, parameterized naturally within the TDQMC approach. At the same time, the nonlocal causality predicted by the exact solution leads to only small oscillations in the quantum trajectories which belong to the idler electron as the driven electron is subjected to a strong high frequency electric field, without interaction between the electrons.     

热库诱导的两比特量子纠缠与量子关联

对比研究了处于热库中的两量子比特的量子纠缠和量子关联随时间的演化。数值模拟的结果表明,尽管热库几乎总能诱导两量子比特的量子纠缠与量子关联,但二者的动力学演化并不完全相同。在与热库的相互作用中,两比特初始形成量子纠缠,随后纠缠逐渐消失,两比特形成纠缠以外的量子关联并得到维持。由此看出,相比于量子纠缠,量子关联对消相干的影响表现得更为稳健。     

Generation of Entanglement for Coherent Excitonic States in Coupled Quantum Dots in a Microcavity

We investigate the generation of entanglement of coherent excitonic states in coupled quantum dots placed in a cavity by meaning of the state preparation fidelity [Nature (London) 404 (2002) 256; Phys. Rev. A 65 (2002) 012107; J. Uffink, Phys. Rev. Lett.88 (2002) 230406.] The effect of the number of excitons and the coherent intensity |α| of the cavity field on the entanglement is also studied.     

On the Entanglement Structure in Quantum Cloning

We study the entanglement properties of the output state of a universal cloning machine. We analyse in particular bipartite and tripartite entanglement of the clones, and discuss the classical limit of infinitely many output copies.     

Many-Agent Controlled Teleportation of Multi-qubit Quantum Information via Quantum Entanglement Swapping

We present a method to teleport multi-qubit quantum information in an easy way from a sender to a receiver via the control of many agents in a network. Only when all the agents collaborate with the quantum information receiver can the unknown states in the sender＇s qubits be fully reconstructed in the receiver＇s qubits. In our method, agents＇s control parameters are obtained via quantum entanglement swapping. As the realization of the many-agent controlled teleportation is concerned, compared to the recent method [G.P. Yang, et al., Phys. Rev. A 70 （2004） 022329], our present method considerably reduces the preparation difficulty of initial states and the identification difficulty of entangled states, moreover, it does not need local Hadamard operations and it is more feasible in technology.     

Many-Agent Controlled Teleportation of Multi-qubit Quantum Information via Quantum Entanglement Swapping

We present a method to teleport multi-qubit quantum information in an easy way from a sender to a receiver via the control of many agents in a network. Only when all the agents collaborate with the quantum information receiver can the unknown states in the sender‘s qubits be fully reconstructed in the receiver‘s qubits. In our method,agents‘s control parameters are obtained via quantum entanglement swapping. As the realization of the many-agent controlled teleportation is concerned, compared to the recent method [C.P. Yang, et al., Phys. Rev. A 70 (2004) 022329],our present method considerably reduces the preparation difficulty of initial states and the identification difficulty of entangled states, moreover, it does not need local Hadamard operations and it is more feasible in technology.     

Continuous-Variable Entanglement in Tunnel-Coupled Double Quantum Wells

This paper investigates the generation and evolution of continuous-variable entanglement in an asymmetric coupled-quantum well （CQW） system. Our numerical results show that this CQW system can be regarded as a source of macroscopic entangled light over a wide range of initial states of the cavity field. This investigation can be used for achieving the macroscopic entangled light in the CQW solid-state medium, which is much more praeticaJ than that in an atomic medium because of its flexible design and the controllable interference strength.     

Greenberger-Horne-Zeilinger Paradox and Quantum Entanglement Swapping in One-Dimensional Lipkin-Meshkov-Glick Model

Based on the ground states of the one-dimensional Lipkin-Meshkov-Glick model (LMGM), we show an all-versus-nothing proof of violation of local realism in this model. Moreover, the quantum entanglement swapping is also investigated in terms of the braiding transformations.     

Multi-state Quantum Teleportation via One Entanglement State

A multi-sender-controlled quantum teleportation scheme is proposed to teleport several secret quantum states from different senders to a distance receiver based on only one Einstein-Podolsky-Rosen （EPR） pair with controlled-NOT （CNOT） gates. In the present scheme, several secret single-qubit quantum states are encoded into a multi-qubit entangled quantum state. Two communication modes, i.e., the detecting mode and the message mode, are employed so that the eavesdropping can be detected easily and the teleported message may be recovered efficiently. It has an advantage over teleporting several different quantum states for one scheme run with more efficiency than the previous quantum teleportation schemes.     

纠缠辅助下三方通信复杂度问题及其量子线路的研究

我们设计了一个实现纠缠辅助下计算特定布尔函数的三方通信复杂度问题的量子线路,该问题中输入变量分布在通信三方。计算分析表明如果通信三方没有分享纠缠态,需要四比特经典通信才能完成计算任务。有纠缠辅助下,仅通过三比特经典通信能够以一定概率完成任务,成功概率与纠缠态的具体形式有关,当选择合适的纠缠态时,成功概率最高可达100%。     

Entanglement Preserving in Quantum Copying of Three-Qubit Entangled State

We study the degree to which quantum entanglement survives when a three-qubit entangled state iscopied by using local and non-local processes, respectively, and investigate iterating quantum copyingfor the three-qubitsystem. There may exist inter-three-qubit entanglement and inter-two-qubit entanglement for the three-qubit system.We show that both local and non-local copying processes degrade quantum entanglement in the three-particle systemdue to a residual correlation between the copied output and the copying machine. We also show that the inter-two-qubitentanglement is preserved better than the inter-three-qubit entanglement in the local cloning process. We find thatnon-local cloning is much more efficient than the local copying for broadcasting entanglement, and output state vianon-local cloning exhibits the fidelity better than local cloning.     

Entanglement Dynamics of Quantum States Undergoing Decoherence from a Driven Critical Environment

In this paper,we have investigated the quantum entanglement of quantum states undergoing decoherence from a spin environment which drives a quantum phase transition.From our analysis,we find that the entanglement dynamics depends not only on the coupling strength but also on the external magnetic field and the number of the freedom degrees of the environment.Specially,our results imply that the decay of the entanglement can be enhanced by the quantum phase transition of the environment when the system is coupled to the environment weakly.Additionally,the discussion of the case of the multipartite states with high dimensions is made.     

应用级联运动方程方法研究并联双量子点的量子纠缠

Quantum dots comprise a type of quantum impurity system. The entanglement and coherence of quantum states are significantly influenced by the strong electron-electron interactions among impurities and their dissipative coupling with the surrounding environment. Competition between many-body effects and transfer couplings plays an important role in determining the entanglement among localized impurity spins. In this work, we employ the hierarchical-equations-of-motion approach to explore the entanglement of a strongly correlated double quantum dots system. The relation between the total system entropy and those of subsystems is also investigated.     

Multi-state Quantum Teleportation via One Entanglement State

A multi-sender-controlled quantum teleportation scheme is proposed to teleport several secret quantum states from different senders to a distance receiver based on only one Einstein-Podolsky-Rosen (EPR) pair with controlled-NOT (CNOT) gates. In the present scheme, several secret single-qubit quantum states are encoded into a multi-qubit entangled quantum state. Two communication modes, i.e., the detecting mode and the message mode, are employed so that the eavesdropping can be detected easily and the teleported message may be recovered efficiently. It has an advantage over teleporting several different quantum states for one scheme run with more efficiency than the previous quantum teleportation schemes.     

Entanglement Dynamics of Quantum States Undergoing Decoherence from a Driven Critical Environment

In this paper, we have investigated the quantum entanglement of quantum states undergoing decoherence from a spin environment which drives a quantum phase transition. From our analysis, we find that the entanglement dynamics depends not only on the coupling strength but also on the external magnetic field and the number of the freedom degrees of the environment. Specially, our results imply that the decay of the entanglement can be enhanced by the quantum phase transition of the environment when the system is coupled to the environment weakly. Additionally, the discussion of the case of the multipartite states with high dimensions is made.