Entanglement Dynamics of Two Qubits Coupled to a Noise Environment

We study the time evolution of two two-state systems （two qubits） initially in the pure entangled states or the maximally entangled mixed states interacting with the individual environmental noise. It is shown that due to environment noise, all quantum entangled states are very fragile and become a classical mixed state in a short-time limit. But the environment can affect entanglement in very different ways. The type of decoherence process for certain entangled states belongs to amplitude damping, while the others belong to dephasing decoherenee.     

Generation and Purification of Atomic Entangled States

Entangled state plays a more and more important role in quantum information, so the generation of entangled state is of scientific value and practical significance.Although the experimental realization of entangled pairs of atoms and polarized photons have been reported recently, the current preparation schemes cannot meet the need of the practical application of entangled state in Quantum Communication and Quantum Computation.At the same time, resulting from the coupling between the quantum systems and its environment, decoherence of the quantum systems is unavoidable, which sets a vital obstacle on the way of the application of entanglement.There exist some entanglement generation and purification schemes, but the range of its application is relative small.So we proposed a more efficient scheme for entanglement generation and purification.The scheme is mainly based on the combination of linear optics and Cavity QED technique.The entanglement generation scheme can entangle two atoms by using MZI plus an optical cavity.Pure maximally entangled atomic states can be generated from product states or mixed states.Using a MZI, we can extract not only two-atom near-maximally entangled states but also four-atom maximally entangled states from less entangled pure or mixed states.     

Generalized projective synchronization of chaotic systems via adaptive learning control

In this paper, a learning control approach is applied to the generalized projective synchronisation (GPS) of different chaotic systems with unknown periodically time-varying parameters. Using the Lyapunov--Krasovskii functional stability theory, a differential-difference mixed parametric learning law and an adaptive learning control law are constructed to make the states of two different chaotic systems asymptotically synchronised. The scheme is successfully applied to the generalized projective synchronisation between the Lorenz system and Chen system. Moreover, numerical simulations results are used to verify the effectiveness of the proposed scheme.     

Concentration of Unknown Atomic Entangled States via Entanglement Swapping through Raman Interaction

We show that entanglement concentration of unknown atomic entangled states is achieved via the implementation of entanglement swapping based on Raman interaction in cavity QED. A maximally entangled state is obtained from a pair of partially entangled states probabilistically. Due to Raman interaction of two atoms with a cavity mode and an external driving field, the influence of atomic spontaneous emission has been eliminated. Because of the virtual excitation of the cavity mode, the decoherence of cavity decay and thermal field is neglected.     

Complex dynamical behavior and chaos control in fractional-order Lorenz-like systems

In this paper, the complex dynamical behavior of a fractional-order Lorenz-like system with two quadratic terms is investigated. The existence and uniqueness of solutions for this system are proved, and the stabilities of the equilibrium points are analyzed as one of the system parameters changes. The pitchfork bifurcation is discussed for the first time, and the necessary conditions for the commensurate and incommensurate fractional-order systems to remain in chaos are derived. The largest Lyapunov exponents and phase portraits are given to check the existence of chaos. Finally, the sliding mode control law is provided to make the states of the Lorenz-like system asymptotically stable. Numerical simulation results show that the presented approach can effectively guide chaotic trajectories to the unstable equilibrium points.     

Dense coding scheme using superpositions of Bell-states and its NMR implementation

Dense coding using superpositions of Bell-states is proposed. The generalized Grover's algorithm is used to prepare the initial entangled states, and the reverse process of the quantum algorithm is used to determine the entangled state in the decoding measurement. Compared with the previous schemes, the superpositions of two Bell-states are exploited. Our scheme is demonstrated using a nuclear magnetic resonance (NMR) quantum computer. The corresponding manipulations are obtained. Experimental results show a good agreement between theory and experiment. We also generalize the scheme to transmit eight messages by introducing an additional two-state system.     

Efficient Scheme for Entangling Two Distant Atoms

A scheme is presented for the generation of entangled states for two atoms trapped in two distant cavities. In the scheme each atom is resonantly coupled with the respective cavity mode and driven by a strong classical field. The detection of a photon decaying from the cavities and passing through a beam-splitter collapses the atoms to an entangled state. The required atom-field interaction time is very short and thus the decoherence effect is suppressed. Our scheme is within the reach of presently available cavity QED techniques.     

Remote Preparation of Multipartite Equatorial Entangled States in High Dimensions with Three Parties

A scheme for probabilistic remotely preparing N-particle d-dimensional equatorial entangled states via entangled swapping with three parties is presented. The quantum channel is composed of N - 1 pairs of bipartite d-dimensional non-maximally entangled states and a tripartite d-dimension non-maximally entangled state. It is shown that the sender can help either of the two receivers to remotely prepare the original state, and the N-particle projective measurement and the generalized Hadamard transformation are needed in this scheme. The total success probability and classical communication cost are calculated.     

Output regulation for linear multi-agent systems with unmeasurable nodesOutput regulation for linear multi-agent systems with unmeasurable nodesOutput regulation for linear multi-agent systems with unmeasurable nodes

The output regulation of linear multi-agent systems with partial unmeasurable agents is investigated in this paper. All the agents except the exosystem can be classified into two groups. Agents in the first group can be measured by themselves and their neighbors. State variables are not fully accessible for direct communication and full order Luenberger observers are constructed for the unmeasurable agents. We give a state feedback control law to solve the output regulation problem under the communication topologies based on both measurable and unmeasurable agents. The heterogeneous agents＇ synchronization problem is a general case of our results. Finally, examples are utilized to show the effectiveness of the obtained results.     

Output regulation for linear multi-agent systems with unmeasurable nodes

The output regulation of linear multi-agent systems with partial unmeasurable agents is investigated in this paper. All the agents except the exosystem can be classified into two groups. Agents in the first group can be measured by themselves and their neighbors. State variables are not fully accessible for direct communication and full order Luenberger observers are constructed for the unmeasurable agents. We give a state feedback control law to solve the output regulation problem under the communication topologies based on both measurable and unmeasurable agents. The heterogeneous agents’ synchronization problem is a general case of our results. Finally, examples are utilized to show the effectiveness of the obtained results.     

噪声环境中两粒子纠缠态的纠缠消相干

借助于共生纠缠度和输入输出保真度考察了初始处于纠缠态的两粒子在联合噪声环境中的消纠缠特性.结果表明:两粒子纠缠态可分为相干保持态和脆弱纠缠态.对于脆弱纠缠态分析了它们在低温条件欧姆型耗散下的纠缠消相干演化动力学. 关键词： 热库 相干保持态 脆弱纠缠态 输入输出保真度     

Robustness of Entanglement During Decoherence

Finding the most robust entangled states during the whole process of decoherence is a particularly fundamental problem for quantum physics and quantum information processing. In this paper, the decoherence process of two-qubit system under two individual identical decoherence channels is investigated systematically. We find that although the robustness of two-qubit states with same initial entanglement is usually different, the Bell-like states are always the most robust entangled states during decoherence. That is to say, affected by the same amount of noise, the remain entanglement of an arbitrary two-qubit state is not more than that of a Bell-like state with the same initial entanglement.     

Experimental investigation of a two-qubit decoherence-free subspace

We thoroughly explore the phenomenon of a decoherence-free subspace (DFS) for two-qubit systems. Specifically, we both collectively and noncollectively decohere entangled polarization-encoded two-qubit states using thick birefringent crystals. These results characterize the basis-dependent effect of decoherence on the four Bell states, the robustness of the DFS state against perturbations in the assumption of collective decoherence, and the existence of a DFS for each type of stable noncollective decoherence. Finally, we investigate the effects of collective and noncollective dissipation.     

Entanglement and decoherence: fragile and robust entanglement

The destruction of entanglement of open quantum systems by decoherence is investigated in the asymptotic long-time limit. For this purpose a general and analytically solvable decoherence model is presented which does not involve any weak-coupling or Markovian assumption. It is shown that two fundamentally different classes of entangled states can be distinguished and that they can be influenced significantly by two important environmental properties, namely, its initially prepared state and its size. Quantum states of the first class are fragile against decoherence so that they can be disentangled asymptotically even if coherences between pointer states are still present. Quantum states of the second type are robust against decoherence. Asymptotically they can be disentangled only if also decoherence is perfect. A simple criterion for identifying these two classes on the basis of two-qubit entanglement is presented.     

Phase transitions of cellular automata

Cellular automata (CA) are simple mathematical models of the dynamics of discrete variables in discrete space and time, with applications in nonequilibrium physics, chemical reactions, population dynamics and parallel computers. Phase transitions of stochastic CA with absorbing states are investigated. Using transfermatrix scaling the phase diagrams, critical properties and the entropy of one-dimensional CA are calculated. The corners of the phase diagrams reduce to deterministic CA discussed by Wolfram (Rev. Mod. Phys.55, 601 (1983)). Three-state models are introduced and, for special cases, exactly mapped onto two-state CA. The critical behaviour of other threestate models with one or two absorbing states and with immunization is investigated. Finally CA with competing reactions and/or with disorder are studied.     

Amplitude-Damping Decoherence Suppression of Two-Qubit Entangled States by Weak Measurements

Taming decoherence is a critical issue in quantum information science. We here investigate amplitude-damping decoherence suppression of two-qubit entangled states by weak quantum measurements. It is shown that the weak measurements can effectively suppress the decoherence for different initial entangled states. More interestingly, we show that the weak measurements have different effects on the entanglement protection for two entangled states which are equivalent under a local unitary operation. This result implies that the entanglement protection effect could be modulated according to different demands.     

Topologically decoherence-protected qubits with trapped ions

We show that trapped ions can be used to simulate a highly symmetrical Hamiltonian with eigenstates naturally protected against local sources of decoherence. This Hamiltonian involves long-range coupling between particles and provides a more efficient protection than nearest neighbor models discussed in previous works. Our results open the perspective of experimentally realizing, in controlled atomic systems, complex entangled states with decoherence times up to 9 orders of magnitude longer than isolated quantum systems.     

多粒子纠缠的保护方案

量子纠缠是量子信息的重要物理资源. 然而当量子系统与环境相互作用时, 会不可避免地产生消相干导致纠缠下降, 因此保护纠缠不受环境的影响具有重要意义. 振幅衰减是一种典型的衰减机制. 如果探测环境保证没有激发从系统中流出, 即视为对系统的一种弱测量. 本文基于局域脉冲序列和弱测量, 提出了一种可以保护多粒子纠缠不受振幅衰减影响的有效物理方案, 保护的对象是在量子通信和量子计算中发挥重要作用的Cluster态和Maximal slice态.     

Blind Quantum Signature with Controlled Four-Particle Cluster States

A novel blind quantum signature scheme based on cluster states is introduced. Cluster states are a type of multi-qubit entangled states and it is more immune to decoherence than other entangled states. The controlled four-particle cluster states are created by acting controlled-Z gate on particles of four-particle cluster states. The presented scheme utilizes the above entangled states and simplifies the measurement basis to generate and verify the signature. Security analysis demonstrates that the scheme is unconditional secure. It can be employed to E-commerce systems in quantum scenario.