Entanglement of a quantum optical elliptic vortex

We calculate the entanglement of a generalized elliptical vortex formed by quantized radiation field, using Wigner quasiprobability distribution function for such states. We find a critical squeezing parameter above which the entanglement is less for higher vorticity, which is counter intuitive.     

Entanglement fidelity of the standard quantum teleportation channel

We consider the standard quantum teleportation protocol where a general bipartite state is used as entanglement resource. We use the entanglement fidelity to describe how well the standard quantum teleportation channel transmits quantum entanglement and give a simple expression for the entanglement fidelity when it is averaged on all input states.     

Entanglement dynamics of qubits in a common environment

We use the quantum jump approach to study the entanglement dynamics of a quantum register, which is composed of two or three dipole-dipole coupled two-level atoms, interacting with a common environment. Our investigation of entanglement dynamics reflects that the environment has dual actions on the entanglement of the qubits in the model. While the environment destroys the entanglement induced by the coherent dipole-dipole interactions, it can produce stable entanglement between the qubits prepared initially in a separable state. The analysis shows that it is the entangled decoherence-free states contained as components in the initial state that contribute to the stable entanglement. Our study indicates how the environmental noise produces the entanglement and exposes the interplay of environmental noise and coherent interactions of qubits on the entanglement.     

Entanglement in S states of two-electron quantum dots with Coulomb impurities at the center

We study a system of two Coulombically interacting electrons in an external harmonic potential in the presence of an on-centre Coulomb impurity. Detailed results for the dependencies of the reduced von Neumann entropy on the control parameters of the system are provided for both the ground state and the triplet S states with the lowest energy. Among other features, it is found that in the weak confinement regime the entanglement is strongly affected by the presence of an acceptor impurity.     

Entanglement propagation and typicality of measurements in the quantum Kac ring

We study the time evolution of entanglement in a new quantum version of the Kac ring, where two spin chains become dynamically entangled by quantum gates, which are used instead of the classical markers. The features of the entanglement evolution are best understood by using knowledge about the behavior of an ensemble of classical Kac rings. For instance, the recurrence time of the quantum many-body system is twice the length of the chain and “thermalization” only occurs on time scales much smaller than the dimension of the Hilbert space. The model thus elucidates the relation between the results of measurements in quantum and classical systems: While in classical systems repeated measurements are performed over an ensemble of systems, the corresponding result is obtained by measuring the same quantum system prepared in an appropriate superposition repeatedly.     

Entanglement and quantum phase transition in a mixed-spin Heisenberg chain with single-ion anisotropy

We study the ground-state and thermal entanglement in the mixed-spin (S,s)=(1,1/2) Heisenberg chain with single-ion anisotropy D using exact diagonalization of small clusters. In this system, a quantum phase transition is revealed to occur at the value D=0, which is the bifurcation point for the global ground state; that is, when the single-ion anisotropy energy is positive, the ground state is unique, whereas when it is negative, the ground state becomes doubly degenerate and the system has the ferrimagnetic long-range order. Using the negativity as a measure of entanglement, we find that a pronounced dip in this quantity, taking place just at the bifurcation point, serves to signal the quantum phase transition. Moreover, we show that the single-ion anisotropy helps to improve the characteristic temperatures above which the quantum behavior disappears.     

Dynamics and protection of tripartite quantum correlations in a thermal bath

We study the dynamics and protection of tripartite quantum correlations in terms of genuinely tripartite concurrence, lower bound of concurrence and tripartite geometric quantum discord in a three-qubit system interacting with independent thermal bath. By comparing the dynamics of entanglement with that of quantum discord for initial GHZ state and W state, we find that W state is more robust than GHZ state, and quantum discord performs better than entanglement against the decoherence induced by the thermal bath. When the bath temperature is low, for the initial GHZ state, combining weak measurement and measurement reversal is necessary for a successful protection of quantum correlations. But for the initial W state, the protection depends solely upon the measurement reversal. In addition, the protection cannot usually be realized irrespective of the initial states as the bath temperature increases.     

Entanglement of a General Formalism V-Type Three-Level Atom Interacting with a Single-Mode Field in the Presence of Nonlinearities

We investigate the evolution of the atomic quantum entropy and the atom-field entanglement in a system of a V-configuration three-level atom interacting with a single-mode field with additional forms of nonlinearities of both the field and the intensity-dependent atom-field coupling. With the derivation of the unitary operator within the frame of the dressed state and the exact results for the state of the system we perform a careful investigation of the temporal evolution of the entropy. A factorization of the initial density operator is assumed, considering the field to be initially in a squeezed coherent or binomial state. The effects of the mean photon number, detuning, Kerr-like medium and the intensity-dependent coupling functional on the entropy are analyzed.     

基于纠缠交换的量子信令中继网络模型及仿真

针对多用户量子信令网络的中继模型,本文研究了环境量子噪音对信令远程传输的影响,分析了量子交换原理,在此基础上提出了基于纠缠交换的量子信令中继策略,给出了总线型信令中继网络模型,并对量子信令远程传输成功率进行了仿真研究.结果表明:量子信令远程传输成功率与噪音功率谱密度、传输距离和中继次数三个因素密切相关;当噪音功率谱密度一定时,随着传输距离的增大,信令传输成功率呈准平抛型曲线下降;在噪音功率谱密度和总的传输距离两个因素都确定的情况下,随着中继次数的增加,信令传输成功率显著增大.因此,为了实现量子信令的可靠传输,在背景噪音一定时,必须准确设计传输距离和中继次数.     

基于纠缠交换的量子信令中继网络模型及仿真

针对多用户量子信令网络的中继模型,本文研究了环境量子噪音对信令远程传输的影响,分析了量子交换原理,在此基础上提出了基于纠缠交换的量子信令中继策略,给出了总线型信令中继网络模型,并对量子信令远程传输成功率进行了仿真研究.结果表明:量子信令远程传输成功率与噪音功率谱密度、传输距离和中继次数三个因素密切相关;当噪音功率谱密度一定时,随着传输距离的增大,信令传输成功率呈准平抛型曲线下降;在噪音功率谱密度和总的传输距离两个因素都确定的情况下,随着中继次数的增加,信令传输成功率显著增大.因此,为了实现量子信令的可靠传输,在背景噪音一定时,必须准确设计传输距离和中继次数.     

Entanglement entropy, decoherence, and quantum phase transitions of a dissipative two-level system

The concept of entanglement entropy appears in multiple contexts, from black hole physics to quantum information theory, where it measures the entanglement of quantum states. We investigate the entanglement entropy in a simple model, the spin-boson model, which describes a qubit (two-level system) interacting with a collection of harmonic oscillators that models the environment responsible for decoherence and dissipation. The entanglement entropy allows to make a precise unification between entanglement of the spin with its environment, decoherence, and quantum phase transitions. We derive exact analytical results which are confirmed by Numerical Renormalization Group arguments both for an ohmic and a subohmic bosonic bath. The entanglement entropy obeys universal scalings. We make comparisons with entanglement properties in the quantum Ising model and in the Dicke model. We also emphasize the possibility of measuring this entropy using charge qubits subject to electromagnetic noise; such measurements would provide an empirical proof of the existence of entanglement entropy.     

单光子纠缠态的纠缠转移和量子隐形传态

使用光学分束器和单光子源,利用单光子态和真空态制备出了纠缠单光子态.利用光学分束器作用和单光子探测,实现了三个通讯伙伴之间的纠缠转移.提出了一个关于纠缠单光子态的量子隐形传态方案.在这个方案中,被传送的是一个未知的单光子纠缠态.通讯双方使用的量子信道是两个单光子纠缠态.通过使用分束器作用和对输出态进行光子测量以及在经典信息的帮助下,纠缠转移和量子隐形传态的过程被完成.     

Control of the entanglement between triple quantum dot molecule and its spontaneous emission fields via quantum entropy

The time evolution of the quantum entropy in a coherently driven triple quantum dot molecule is investigated. The entanglement of the quantum dot molecule and its spontaneous emission field is coherently controlled by the gate voltage and the rate of an incoherent pump field. The degree of entanglement between a triple quantum dot molecule and its spontaneous emission fields is decreased by increasing the tunneling parameter.     

两Jaynes-Cummings原子间的纠缠动力学和转移特性

采用两种不同的纠缠度量方法(并发度和负值度),研究了两Jaynes-Cummings原子之间的纠缠演化以及各子系统之间的纠缠转移,分析了两原子之间初始纠缠度对纠缠的影响.结果表明纠缠的幅值依赖于初始纠缠度,而解纠缠时间长度与初始纠缠度无关.制备了两个腔场之间的最大纠缠态,数值分析显示两原子之间的初始纠缠流入了其它各个子系统,导致演化过程中的纠缠突然死亡和纠缠突然产生现象.     

Entanglement of formation for a class of quantum states

Entanglement of formation for a class of higher-dimensional quantum mixed states is studied in terms of a generalized formula of concurrence for N-dimensional quantum systems. As applications, the entanglement of formation for a class of 16×16 density matrices are calculated.     

Entanglement in the Quantum Ising Model

We study the asymptotic scaling of the entanglement of a block of spins for the ground state of the one-dimensional quantum Ising model with transverse field. When the field is sufficiently strong, the entanglement grows at most logarithmically in the number of spins. The proof utilises a transformation to a model of classical probability called the continuum random-cluster model, and is based on a property of the latter model termed ratio weak-mixing. In an intermediate result, we establish an exponentially decaying bound on the operator norm of differences of the reduced density operator. Of special interest is the mathematical rigour of this work, and the fact that the proof applies equally to a large class of disordered interactions.     

Induced entanglement revival and entanglement protection in a two qubit system

A system of two initially entangled qubits interacting with a bosonic environment is considered. The interaction induces a loss of the initial entanglement of the two qubits, and for specific initial states it causes entanglement sudden death. An investigation of the modifications on the entanglement dynamics by a single pulse control field, performed in the two qubit system, shows that the control field can not only protect entangled states against sudden death but also induce a revival of entanglement in the two qubit system.     

Limiting distribution of periodic position measurements of a quantum harmonic oscillator

We consider a particle in harmonic oscillator potential, whose position is periodically measured with an instrument of finite precision. We show that the distribution of the measured positions tends to a limiting distribution when the number of measurements tends to infinity. We derive the expression for the limiting position distribution and validate it with numerical simulation.     

Quantum computers and quantum coherence

If the states of spins in solids can be created, manipulated, and measured at the single-quantum level, an entirely new form of information processing, quantum computing, will be possible. We first give an overview of quantum information processing, showing that the famous Shor speedup of integer factoring is just one of a host of important applications for qubits, including cryptography, counterfeit protection, channel capacity enhancement, distributed computing, and others. We review our proposed spin-quantum dot architecture for a quantum computer, and we indicate a variety of first generation materials, optical, and electrical measurements which should be considered. We analyze the efficiency of a two-dot device as a transmitter of quantum information via the propagation of qubit carriers (i.e. electrons) in a Fermi sea.     

Non-extensive statistical entropy,quantum groups and quantum entanglement

In this work, we explore a new connection between quantum groups and Tsallis entropy through the energy spectrum of a Hamiltonian with S_Uq(2)$S{U}_q\left(2\right)$ symmetry. Identifying the deformation parameter of the entropy with the parameter of deformation of the associated quantum group, we deduce Tsallis entropy for states related to such a system with S_Uq(2)$S{U}_q\left(2\right)$ symmetry and conducted an investigation of quantum entanglement.