利用两能级原子与腔场的相互作用转移纠缠

分析了大失谐情况下一个两能级原子和相干态腔场相互作用的特点;讨论了利用两能级原子和相干态腔场相互作用制备纠缠相干态的方法;提出了一个关于纠缠相干态的纠缠转移的方案。在这个纠缠转移的方案里,通讯伙伴之间使用的量子信道是由两个振幅相同位相相反的相干态构成的纠缠态。通过使用两能级原子和腔肠相干态的相互作用和两模正交态测量并在经典信息的帮助下完成了三个通讯伙伴之间的纠缠转移。随着近来腔量子电动力学技术的发展,这个方案是能够被实行的。     

Thermal entanglement in the XYZ model for a two-qutrit system

The negativity as a measure of thermal entanglement was studied for a two-qutrit spin-1 anisotropic Heisenberg XYZ chain with Dzyaloshinskii-Moriya (DM) interaction in a homogeneous magnetic field in detail. An analytical expression was found for the negativity and then the thermal variations of negativity were investigated in full detail for given values of the DM interaction parameter D_z, the external magnetic field B, the measure of the anisotropy on the xy-plane γ = (J_x − J_y)/(J_x + J_y), a parameter J = (J_x + J_y)/2 and the bilinear interaction parameter J_z along the z-axis.     

Kerr nonlinear switch based on ultra-compact photonic crystal directional coupler

In this paper, an all optical switch based on nonlinear photonic crystal directional coupler has been simulated and analyzed by the finite difference time domain (FDTD) method. The lunched pump signal increases the refractive indices of the central row of the coupler, due to nonlinear Kerr effect, hence the coupler works in the nonlinear conditions and lightwave guides to the other output port. We have tried to increase the coupling efficiency and reduce the required power in the nonlinear status by optimizing the bends structure and increasing the interaction between dielectric and lightwave signal. Therefore, the input signal beam can be controlled to be exchanged between two output ports to earn the highest output power ratio and the smallest amount of power required for nonlinear performance, the physical length of the coupler is determined to be 20a, where a is the structure lattice constant.     

Thermal atom–atom entanglement in a bichromatic Kerr nonlinear coupler

In this paper thermal entanglement between two identical two-level atoms within a bichromatic cavity including Kerr nonlinear coupler is investigated. In this study, besides atom–field interaction, the field–field (via linear and Kerr-type couplings) and atomic dipole–dipole interactions are also included. It is also assumed that the cavity is held at a temperature T$T$, so that all atom–photon states with probabilities defined by Boltzmann factor are present. Using a canonical transformation, the presented model is converted to a generalized form of Jaynes–Cummings model. After introducing Casimir operators of the system, it is shown that the Hamiltonian representation is block-diagonal. Diagonalizing each block, the thermal (Gibb’s) density matrix, written in the bases of total Hamiltonian, is obtained. The reduced atomic density matrix and consequently the concurrence, as a measure of entanglement, are obtained by partial tracing of thermal density matrix over the bichromatic photonic states. The concurrence vanishes at zero temperature, indicating that the ground state is separable, exhibits a maximal at a critical temperature and terminates at a finite temperature. The influences of coupler nonlinearities and dipole–dipole coupling on the thermal atom–atom entanglement are also addressed in detail.     

Analysis of a gained nonlinear directional coupler pulse switch

We have investigated the switching performance of a gained (nonlinear directional coupler) NLDC switch in the presence of both 2nd and 4th order gain nonlinearities. In this system, we have achieved a nearly complete pulse switching at half beat length of the coupler which implies about 40% reduction in the switching length as compared to the switching length reported in Trillo et al. (1988). We have shown that at the half beat length the output energy of each branch is equal to that of the input energy and hence the gained NLDC switches have ability to be cascaded. Our initial investigations reveal that this gained NLDC switch has remarkable performance and potential to be used in ultra-fast optical communication systems.     

Vector soliton switching in a fiber nonlinear directional coupler

This paper reports a detailed numerical study of soliton switching in a high as well as low birefringent nonlinear coupler. It is shown that by controlling the polarization angle one can have nearly 100% transmission with excellent switching characteristics. It is shown that soliton remains stable during its propagation inside the coupler. However it is observed that high birefringent coupler exhibits relatively better soliton stability. We show that the coupler could be used as a soliton switch even at an input peak power less than the critical power, the power at which 50-50 power sharing takes place between the two cores, just by a judicious choice of the polarization angle.     

Chirp controllable all-optical router in a nonlinear directional coupler

We analyzed the evolution of chirped optical pulses in a nonlinear directional coupler by coupled nonlinear Schrödinger equations. It is shown that an optical pulses launched into one channel can couple out from either of the two channels of the nonlinear directional coupler, with the routing determined by the chirp of the pulse. This is different from the previous routing mechanism of the nonlinear directional coupler where routing is determined by the peak power of the pulse. Based on the chirp-dependent feature, all-optical router and switch could be implemented by the pulse chirp modulation.     

Two-mode excited entangled coherent states and their entanglement properties

This paper introduces two types of two-mode excited entangled coherent states (TMEECSs) |Ψ_±(α,m,n)>, studies their entanglement characteristics, and investigates the influence of photon excitations on quantum entanglement. It shows that for the state |Ψ₊(α,m,m)> the two-mode photon excitations affect seriously entanglement character while the state |Ψ_-(α,m,m)> is always a maximally entangled state, and shows how such states can be produced by using cavity quantum electrodynamics and quantum measurements. It finds that the entanglement amount of the TMEECSs is larger than that of the single-mode excited entangled coherent states with the same photon excitation number.     

Intrinsic decoherence of entanglement of a single quantized field interacting with a two-level atom

How the mean photon number, the probability of excited state and intrinsic decoherenee coefficient in-fluence the time evolution of entanglement is unknown, when a single-mode quantized optic field and a two-levelatom coupling system is governed by Milburn equation. The Jaynes-Cummings model is considered. A lowerhound of concurrence is proposed to calculate the entanglement. Simulation results indicate that the entanglementof system increases following the increasing of intrinsic decoherence coefficient or the decreasing of the mean photon number. Besides that, the entanglement of system decreases, while the probability of exited state increasesfrom 0 to 0.1, and increases, while the probability of exited state increases from 0.1 to 1.     

Intrinsic decoherence of entanglement of a single quantized field interacting with atwo-level atom

How the mean photon number, the probability of excited state and intrinsic decoherence coefficient influence the time evolution of entanglement is unknown, when a single-mode quantized optic field and a two-level atom coupling system is governed by Milburn equation. The Jaynes-Cummings model is considered. A lower bound of concurrence is proposed to calculate the entanglement. Simulation results indicate that the entanglement of system increases following the increasing of intrinsic decoherence coefficient or the decreasing of the mean photon number. Besides that, the entanglement of system decreases, while the probability of exited state increases from 0 to 0.1, and increases, while the probability of exited state increases from 0.1 to 1.     

单模光场与二能级原子的纠缠的内禀退相干

在一个考虑了内禀退相干的单模光场与二能级原子的耦合系统中,平均光子数,激发态的概率和退相干系数如何影响纠缠随时间演化还并不清楚.Jaynes-Cummings模型被用来描述光场与原子间相互作用.用concurrence下限来计算纠缠度.模拟的结果表明纠缠度随着退相干系数的增加或平均光子数的减少而增加.当激发态的概率在0至0.1之间,纠缠度随着的概率增加而减少;当激发态的概率在0.1至1之间,纠缠度随着的概率增加而增加.无论以上参数取何值,纠缠度都随着时间而减少.     

六光子量子纠缠态的制备

我们提出了一个完全基于量子擦除概念上的六光子纠缠态的制备方案。首先，提出了利用一组四纠缠光子来制备六光子纠缠态的方案。接着，运用同样的技术，通过五粒子纠缠和Bell态的相互作用来制备六光子纠缠态。我们的实验方案可以应用于以测量为基础的量子计算和多体量子通信领域的研究。我们发现该方案成功的概率取决于纠缠态的系数。     

Design and simulation of a switch based on nonlinear directional coupler

In this paper, we have analyzed a nonlinear photonic crystal directional coupler by the finite difference time domain method. We have tried to increase the coupling efficiency and also reduce the coupling length in linear and nonlinear states in this device. In this coupler, refractive index of the rods of the central row is tuned by input signal power due to nonlinear Kerr effect; therefore, input signal beam can be controlled so as to be exchanged between two output ports. Physical length is chosen to be 24a so as to have the highest output power ratio and also the smallest amount of power required for nonlinear performance.     

陆续通过一个双模腔的两原子之间纠缠的突然产生和调控

通过计算并发度研究了陆续通过一个双模腔的两个原子之间的纠缠动力学特性, 讨论了第一个原子的相干性以及腔场初始纠缠度对两原子之间纠缠的影响. 结果表明系统在一定条件下可以出现两原子之间纠缠突然产生现象, 两原子之间产生纠缠的最大值依赖于双模腔场初始纠缠度; 并且可以通过改变原子的振幅来控制产生纠缠的阈值时间和纠缠的最大值, 理论上提供了一种调控纠缠的方式.     

Von Neumann entropy and entropy squeezing of a two-level atom and the superposition of squeezed displaced fock states

The work presents some aspects of studying the two-level atom in a Kerr media interacting with a single-mode electromagnetic field, which is the superposition of squeezed displaced Fock states. We discuss the effect of the new field on quantum entropy and the entanglement of the atom-field system in the two-photon process. The exact results are employed to perform a careful investigation of the temporal entropy evolution. The position and momentum entropy squeezing of the optical field are investigated. The general conclusions obtained are illustrated by numerical results.     

Tunable entanglement sudden death and three-partite entanglement in Tavis-Cummings model with an added nonlinear kerr-like medium

We investigate the entanglement dynamics and the phenomenon of entanglement sudden death in the two-photon Tavis-Cummings model with an added nonlinear kerr-like medium. It is shown that the phenomenon of entanglement sudden death can be controlled by the nonlinear kerr-like medium. Furthermore, the influence of dipole-dipole interaction on entanglement between atoms is also discussed. It is found that the phenomenon of entanglement sudden death can be weakened by the dipole-dipole interaction. Finally, we investigate the influence of kerr medium and dipole-dipole interaction on the three-partite entanglement of the system by making use of the state preparation fidelity.     

Remote atom entanglement in a fibre-connected three-atom system

An Ising-type atom-atom interaction is obtained in a fibre-connected three-atom system. The interaction is effective when △ ≈γ0 〉〉 g. The preparations of remote two-atom and three-atom entanglements governed by this interaction are discussed in a specific parameter region. The overall two-atom entanglement is very small because of the existence of the third atom. However, the three-atom entanglement can reach a maximum very close to 1.     

Three-body entanglement induced by spontaneous emission in a three two-level atoms system

We study three-body entanglement induced by spontaneous emission in a three two-level atoms system by using the entanglement tensor approach. The results show that the amount of entanglement is strongly dependent on the initial state of the system and the species of atoms. The three-body entanglement is the result of the coherent superposition of the two-body entanglements. The larger the two-body entanglement is, the stronger the three-body entanglement is. On the other hand, if there exists a great difference in three two-body entanglement measures, the three-body entanglement is very weak. We also find that the maximum of the two-body entanglement obtained with nonidentical atoms is greater than that obtained with identical atoms via adjusting the difference in atomic frequency.     

非线性一维光学晶格链中的热纠缠

本文对非线性光学晶格链中自旋为1的原子，在两原子以及多原子链中紧邻原子之间的热纠缠问题作了详尽的研究，结果表明非线性耦合和线性耦合系数之间的关系对界定热纠缠是否存在起到关键作用，由于不同的原子这两种耦合对应着不同的关系，这一研究揭示在那些种类的原子中存在热纠缠；这一结果对两粒子以及多粒子体系都是成立的。在多粒子情况下临界温度以及热纠缠仅有少许下降。