Quantum entanglement and quantum nonlocality for N-photon entangled states

Quantum entanglement and quantum nonlocality of N-photon entangled states |\psi_{N m}\rangle =C_m[\cos\gamma|N-m\rangle₁|m\rangle₂ +\e^{\i\θ_m}\sin\gamma|m\rangle₁|N-m\rangle₂] and their superpositions are studied. We point out that the relative phase θ_m affects the quantum nonlocality but not the quantum entanglement for the state |\psi_{N m}\rangle. We show that quantum nonlocality can be controlled and manipulated by adjusting the state parameters of |\psi_{N m}\rangle, superposition coefficients, and the azimuthal angles of the Bell operator. We also show that the violation of the Bell inequality can reach its maximal value under certain conditions. It is found that quantum superpositions based on |\psi_{N m}\rangle can increase the amount of entanglement, and give more ways to reach the maximal violation of the Bell inequality.     

Quantum nonlocality for two-mode correlated states based on generalized pseudospin operators

In this Letter, we investigate the quantum nonlocality of two-mode correlated states. We find that the pseudospin formalism [Z.B. Chen, J.W. Pan, G. Hou, Y.D. Zhang, Phys. Rev. Lett. 88 (2002) 040406] generally fails to depict the nonlocality of the states when the photon number difference between the two modes is odd. The formalism is then generalized such that the nonlocality of a two-mode correlated state can be well revealed without regard to the difference. Later we consider the nonlocality of the two-mode intelligent SU(1,1) states in the generalized formalism and compare our results with the entanglement of the corresponding states.     

基于双模压缩态的量子投票协议

提出了一种基于双模压缩态的基本量子投票协议,该协议通过随机选择信号加载的方式,充分利用量子信号测不确定性原理实现了分布式投票系统.并在此基础上分析可能遇到的攻击.双模压缩态的模间关联性保证了该方案的安全性. 关键词： 量子投票协议 双模压缩态 不确定性原理     

Preparation of entangled squeezed states and quantification of their entanglement

We propose a scheme for generating bipartite and multipartite entangled squeezed states via the Jaynes－Cummings model with large detuning. Bipartite entanglement of these entangled states is quantified by the concurrence. We also use the N-tangle to compute multipartite entanglement of these multipartite entangled squeezed states. Finally we discuss two limiting cases which arise from r→∞ and r→0, in which the multipartite entangled squeezed state reduces correspondingly into an N-qubit Greenberger－Horne－Zeilinger state and an N-qubit W state.     

Gaussian entanglement of symmetric two-mode Gaussian states

A Gaussian degree of entanglement for a symmetric two-mode Gaussian state can be defined as its distance to the set of all separable two-mode Gaussian states. The principal property that enables us to evaluate both Bures distance and relative entropy between symmetric two-mode Gaussian states is the diagonalization of their covariance matrices under the same beam-splitter transformation. The multiplicativity property of the Uhlmann fidelity and the additivity of the relative entropy allow one to finally deal with a single-mode optimization problem in both cases. We find that only the Bures-distance Gaussian entanglement is consistent with the exact entanglement of formation.     

Relative entropy as a measure of entanglement for Gaussian states

In this paper, we derive an explicit analytic expression of the relative entropy between two general Gaussian states. In the restriction of the set for Gaussian states and with the help of relative entropy formula and Peres--Simon separability criterion, one can conveniently obtain the relative entropy entanglement for Gaussian states. As an example, the relative entanglement for a two-mode squeezed thermal state has been obtained.     

基于双模压缩信道的双模高斯态量子隐形传态

研究了双模高斯态的两个模经由两个双模压缩量子信道的隐形传态.结果表明，当输入态有纠缠时，若要输出态的纠缠不为零，量子信道的纠缠必须大于一确定值，其大小在输入态为纯态时依赖于输入态的纠缠度；在输入态为混合态时不仅与输入态的纠缠有关，还依赖于输入态的整体纯度. 关键词： 量子隐形传态 量子纠缠 双模高斯态     

Photon statistical properties of photon-added two-mode squeezed coherent states

We investigate photon statistical properties of the multiple-photon-added two-mode squeezed coherent states(PATMSCS).We find that the photon statistical properties are sensitive to the compound phase involved in the TMSCS.Our numerical analyses show that the photon addition can enhance the cross-correlation and anti-bunching effects of the PATMSCS.Compared with that of the TMSCS,the photon number distribution of the PA-TMSCS is modulated by a factor that is a monotonically increasing function of the numbers of adding photons to each mode;further,that the photon addition essentially shifts the photon number distribution.     

关联噪音通道中连续变量高斯态的纠缠动力学

本文研究了连续变量的双模压缩真空态在关联噪音通道中的纠缠演化. 研究发现此类关联环境能有效的抑止系统的纠缠的衰减. 而且系统在关联环境中的纠缠时间与环境的关联特性密切相关.     

Effects of photon addition on quantum nonlocality of squeezed entangled coherent states

Effects of photon addition on quantum nonlocality of squeezed entangled coherent states for Bell-inequality tests are studied theoretically. By utilizing the method of photon-parity measurement, it is found that photon addition can always increase the degrees of Bell violations within a certain parameter range. A possible scheme for generating photon-added squeezed entangled coherent states is proposed.     

Effects of photon addition on the quantum nonlocality of squeezed entangled-coherent states

Effects of photon addition on the quantum nonlocality of squeezed entangled coherent states for Bell-inequality tests are studied theoretically. By utilizing the method of photon-parity measurement, it is found that photon addition can always increase the degrees of Bell violations within a certain parameter range. A possible scheme for generating photon-added squeezed entangled coherent states is proposed.     

压缩真空态中原子与场的量子纠缠和量子discord

在压缩真空库中,研究了原子与场的相互作用下二比特体系的量子discord和量子纠缠的动力学行为.重点分析了原子的初始态系数和压缩真空库的压缩系数对量子纠缠和量子discord的影响.通过数值分析我们得到,随着原子的初始态系数和压缩真空库的压缩系数值的增加,量子纠缠和量子discord都会减小,但量子纠缠比量子discord减小的更快一些.最后研究了在原子的初始态系数和压缩真空库的压缩系数的值相同的情况下,量子discord比量子纠缠存在的时间更长些.     

Relativistic motion on Gaussian quantum steering for two-mode localized Gaussian states

Realistic quantum systems always exhibit gravitational and relativistic features. In this paper, we investigate the properties of Gaussian steering and its asymmetry by the localized two-mode Gaussian quantum states, instead of the traditional single-mode approximation method in the relativistic setting. We find that the one-side Gaussian quantum steering will monotonically decrease with increasing observers of acceleration. Meanwhile, our results also reveal the interesting behavior of the Gaussian steering asymmetry, which increases for a specific range of accelerated parameter and then gradually approaches to a finite value. Such finding is well consistent and explained by the well-known Unruh effect, which could significantly destroy the one-side Gaussian quantum steering. Finally, our results could also be applied to the dynamical studies of Gaussian steering between the Earth and satellites, since the effects of acceleration are equal to the effects of gravity according to the equivalence principle.     

The entanglement properties of non-Gaussian states prepared by photon subtraction from two-mode squeezed thermal states

We analyze the entanglement condition of a new kind of non-Gaussian quantum state, which is prepared by photon number subtraction from the two mode squeezed thermal state. Fock space criterion and Shchukin-Vogel criterion are applied, and they give the same condition which is also that of the input state.     

广义GHZ态的纠缠与非定域性

对GHZ态与W态叠加的三粒子广义GHZ态,用Svetlichny不等式,给出了其非定域性的解析表达式.用数值计算方法讨论了纠缠与非定域性之间的关系. 关键词： 广义GHZ态 剩余纠缠 Svetlichny不等式 非定域性     

三模高斯态光场非局域性的增强

提出了增强三模高斯态光场非局域性的方案. 结果表明：通过一个IPS（inconlusive photon subtraction）过程的非高斯操作,可以将三模高斯态退变为非高斯态,利用Bell不等式检测发现对于较弱的输入高斯态非局域性能够得到加强. 关键词： 非局域性 三模高斯态     

Quantum optical interferometry via general photon-subtracted two-mode squeezed states

We investigate the sensitivity of phase estimation in a Mach–Zehnder interferometer with photon-subtracted twomode squeezed vacuum states.Our results show that, for given initial squeezing parameter, both symmetric and asymmetric photon subtractions can further improve the quantum Cramér–Rao bound(i.e., the ultimate phase sensitivity), especially for single-mode photon subtraction.On the other hand, the quantum Cramér–Rao bound can be reached by parity detection for symmetric photon-subtracted two-mode squeezed vacuum states at particular values of the phase shift, but it is not valid for asymmetric photon-subtracted two-mode squeezed vacuum states.In addition, compared with the two-mode squeezed vacuum state, the phase sensitivity via parity detection with asymmetric photon-subtracted two-mode squeezed vacuum states will be getting worse.Thus, parity detection may not always be the optimal detection scheme for nonclassical states of light when they are considered as the interferometer states.