Pairwise Quantum Correlations for Superpositions of Dicke States

Pairwise correlation is really an important property for multi-qubit states. For the two-qubit X states extracted from Dicke states and their superposition states, we obtain a compact expression of the quantum discord by numerical check. We then apply the expression to discuss the quantum correlation of the reduced two-qubit states of Dicke states and their superpositions, and the results are compared with those obtained by entanglement of formation, which is a quantum entanglement measure.     

Spin Squeezing in Superpositions of GHZ State and W States

We investigate the spin squeezing in superpositions of a N-qubit GHZ state and two W states. The spin squeezing parameter is determined by the superposition coefficients and the relative phase. It is shown that there is squeezing in the n ₂ direction and there is no squeezing in the n ₁ direction. It is also shown that the state with more number of qubits has smaller spin squeezing parameter.     

相位差与一般相干迭加态压缩特性

研究一般相干态的迭加态|ψ＞=a|β＞ beiψ|mβeiδ＞的k次方压缩特性,结果表明:一般相干态的迭加态和量子涨落的k次方压缩,在m=1时可表为k≠2nπ/δ,此处n是整数;在m≠1时,压缩的次方数可以是偶数也可以是奇数;当δ=π时,无论m[m∈(0.∞)]取何值均存在奇次方压缩,当δ=π/2时,无论m取何值均存在奇次方压缩和偶次方压缩.这说明参数相位差δ对一般相干态的k次方压缩的次方数起关键性的作用.     

相位差与q变形广义相干叠加态的压缩特性

对于q变形的非简谐振子广义相干态的叠加态β〉+eiφβeiδ〉,其量子涨落的可能高阶压缩阶数可以表示为k≠2πn/δ,这里n是整数.当δ=π时,压缩阶数不能是偶数即只能是奇数,这正是q变形非简谐振子广义奇偶相干态的结果.由此表明参数相位差δ对决定q变形的非简谐振子广义相干态叠加态的高阶压缩阶数起决定性作用.     

Spin Squeezing in Superposition of Four-Qubit Symmetric State and W States

We investigate the entanglement property of a four-qubit state superposed by a four-qubit GHZ state and a symmetric state with spin squeezing. Numerically and analytic solutions for spin squeezing parameters are given. The squeezed parameters depend on the superpose coefficients and the relative phases. It is shown that there is no spin squeezing at n ₁ direction and it occurs at n ₂ direction under some conditions.     

Spin Squeezing of Superposition

We investigate the mean spin direction, length of mean spin and spin squeezing in a general superposition of a GHZ state and a W state. It is shown that the mean spin direction and spin squeezing parameter are determined by the coefficients of superposition and the relative phase between the GHZ state and the W state; while the length of mean spin is only determined by coefficients of superposition.     

Spin Squeezing in Multi-Qubit State

We investigate the spin squeezing of a multi-qubit state superposed by four Dicke state. Numerically solutions for optimally squeezed angle and spin squeezing parameter are derived. Optimally squeezed angle and spin squeezing parameter are determined by the superposition coefficients and the relative phase.     

Spin Squeezing and Fixed-Point Bifurcation

We study spin squeezing and classical bifurcation in a nonlinear bipartite system. We show that the spin squeezing can be associated with a fixed-point bifurcation in the classical dynamics, namely, it acts as an indicator of the classical bifurcation. For the ground state of a system with coupled giant spins, we find that the spin squeezing achieves its minimum value near the bifurcation point. We also study the dynamics of the spin squeezing, for an initial state corresponding to one of the fixed point, we find that in the stable regime, the spin squeezing exhibits periodic oscillation and always persists except at some fixed times, while in the unstable regime, the periodic oscillation phenomenon disappears and the spin squeezing survives for a short time. Finally, we show that the mean spin squeezing, which is defined to be averaged over time, attains its minimum value near the bifurcation point.     

Spin Squeezing and Fixed-Point Bifurcation

We study spin squeezing and classical bifurcation in a nonlinear bipartite system. We show that the spin squeezing can be associated with a fixed-point bifurcation in the classical dynamics, namely, it acts as an indicator of the classical bifurcation. For the ground state of a system with coupled giant spins, we find that the spin squeezing achieves its minimum value near the bifurcation point. We also study the dynamics of the spin squeezing, for an initial state corresponding to one of the fixed point, we find that in the stable regime, the spin squeezing exhibits periodic oscillation and always persists except at some fixed times, while in the unstable regime, the periodic oscillation phenomenon disappears and the spin squeezing survives for a short time. Finally, we show that the mean spin squeezing, which is defined to be averaged over time, attains its minimum value near the bifurcation point.     

Spin Squeezing of One-Axis Twisting Model

We investigate spin squeezing of the one-axis twisting model. By using short-time approximation solutions of the angular momentum operators, we analytically and numerically calculate the spin squeezing parameter. It is shown that smaller linear interaction can produce a stronger spin squeezing and maintain a longer time interval. It is also shown that the stronger spin squeezing can be achieved by increasing the number of particles.     

Spin Squeezing of 4-Qubit State

We investigate the spin squeezing of a 4-qubit state, which is superposed by a 4-qubit GHZ state and two W states with a relative phase. Numerically solution for spin squeezing parameter is given. It is shown that the parameter depends on the superposition coefficients and the relative phase. It is shown that spin squeezing exists over a relatively long time with increasing superposition coefficient γ and the smaller the value of relative phase is, the longer the time of existing spin squeezing.     

Entanglement of Superpositions of Orthogonal Maximally Entangled States

We study the entanglement properties of the superposed state of orthogonal maximally entangled states. It is shown that the superposed state is maximally entangled and the superposed state is separable. The relation between the superposed state and the mutually unbiased state is discussed.     

Spin Squeezing Base on Four Wave Mixing

International Journal of Theoretical Physics - We investigate how to prepare spin squeezing state base on four wave mixing. By the bosonization method, we analytically and numerically calculate the...     

Entanglement Teleportation via Two-Qubit Spin Squeezing Model

Entanglement teleportation via two spins coupled to each other by one axis twisting spin squeezing interaction is investigated. We mainly concentrate on the properties of the channel entanglement, the output entanglement and the teleportation fidelity. It shows that the output entanglement increases linearly with increasing the value of the input entanglement. With the increasing of T, the teleportated entanglement increases sharply from zero to a maximum value and then decreases slowly to zero when the temperature is improved to one threshold value of T _c , and the threshold value of T _c increases with the increasing of the input entanglement. When enlarging the external magnetic, F _a firstly decreases quickly to a minimum value as the critical magnetic field Ω _c is reached, then it increases abruptly to a maximum value and finally it will be a certain value. Besides, the critical external magnetic Ω _c increases when μ is larger. For Ω<Ω _c the value of the average fidelity increases with the increasing of μ, but for Ω>Ω _c the value of it decreases from a maximum value. The influence of T on the average fidelity is similar with μ.     

Coherently Controlling Spin Squeezing in Optomechanical System

We investigate spin squeezing in optomechanical system. We first derive the mean spin direction, the optimally squeezed angle and then calculate the spin squeezing parameter, which is independent of the frequency of the cavity field. The lager coupling strength more rapidly generates spin squeezed state, but the corresponding spin squeezed state maintains shorter time interval.     

Teleportation of Unknown Superpositions of Collective Atomic Coherent States

We propose a scheme to teleport an unknown superposition of two atomic coherent states with different phases.Our scheme is based on resonant and dispersive atom-field interaction.Our scheme provides a possibility of teleporting macroscopic superposition states of many atoms first time.     

Spin Squeezing and Quantum Fisher Information for Mixed Hamiltonian Model

We investigate spin squeezing and the mean quantum Fisher information per particle χ ² for mixed Hamiltonian model. By adopting frozen-spin approximation, we first derive a analytical expression of spin squeezing parameter ξ ² and then numerically calculate ξ ² and χ ². It is shown that most of the time the spin squeezing appears alternatively in the x and y directions, but it cannot appear simultaneously in the two directions. It is also shown that the smaller external field strength induces better entanglement and larger period.     

Spin Squeezing of the Generalized One-Axis Twisting Model

We investigate spin squeezing of the generalized one-axis twisting model. By using the perturbation techniques, we analytically and numerically calculate spin squeezing parameter defined by Kitagawa and Ueda. It shows that more squeezing may be achieved by strengthening linear interactions and increasing the total number of particles.