Spin Squeezing and Quantum Fisher Information in the Lipkin-Meshkov-Glick Model

We investigate spin squeezing and quantum Fisher information in the Lipkin-Meshkov-Glick model. Approximate analytical expressions and the numerical analysis of spin squeezing and quantum Fisher information are derived. Spin squeezing and quantum Fisher information depend on the strength of the external transverse magnetic field and the anisotropic parameter     

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 and Quantum Fisher Information in Generalized Two-Axis Twisting Model

We investigated spin squeezing and quantum Fisher information in generalized two-axis twisting model; which generalizes the two-axis twisting model including a linear interaction controlled by an external field. In particular, we are interested in the dependence of spin squeezing and quantum Fisher information on the external field. By adopting frozen-spin approximation, we derive the theoretical and numerical results for spin squeezing and quantum Fisher information. Except certain special conditions, the stronger external field induces to stronger squeezing. Spin squeezing parameter and the reciprocal of the mean quantum Fisher information per particle are periodic function; but the external field has not important effect on the period.     

Spin Squeezing and Quantum Fisher Information in One-Axis Twisting Model with Decay

We investigate spin squeezing and the reciprocal of the mean quantum Fisher information per particle (RMQFI) of a collective spin governed by a one-axis twisting Hamiltonian with decay. In particular, we are interesting in the dependence of spin squeezing and RMQFI on the decay rate. The larger decay rate induces to stronger squeezing in certain regime and the larger period of RMQFI.     

Quantum Coherence and Spin Squeezing in Optomechanical System

We investigate quantum coherence and spin squeezing in optomechanical system. We first determine the mean spin direction, the optimally squeezed angle and then calculate the first-order temporal correlation function, the squeezing parameter, which are 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.     

Quantum Discord Behaviors in Two Qubits Spin Squeezing Model with Intrinsic Decoherence

By taking into account the intrinsic decoherence and the external magnetic field, quantum discord(QD) behaviors in two-qubit spin squeezing model are investigated in detail. It is found that the magnitude of quantum discord is strongly dependent on the initial states, the squeezing interaction μ, the magnetic field Ω and the purity r of initial states. With t → ∞, one can obtain the steady quantum discord (SQD) value, the environmental decoherence cannot entirely destroy the quantum correlation. Based on the analysis of the SQD, the conditions about the existence of SQD are obtained with different initial states. Varying the parameters μ, Ω and r not only can weaken the effects of decoherence but also can improve the magnitude of QD and SQD. The effects of the parameters μ and Ω on the QD and SQD display so different and complicated features that one cannot get an uniform law about them, while the values of QD and SQD are improved with increasing r. Properly tuning the parameters μ, Ω and r, one can obtain a larger value of QD or SQD.     

量子混沌系统中的自旋压缩性质

量子信息是21世纪的一门新兴交叉学科,现已经成为世界关注的热门研究领域.近年来,量子计算机的研究正成为大家十分感兴趣的课题.在寻找量子计算的实现方案过程中,量子混沌引起了研究人员的极大关注,因为在量子计算机执行一些量子运算法则的过程中可能产生量子混沌,并可能破坏量子计算机的运算操作条件.近期有关量子纠缠与量子混沌之间的关系已经有所报道,而自旋压缩作为另外一种典型的纯量子效应,是否也与量子混沌之间存在一定关系呢?讨论了量子混沌研究中一个非常典型的QKT模型,研究了量子混沌系统中自旋压缩的性质.通过数值模拟计算,给出了两种不同定义的自旋压缩系数与混沌系数κ之间的变化关系,结果发现在经典相空间中,如果在规则区域占优势的情况下,当初始自旋相干态波包位于椭圆形中心时,随着时间的演化,系统压缩行为表现得非常强;而对于经典相空间中混沌区域占优势的情况下,初始自旋相干态波包同样位于椭圆形中心,则系统的压缩行为表现得非常弱,说明自旋压缩对相应的经典混沌非常敏感.通过比较还发现,采用Wineland等定义的自旋压缩系数比采用Kitagawa和Ueda等定义的自旋压缩系数对经典混沌更敏感一些,从而得出用自旋压缩可以刻画量子混沌的结论.     

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.     

Quantum Coherence and Spin Squeezing in Bose-Einstein Condensate with Josephson-Like Coupling

We consider single-particle coherence and spin squeezing in Bose-Einstein Condensate (BEC) with Josephson-like coupling when the BEC is initially in a coherent spin state. Josephson-like coupling gives vanishing contribution to single-particle coherence and spin squeezing. The stronger squeezing can be achieved by increasing the number of atoms.     

Quantum Entanglement and Spin Squeezing of Two Species Bose-Einstein Condensates

We investigate quantum entanglement and spin squeezing of two species Bose-Einstein condensates. By the rotating-wave approximation, we obtain the effective Hamiltonian and the wave function of the system. It’s shown that more entanglement and squeezing may be achieved by increasing the population difference of particles.     

Steady-State Spin Squeezing Generated in Diamond Nanostructures Coupled to Carbon Nanotubes

International Journal of Theoretical Physics - Spin squeezed state, as an important quantum resource, can be used to implement the high precise measurement beyond the standard quantum limit. Based...     

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.     

Spin Squeezing and Entanglement of Two-Axis Twisting Model

We investigate spin squeezing and entanglement of the two-axis twisting model with the bosonization method, The smaller linear interaction can produce a stronger spin squeezing, the better entanglement and the squeezing and entanglement can maintain a longer time interval. The stronger spin squeezing and the better entanglement can also be achieved by increasing the number of particles.     

Quantum to Classical Transition in a System of Two Coupled Kicked Rotors

We investigate the quantum-classical transition in a system of two coupled kicked rotors. We lind that when the mass of one kicked rotor is much smaller than the other＇s, the influence of the light kicked rotor is still able to make decoherence of the heavy one. This leads to the quantum-classical transition of the heavy kicked rotor. We demonstrate this by two different coupling potentials.     

Various Quantum Correlations in Two-Qubit Two-Axis Spin Squeezing Model in Thermal Equilibrium under an External Magnetic Field

This paper investigates the influence of the spin squeezing parameter γ, the external magnetic field B and the temperature T on the concurrence (C), the quantum discord (QD), and the geometric quantum discord (GQD) in the two-qubit two-axis spin squeezing model in thermal equilibrium under an external magnetic field. The results show that the spin squeezing parameter γ has a positive effect on all three correlations. When the system is in the ground state, the external magnetic field B has a weakening effect on the three types of quantum correlations. Particularly, the spin squeezing parameter can be used to alleviate the destructive effect of the magnetic field on the geometric quantum discord. At a relatively high temperature, the externally applied magnetic field B helps enhance the quantum discord (QD). Further, the quantum discord is more robust than concurrence, and thus is more suitable for use as a quantum resource in information processing.     

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.

     

Controlling Spin Squeezing and Quantum Fisher Information with a Strong External Field

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.     

Phase Coherence and Spin Squeezing in One-Axis Twisting Model with Decay

We investigate phase coherence and spin squeezing of a collective spin governed by one-axis twisting Hamiltonian with decay. Here we are interested in the dependence of phase coherence and spin squeezing parameter on the decay. The analytical expressions of phase coherence and spin squeezing parameter are obtained. The stronger decay can induce a stronger spin squeezing, and the squeezing can maintain a longer time interval. Moreover, more squeezing can be achieved by increasing the number of particles.     

Dense Coding in Two Kinds of Two-Qubit Spin Squeezing Model

International Journal of Theoretical Physics - We investigate the effects of spin squeezing interaction, external magnetic field and temperature on dense coding capacity properties in two kinds of...