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 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.     

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.     

Global and Local Spin Squeezing in Coupled Quantum Kicked Tops Model

We investigate the global and the local spin squeezing in a weakly coupled quantum kicked tops system. Two different situations are considered: (i) N=1 and (ii) N=30 for each subsystem, corresponding to quantum and classical cases, respectively. In the first case, since the two subsystems have no spin squeezing, the global squeezing completely originates from quantum correlations. For the second one, the global spin squeezing is enhanced over the local one. Due to the chaotic nature of the system, the spin squeezing is sensitive to the initial state. In chaotic region, the squeezing vanished time is much shorter than that in the regular region.     

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.     

Spin Squeezing in Mixed Model of the One-Axis Twisting and Two-Axis Counter-Twisting with Decay

We investigate spin squeezing of a collective spin system governed by mixed Hamiltonian of the one-axis twisting and two-axis counter-twisting with decay. The analytic solutions of the optimally squeezed angle and the spin squeezing parameter are derived. It is shown that the two-axis counter-twisting Hamiltonian induces stronger spin squeezing than the one-axis twisting one. It is also shown that the decay rate has no effect on the size of the squeezing parameter, it only affects the period of squeezing parameter.     

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 Fisher Information in the Generalized One-axis Twisting Model

We investigate the quantum Fisher information (QFI) of symmetric states for spin-s particles. We derive the maximal QFI, and find that quantum spin correlations are essential ingredients of the maximal QFI. We make applications to the generalized one-axis twisting model. The results show that the redistributions of uncertainties on the basis of the quantum correlations in the multiqubit system are useful for sub-shot-noise phase sensitivity. Furthermore, for high-spin (s>1/2) composite systems, we find a sufficient criterion for entanglement.     

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.     

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.     

Quantum Fisher Information and Uncertainty Relations

It is well known that the Cramér–Rao inequality places a lower bound for quantum Fisher information in terms of the variance of any quantum measurement. We establish an upper bound for quantum Fisher information of a parameterized family of density operators in terms of the variance of the generator. These two bounds together yield a generalization of the Heisenberg uncertainty relations from statistical estimation perspective.     

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 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.     

Protecting Quantum Fisher Information in Correlated Quantum Channels

Quantum Fisher information (QFI) has potential applications in quantum metrology tasks. QFI is investigated when the consecutive actions of a quantum channel on the sequence of qubits have partial classical correlations. The results showed that while the decoherence effect is detrimental to QFI, effects of such classical correlations on QFI are channel-dependent. For the Bell-type probe states, the classical correlations on consecutive actions of the depolarizing and phase flip channels can be harnessed to improve QFI, while the classical correlations in the bit flip and bit-phase flip channels induce a slight decrease of QFI. For a more general parameterization form of the probe states, the advantage of using the initial correlated system on improving QFI can also remain in a wide regime of the correlated quantum channels.     

Wave-Particle Duality via Quantum Fisher Information

Quantum Fisher information(QFI) plays an important role in quantum metrology, placing the ultimate limit to how precise we can estimate some unknown parameter and thus quantifying how much information we can extract. We observe that both the wave and particle properties within a Mach–Zehnder interferometer can naturally be quantified by QFI. Firstly, the particle property can be quantified by how well one can estimate the a priori probability of the path taken by the particle within the interfer...     

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.

     

Learning the Effective Spin Hamiltonian of a Quantum Magnet

To understand the intriguing many-body states and effects in the correlated quantum materials,inference of the microscopic effective Hamiltonian from experiments constitutes an important yet very challenging inverse problem.Here we propose an unbiased and efficient approach learning the effective Hamiltonian through the many-body analysis of the measured thermal data.Our approach combines the strategies including the automatic gradient and Bayesian optimization with the thermodynamics many-body solvers including the exact diagonalization and the tensor renormalization group methods.We showcase the accuracy and powerfulness of the Hamiltonian learning by applying it firstly to the thermal data generated from a given spin model,and then to realistic experimental data measured in the spin-chain compound copper nitrate and triangular-lattice magnet TmMgGaO_4.The present automatic approach constitutes a unified framework of many-body thermal data analysis in the studies of quantum magnets and strongly correlated materials in general.