有限深势阱下极性晶体膜量子比特及其声子效应

采用Peaker变分法,研究有限深势阱下极性晶体膜中量子比特及其声子效应.晶体膜中这样的二能级体系可作为一个量子比特.当膜中电子处于基态和第一激发态的叠加态时,电子的概率密度在空间作周期性震荡,得出振荡周期随耦合强度的增加而减小,随膜厚的增加而增大.     

原子玻色-爱因斯坦凝聚体对光场压缩特性的影响

运用全量子理论,研究了单模压缩相干态光场与[I]型三能级原子玻色-爱因斯坦凝聚体(BEC)相互作用系统中光场的压缩特性.结果表明:光场在与原子BEC相互作用过程中呈现周期性压缩,BEC原子能级的初始占据率对光场的压缩特性具有重要影响,光场压缩的振荡周期与光场圆频率有关.当初始时刻原子全部处于基态时,光场压缩呈现规则的周期性振荡.而当初始时刻激发态上有原子占据时,光场压缩的时间演化曲线产生明显的畸变,且这种畸变随初始时刻激发态原子占据率的增大而增大.激发态原子占据率还对光场的最大压缩深度有一定的影响,激发态原子占据率较大时,光场的最大压缩深度也较大.     

玻色-爱因斯坦凝聚体系中的混沌隧穿行为

研究了双势阱玻色-爱因斯坦凝聚(BEC)系统在外加周期调制下的混沌相变过程,着重讨论了混沌现象对BEC系统隧穿的影响.当外加调制频率与系统固有频率达到共振时,相平面会出现不稳定现象,即混沌现象.在量子情况下,研究了系统的Husimi函数随时间的演化.研究发现：当混沌现象出现时,系统中粒子间相互作用增大,使得混沌区域扩大,进而引起混沌辅助隧穿程度的变化. 关键词： 玻色-爱因斯坦凝聚 混沌 隧穿     

双势阱玻色-爱因斯坦凝聚体系的自俘获现象及其周期调制效应

研究了双势阱玻色-爱因斯坦凝聚体系(BEC)的自俘获现象(self-trapping). 在平均场近似下通过相平面(phase space)分析的方法研究了两种自俘获的机理：1)势阱中的粒子数在平衡位置附近振动，而相对相位随时间单调变化(running-phase); 2) 势阱中的粒子数和相对相位都在平衡点附近振动. 研究了周期调制场对自俘获现象的影响，发现发生自俘获现象的相变参数能够被周期场非常有效的调制，从而在弱相互作用BEC体系中也可以观察到自俘获现象. 还研究了多体量子涨落对自俘获现象的影响，讨论了在现有的实验条件下对凝聚体自俘获现象进行观察和周期调制. 关键词： 玻色-爱因斯坦凝聚 自俘获 双势阱 周期调制     

三维介观超导环的涡旋结构

在Ginzburg-Landan理论的框架下, 运用有限差分法研究了在圆环电流产生磁场下的介观超导圆环内的涡旋结构, 讨论了超导圆环尺寸和不同空间分布的磁场对涡旋形成的影响, 得到在一般超导圆环体内的基态多是巨涡旋态、而多涡旋态多以激发态形式存在的结论, 说明磁场一般从超导圆环的环孔穿过, 而很难穿过超导圆环体.     

一维有限深方势阱中的束缚态

主要对量子力学一维有限深方势阱中运动粒子的束缚态存在条件进行讨论.通过求解定态薛定谔方程,得到粒子运动满足的超越方程,借助于Mathematica软件求解该超越方程得到粒子的能级结构,对粒子束缚态存在条件进行分析.结果表明,对于在一维有限深对称方势阱中运动的粒子,总会有束缚态的存在,和势阱的宽度、深度及粒子的质量无关.而在半壁有限深势阱中运动的粒子,出现束缚态是有条件的.并且分别给出了在两种势阱中粒子存在多个束缚态需要满足的条件.     

具有局部畸变的常规势阱中玻色-爱因斯坦凝聚孤子的演化特性

运用数值模拟方法解Gross—Pitaevskii方程,研究了具有局部畸变（impurity）的常规势阱中玻色-爱因斯坦凝聚（BEC）孤子的演化特性,发现势阱的局部畸变对BEC孤子随时间演化的特性有重要影响,并且其影响与畸变的强度、势阱的幅度和势阱的类型有关。     

二维激子极化激元凝聚中涡旋叠加态稳态及动力学特性研究

利用分步Crank-Nicolson方案的虚时和实时有限差分方法求解耗散系统的Gross-Pitaevskii(GP)方程,研究了二维激子极化激元凝聚(exciton-polariton condensates)体系中正反涡旋叠加态的稳态结构并直观地验证这种稳态结构在半导体微腔旋转下的稳定性和动力学特性.通过虚时和实时演化相结合的方法求解出几种角动量情况下所对应的稳定涡旋叠加态.然后利用实时演化方法研究在半导体微腔旋转的情况下,正反涡旋叠加态的稳定性及其旋转角速率和半导体微腔旋转角速率之间的定量关系.最后研究了单涡旋态在半导体微腔旋转时形成涡旋阵列的动力学过程,并给出了泵浦光宽度和增益项对涡旋阵列结构的影响.研究表明,系统的泵浦,损耗和增益对稳定性和动力学特性有重要影响.     

有限深势阱下抛物量子线量子比特及其声子效应

采用Peaker变分法,研究了在有限深势阱下抛物量子线量子比特及其声子效应。量子线中这样的二能级体系可作为一个量子比特。当量子线中电子处于基态和第一激发态的叠加态时,电子的概率密度|ψ01(t,x,y,0)|2随着势阱宽度的增加而减小;电子的振荡周期T0随耦合强度α的增大而减小;振荡周期T0随受限长度l0的增加而增加。     

两模光机械系统中超冷原子的量子相变

Dicke模型(DM)用于描述单模玻色光场与多个全同二能级原子相互作用。本文利用自旋相干态变分法得到两模光机械系统中基态能量的精确解,并通过变分法求得相变点并画出基态相图,并在此基础上研究原子-场耦合强度等系统参数对基态稳定性的影响。通过稳定性讨论,我们发现:原子-光子耦合常数g和光子-声子耦合常量ζ都会对光机械系统的基态特性产生影响。当双模光腔变成单模光腔时,机械振子能诱导超辐射相的塌缩;而且当光子-声子耦合强度大时,超辐射相被完全压制,而直接出现两原子能级之间的转移;存在不稳定的非零光子态,类似于超辐射态。光机械腔中光子-声子耦合诱导的超辐射态的塌缩和复苏是不同于光腔内囚禁的BEC系统,即机械振子不存在时的情况,而双模光腔对量子相变点和相图预期也会有影响。可见,分析机械振子的对多稳性和相关的量子相变的影响是非常有意义的课题。     

Vortex-lattice dynamics in rotating spinor Bose-Einstein condensates

We observe interlaced square vortex lattices in rotating dilute-gas spinor Bose-Einstein condensates (BEC). After preparing a hexagonal vortex lattice in a one-component BEC in an internal atomic state |1, we coherently transfer a fraction of the superfluid to a different state |2. The subsequent evolution of this pseudo-spin-1/2 superfluid towards a state of offset square lattices involves an intriguing interplay of phase-separation and -mixing dynamics, both macroscopically and on the length scale of the vortex cores, and a stage of vortex turbulence. The stability of the square structure is proved by its response to applied shear perturbations. An interference technique shows the spatial offset between the two vortex lattices. Vortex cores in either component are filled by fluid of the other component, such that the spin-1/2 order parameter forms a Skyrmion lattice.     

Stability of Two-Component Bose-Einstein Condensates in Bessel Optical Lattices

The stability of the ground state of two-component Bose-Einstein condensates （BEGs） loaded into the central well of an axially symmetric Bessel lattices （BLs） potential with attractive or repulsive atoms interactions is studied using the time-dependent Gross-Pitaevskii equation （GPE）. By using the variational method, we find that stable ground state of two-component BEGs can exist in BLs. The BLs＇s depth and the intra-species atom interaction play an important role in the stability of ground state. The collapse of two-component BEGs in BLs is also studied and a collapse condition for trapped two-component BEGs is obtained. It is shown that the two-component BEGs exhibit rich collapse dynamics. That is, the two-component BEGs can collapse in the system with both intra- and inter-attractive, or with intra-attractive and inter-repulsive, or with intra-repulsive and inter-attractive atom interactions. Furthermore, the control of the collapse of the two-component BEGs in BLs is discussed in detail. The stability diagram of the ground state in parameter space is obtained. The results show that the collapse of two-component BEGs can be controlled by temporal modulation of the atom interaction.     

Stability Diagrams of a Bose-Einstein Condensate in Excited Bloch Bands

Landau and dynamical instabilities o/a Bose-Einstein condensate （BEC） in the excited bands of a one-dimensional optical lattice are investigated by the Gross Pitaevskii theory. Our results show that there always exists Landau instability for a BEC in the whole region of excited bands. We also map out the dangerous zones of the dynamical instability. The experimental implications of the stability diagram are discussed.     

Vortex phase qubit: generating arbitrary, counterrotating, coherent superpositions in Bose-Einstein condensates via optical angular momentum beams

We propose a scheme for the generation of arbitrary coherent superpositions of vortex states in Bose-Einstein condensates (BEC) using the orbital-angular-momentum states of light. We devise a scheme to generate coherent superpositions of two such counterrotating states of light using well-known experimental techniques. We show that a specially designed Raman scheme allows for transfer of the optical vortex-superposition state onto an initially nonrotating BEC. This creates an arbitrary and coherent superposition of a vortex and antivortex pair in the BEC. The ideas presented here could be extended to generate entangled vortex states, design memories for the orbital-angular-momentum states of light, and perform other quantum information tasks. Applications to inertial sensing are also discussed.     

Band structures of a slowly rotating dipolar Bose-Einstein condensate with a quantized vortex along a one-dimensional optical lattice

We derive the effective Gross-Pitaevskii equation for a slowly rotating dipolar Bose-Einstein condensate (BEC) with a quantized vortex along a one-dimensional optical lattice and calculate its band structures. The band structure of a slowly rotating BEC in a lattice becomes interesting when dipole-dipole interaction (DDI) is involved. Under rotation, a dipolar rotating term emerges from the DDI potential. The dipolar rotating term makes a BEC with an attractive DDI more stable than one with a repulsive DDI. The dipolar rotating term changes and generalizes the definition for the type of BEC, which cannot be simply determined by an s-wave scattering length or an effective contact interaction term. The dipolar rotating term also makes the band structure fascinating and tunable. A so-called swallowtail band structure, i.e., a multi-valued solution due to nonlinear interaction, can either elongate or shrink as the band index increases, in contrast to a non-rotating dipolar BEC system with a monotonic dependence. With the dipolar rotating term, various band structures as well as an attractive BEC without collapse can be easily achieved. We demonstrate that a rotating dipolar BEC system subject to an optical lattice combines features of a crystal and a superfluid and promises wide applications.     

Evolution of the vortex state in the BCS-BEC crossover of a quasi two-dimensional superfluid Fermi gas

We use the path-integral formalism to investigate the vortex properties of a quasi-two dimensional(2D) Fermi superfluid system trapped in an optical lattice potential.Within the framework of mean-field theory,the cooper pair density,the atom number density,and the vortex core size are calculated from weakly interacting BCS regime to strongly coupled while weakly interacting BEC regime.Numerical results show that the atoms gradually penetrate into the vortex core as the system evolves from BEC to BCS regime.Meanwhile,the presence of the optical lattice allows us to analyze the vortex properties in the crossover from three-dimensional(3D) to 2D case.Furthermore,using a simple re-normalization procedure,we find that the two-body bound state exists only when the interaction is stronger than a critical one denoted by G_c which is obtained as a function of the lattice potential's parameter.Finally,we investigate the vortex core size and find that it grows with increasing interaction strength.In particular,by analyzing the behavior of the vortex core size in both BCS and BEC regimes,we find that the vortex core size behaves quite differently for positive and negative chemical potentials.     

偶极玻色-爱因斯坦凝聚体在类方势阱中的Bénard-von Kármán涡街

对偶极玻色-爱因斯坦凝聚体（Bose-Einstein condensate,BEC）在类方势阱中的Bénard-von Kármán涡街现象进行了数值研究.结果表明,当障碍势在BEC中的运动速度与尺寸在适当范围内时,系统中会出现稳定的两列涡旋对阵列,即Bénard-von Kármán涡街.研究了偶极相互作用强弱、障碍势尺寸以及运动速度对尾流中产生的涡旋结构的影响,得到了相图结构.对障碍势所受拖拽力进行计算,分析了涡旋对产生的力学机理.     

Exploring ground states and excited states of spin-1 Bose–Einstein condensates by continuation methods

A pseudo-arclength continuation method (PACM) is employed to compute the ground state and excited state solutions of spin-1 Bose–Einstein condensates (BEC). The BEC is governed by the time-independent coupled Gross–Pitaevskii equations (GPE) under the conservations of the mass and magnetization. The coupling constants that characterize the spin-independent and spin-exchange interactions are chosen as the continuation parameters. The continuation curve starts from a ground state or an excited state with very small coupling parameters. The proposed numerical schemes allow us to investigate the effect of the coupling constants and study the bifurcation diagrams of the time-independent coupled GPE. Numerical results on the wave functions and their corresponding energies of spin-1 BEC with repulsive/attractive and ferromagnetic/antiferromagnetic interactions are presented. Furthermore, we reveal that the component separation and population transfer between the different hyperfine states can only occur in excited states due to the spin-exchange interactions.     

Analytic Study of a Bose--Einstein Condensate in Waveguide with an Obstacle Potential

We investigate the exact solutions of one-dimensional (1D) time-independent Gross-Pitaevskii equation (GPE), which governs a Bose-Einstein condensate (BEC) in the magnetic waveguide with a square-Sech potential. Both the bound state and transmission state are found and the corresponding spatial configurations and transport properties of BEC are analyzed. It is shown that the well-known absolute transmission of the linear system can occur in the considered nonlinear system.     

Demonstration of quantum anti-Zeno effect with a single trapped ion

We experimentally demonstrate the quantum anti-Zeno effect in a two-level system based on a single trapped ion ~(40)Ca~+. In the large detuning regime, we show that the transfer from the ground state to the excited state can be remarkably enhanced by the inserted projection measurements. The inserted measurements in our experiment are realized by the electron shelving technique. Compared to the ideal projection measurement, which makes the quantum state collapse instantaneously, a practical electron shelving process needs a finite time duration. The minimum time for this collapse process is shown to be inversely proportional to the square of the coupling strength between the measurement laser and the system.