铁磁绝缘体间的极薄Bi2Se3薄膜的相变研究

研究一个极薄三维拓扑绝缘体Bi₂Se₃薄膜处于两个铁磁绝缘体层之间, 其铁磁层的磁化方向都处于竖直平面, 系统拓扑性质随磁化方向夹角的变化. 从表面态电子低能有效哈密顿量出发计算系统的Chern 数, 和运用一个具有Armchair边界的单层六角晶格带的紧束缚模型模拟系统的体能带和边缘态, 来确定系统所处的拓扑相. 发现两个铁磁层的磁化方式从平行转到反平行的某一临界角度, 系统经历从反常量子霍尔相到普通绝缘相的转变.     

Bose-Einstein condensates in rotating lattices

Strongly interacting bosons in a two-dimensional rotating square lattice are investigated via a modified Bose-Hubbard Hamiltonian. Such a system corresponds to a rotating lattice potential imprinted on a trapped Bose-Einstein condensate. Second-order quantum phase transitions between states of different symmetries are observed at discrete rotation rates. For the square lattice we study, there are four possible ground-state symmetries.     

Continuous weak measurement and nonlinear dynamics in a cold spin ensemble

A weak continuous quantum measurement of an atomic spin ensemble can be implemented via Faraday rotation of an off-resonance probe beam, and may be used to create and probe nonclassical spin states and dynamics. We show that the probe light shift leads to nonlinearity in the spin dynamics and limits the useful Faraday measurement window. Removing the nonlinearity allows a nonperturbing measurement on the much longer time scale set by decoherence. The nonlinear spin Hamiltonian is of interest for studies of quantum chaos and real-time quantum state estimation.     

线型分子的电子能级之Λ分裂的新解释

认为电子态能级发生Λ分裂虽然是角动量引起的,但不是耦合而是运动牵连作用的结果.从这一认识出发,利用欧拉方程和拉格朗日方程分别得到了包含Λ分裂的分子转动哈密顿函数和哈密顿算符,所得结果与Van.Vleck的著名结论完全一致 关键词： Λ分裂 线型分子 欧拉方程 哈密顿算符     

Chiral bands, dynamical spontaneous symmetry breaking, and the selection rule for electromagnetic transitions in the chiral geometry

A model for a special configuration in triaxial odd-odd nuclei is constructed which exhibits degenerate chiral bands with a sizable rotation, a manifestation of dynamical spontaneous symmetry breaking. A quantum number obtained from the invariance of the model Hamiltonian, which characterizes observable states, is given and selection rules for electromagnetic transition probabilities in chiral bands is derived in terms of this quantum number. The degeneracy of the lowest two bands is indeed obtained in the numerical diagonalization of the Hamiltonian at an intermediate spin range, over which electromagnetic transitions follow exactly the selection rule expected for the chiral geometry.     

双棱镜改变写入Bragg波长的相位掩模技术

将一种可旋转的双棱镜引入到相位掩模技术中以改变光栅的写入Bragg波长.在该系统中,光纤光栅是由来自可旋转双棱镜所形成的波长为248 nm的紫外干涉条纹写入的,其中,相位掩模被用作 1级衍射光的分束器,通过双棱镜的旋转可改变两写入光束的交叉角.为了初始化Bragg波长的参考值,双光栅的顶角由相位掩模的 1级衍射角和双棱镜的折射率确定.因为在～100 nm范围内两光束的非对称旋转对光栅周期的改变是5×10~(-4) nm,双棱镜引入的光栅的闪耀可忽略.当Bragg波长的移位为1 nm时,棱镜最大的旋转角为～1 degree,最小的旋转角是～2.4 min.与Talbot干涉仪中平面镜的旋转角～23 s/nm相比,该相位干涉仪中棱镜的旋转精度降低了2～3个数量级.     

基于旋转波片的斯托克斯参量检测与精度分析

用于偏振光学遥感器定标的参考光源,其偏振态的检测精度会直接影响偏振光学遥感器的定标精度,进而影响目标特性的反演水平。选用870 nm波段的水平线偏振光作为被测试的定标参考光源,通过旋转1/4波片(quarter-wave plate, QWP)对其光强进行调制。调制光强可表达为波片快轴旋转角度的傅里叶级数,采用傅里叶变换法反演出级数的系数,根据该系数即可计算出被测试光源的Stokes参量。给出10次测量各参量及偏振度的平均值、标准偏差、合成不确定度以及测量平均值与理论值的相对偏差。为提高测试精度,通过对波片快轴初始定位角度偏差Δα、延迟量偏差Δδ与检偏器透光轴角度偏差Δβ进行分析,提出了偏差修正模型。该模型通过Stokes参量检测偏差随Δδ和Δβ的变化趋势及实际偏差值,确定Δδ和Δβ的大小。结合模拟出的波片快轴初始定位角度偏差Δα,对实验装置加以调整,再次对光源的偏振态进行检测。结果表明,基于该修正模型测得光源的各Stokes参量与理论值最大偏差从未经修正的3.77%降低至1.41%。证实了基于本实验的原理、装置、测量方法及所提出的偏差修正模型可有效提高定标参考光源偏振态检测的精度。     

基于Zernike多项式拟合三平面互检的误差分析

基于Zernike多项式拟合三平面互检法,在理论上对光轴偏离、旋转角度以及有效面积对不同面型精度平面的测量误差进行了分析,并分别进行了实验验证.研究结果表明,旋转角度误差和有效面积对测量影响不敏感,可适当降低标定旋转角度成本;而光轴偏离对测量精度影响明显,在高精度面型绝对检测中应对其进行严格校准.     

莫尔条纹技术的三维测角方法研究

针对传统莫尔条纹技术在测量物体偏转角度中不能对多维角度变化进行测量的问题,采用了一种分区间设计的组合光栅方法,并结合光学自准直成像方法构建了一种新的实验系统装置,利用该系统通过测量莫尔条纹的变化实现了对被测量物体偏转角度的三维测量,对合作光学反射镜的偏转角度测量精度优于1”,绕轴转动测量精度优于2”。     

Universality of decoherence

We consider environment induced decoherence of quantum superpositions to mixtures in the limit in which that process is much faster than any competing one generated by the Hamiltonian H(sys) of the isolated system. While the golden rule then does not apply we can discard H(sys). By allowing for couplings to different reservoirs, we reveal decoherence as a universal short-time phenomenon independent of the character of the system as well as the bath and of the basis the superimposed states are taken from. We discuss consequences for the classical behavior of the macroworld and quantum measurement: For decoherence of superpositions of macroscopically distinct states H(sys) is always negligible.     

Approximate Real–Time Visualization of a Quantum Transition by Means of Continuous Fuzzy Measurement

For the detection of gravitational waves the quantum mechanical properties of the detector have to be taken into account. Not all gravitational wave detectors allow a quantum nondemolition (QND) measurement. Continuous weak or fuzzy measurements are an alternative to study the evolution of a quantum mechanical system under the influence of an external field. In the present paper we investigate this alternative by applying it to a simplified system. We numerically simulate continuous fuzzy measurements of the oscillations of a two-level atom subjected to a resonant external light field. We thereby address the question whether it is possible to measure characteristic features of the evolution of a single quantum system in real time without relying on a QND scheme. We compare two schemes of continuous measurement: continuous measurement with constant fuzziness and with fuzziness changing in the course of the measurement. Because the sensitivity of the two-level atom to the influence of the measurement depends on the state of the atom, it is possible to optimize the continuous fuzzy measurement by varying its fuzziness.     

Jaynes-Cummings dynamics with a matter wave oscillator

We propose to subject two Bose-Einstein condensates to a periodic potential, so that one condensate undergoes the Mott-insulator transition to a state with precisely one atom per lattice site. We show that photoassociation of heteronuclear molecules within each lattice site is described by the quantum optical Jaynes-Cummings Hamiltonian. In analogy with studies of this Hamiltonian with cavity fields and trapped ions, we are thus able to engineer quantum optical states of atomic matter wave fields and we are able to reconstruct these states by quantum state tomography.     

二维介电光子晶体中的赝自旋态与拓扑相变

基于背散射抑制且对缺陷免疫的传输性质,光子拓扑绝缘体为电磁传输调控提供了一种新颖的思路.类比电子体系中的量子自旋霍尔效应,本文设计出一种简单的二维介电光子晶体,以实现自旋依赖的光子拓扑边界态.该光子晶体是正三角环形硅柱子在空气中排列而成的蜂窝结构.将硅柱子绕各自中心旋转60°,可实现二重简并的偶极子态和四极子态之间的能带翻转.这两对二重简并态的平均能流密度围绕原胞中心的手性可充当赝自旋自由度,其点群对称性可用来构建赝时间反演对称.根据k·p微扰理论,给出了布里渊区中心附近的有效哈密顿量以及对应的自旋陈数,由此证实能带翻转的实质是拓扑相变.数值计算结果揭示,在拓扑非平庸和平庸的光子晶体分界面上可实现单向传输且对弯曲、空穴等缺陷免疫的拓扑边界态.本文中的光子晶体只由电介质材料组成并且晶格结构简单,实现拓扑相变时无需改变柱子的填充率或位置,只需转动一个角度.因此,这种结构在拓扑边界态的应用中更为有效.     

含时边界条件和Berry相位

对一具有含时边界条件的量子体系,有效哈密顿算符可用一种简单的方式去构造而不涉及任何几何处理。用这种方法构造了一个在半径随时间变化的球形盒子内的量子粒子的有效哈密顿算符,并用它来计算波函数的Berry相位。发现有效哈密顿算符与原哈密顿算符在形式上由一静态规范变换相关联。这两个哈密顿算符的量子态差Berry相位。 关键词：     

Nonsymmetric entanglement of atomic ensembles

The entanglement of multiatom quantum states is considered. In order to cancel noise due to inhomogeneous light-atom coupling, the concept of matched multiatom observables is proposed. As a means to eliminate an important form of decoherence this idea should be of broad relevance for quantum information processing with atomic ensembles. The general approach is illustrated on the example of rotation angle measurement, and it is shown that the multiatom states that were thought to be only weakly entangled can exhibit near-maximum entanglement.     

平面自旋压缩态的产生与原子干涉的机理

在玻色-爱因斯坦凝聚体中实现自旋压缩和量子纠缠, 对于提高原子干涉测量相位灵敏度和原子钟精度有着非常重要的意义. 基于一种新的平面自旋分量的不确定性关系, 介绍了如何利用两分量玻色-爱因斯坦凝聚系统中原子间相互作用提供的非线性效应和原子内部能级间线性耦合, 实现量子平面自旋压缩(挤压)和模式纠缠. 描述了一项关于平面压缩态的理论工作, 该工作利用哈密顿量的精确对角化求解系统基态, 优化非线性作用和线性耦合强度比值, 使得包含平均自旋方向在内的两个正交自旋分量的不确定度同时压缩, 因此在平面上所有相位角度的涨落都受到压制, 而在与该平面垂直的第三个自旋分量方向反压缩. 利用传统自旋压缩判定纠缠, 只能判断多个不可分辨的原子处于纠缠态, 而平面自旋压缩可以检测两个可区分模式(比如, 原子内态)间的纠缠, 从而在不同模式间进行量子信息处理. 同时, 为实现超越标准量子极限的原子干涉相位精密测量, 传统方式是利用单个自旋分量压缩, 但需要对待测相位角度有很好的估计, 或者可以进行多次测量以逐渐逼近可获得的最大压缩极限, 这就要求量子态可以被精确的重复制备. 而利用平面自旋压缩, 对任意未知相位角度只需要测量两个垂直自旋分量就可以实现高的相位测量灵敏度.     

Entanglement, quantum phase transition and fixed-point bifurcation in the N-atom Jaynes-Cummings model with an additional symmetry breaking term

In the present work we analyze the quantum phase transition (QPT) in the N-atom Jaynes-Cummings model (NJCM) with an additional symmetry breaking interaction term in the Hamiltonian. We show that depending on the type of symmetry breaking term added the transition order can change or not and also the fixed point associated to the classical analogue of the Hamiltonian can bifurcate or not. We present two examples of symmetry broken Hamiltonians and discuss based on them, the interconnection between the transition order, appearance of bifurcation and the behavior of the entanglement.     

Quantum deleting and cloning in a pseudo-unitary system

In conventional quantum mechanics, quantum no-deleting and no-cloning theorems indicate that two different and nonorthogonal states cannot be perfectly and deterministically deleted and cloned, respectively. Here, we investigate the quantum deleting and cloning in a pseudo-unitary system. We first present a pseudo-Hermitian Hamiltonian with real eigenvalues in a two-qubit system. By using the pseudo-unitary operators generated from this pseudo-Hermitian Hamiltonian, we show that it is possible to delete and clone a class of two different and nonorthogonal states, and it can be generalized to arbitrary two different and nonorthogonal pure qubit states. Furthermore, state discrimination, which is strongly related to quantum no-cloning theorem, is also discussed. Last but not least, we simulate the pseudo-unitary operators in conventional quantum mechanics with post-selection, and obtain the success probability of simulations. Pseudo-unitary operators are implemented with a limited efficiency due to the post-selections. Thus, the success probabilities of deleting and cloning in the simulation by conventional quantum mechanics are less than unity, which maintain the quantum no-deleting and no-cloning theorems.     

Resolution of ghost imaging with entangled photons for different types of momentum correlation

We present an analytical analysis of the spatial resolution of quantum ghost imaging implemented by entangled photons from a general, spontaneously parametric, down-conversion process. We find that the resolution is affected by both the pump beam waist and the nonlinear crystal length. Hence, we determined a method to improve the resolution for a certain imaging setup. It should be noted that the resolution is not uniquely related to the degree of entanglement of the photon pair since the resolution can be optimized for a certain degree of entanglement. For certain types of Einstein-Podolsky-Rosen(EPR) states——namely the momentum-correlated or momentum-positively correlated states——the resolution exhibits a simpler relationship with the pump beam waist and crystal length. Further, a vivid numerical simulation of ghost imaging is presented for different types of EPR states,which supports our analysis. This work discusses applicable references to the applications of quantum ghost imaging.     

Ehrenfest's theorem reinterpreted and extended with Wigner's function

For a wave packet evolving quantum mechanically, the rates of change of the expectations and uncertainties of the position and momentum are exactly the same as if Wigner's function instantaneously obeyed a classical Liouville equation (whatever the Hamiltonian). This extension of Ehrenfest's theorem should be useful for dealing with the evolution and manipulation of quantum states.