1.College of Physics and Materials Science, Henan Normal University, Xinxiang 453007, China 2.Laboratory of Condensed Matter Theory and Materials Computation, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Fund Project:Project supported by the National Key R&D Program of China (Grant No. 2016YFA0301500), the Nantional Natueral Science Foundation of China (Grant Nos. 11434015, 61835013, 11728407, KZ201610005011, 11347159,11604086), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant Nos. XDB01020300, XDB21030300), and the Foundation of He'nan Educational Committee, China (Grant Nos. 01026631082,14A140032).
Received Date:27 January 2019
Accepted Date:11 February 2019
Available Online:19 February 2019
Published Online:20 February 2019
Abstract:Spinor condensates trapped in optical lattices have become potential candidates for multi-bit quantum computation due to their long coherence and controllability. But first, we need to understand the generation and regulation of spin and magnetism in the system. This paper reviews the origin and manipulation of the magnetism of atomic spin chains in optical lattices. The theoretical study of the whole process is described in this paper, including laser cooling, the spinor Bose-Einstein condensate preparations, the optical lattice, and the atomic spin chain. Then, the generation and manipulation of magnetic excitations are discussed, including the preparation of magnetic solitons. Finally, we discuss how to apply atomic spin chains to quantum simulation. The theoretical study of magnetic excitations in optical lattices will play a guiding role when the optical lattice is used in cold atomic physics, condensed matter physics and quantum information. Keywords:spinor Bose-Einstein condensates/ optical lattice/ magnetic soliton/ quantum simulation
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2.激光冷却与玻色-爱因斯坦凝聚当激光的频率与原子的固有频率相同时, 激光射向运动着的原子, 原子就会共振吸收迎面射来的光子从低能级跃迁到高能级(图1(a)), 由于整个过程动量守恒, 原子在得到光子以后的动量等于原子的初始动量和光子动量的矢量和, 由于二者反号, 被激发的原子的动量就会变小(图1(b)). 同时处于激发态的原子会自发辐射出光子而回到低能级的初态并获得反冲动量, 因为自发辐射出的光子的方向是随机的, 所以多次自发辐射的平均结果并不会增加原子的动量(图1(c)). 所谓激光冷却, 实际上就是通过激光与原子的作用下使原子减速, 温度是物体分子热运动的平均动能的标志, 从微观角度看, 原子减速了, 温度也就降低了. 图 1 (a)运动的原子和反向传输的激光; (b)吸收光子动量减少的原子; (c)原子随机辐射光子 Figure1. (a) Moving atoms and counter propagating laser; (b) atoms with reduced momentum after absorbing photons; (c) atoms radiate photons in random directions.
得到, 这里$p$和$q$表征塞曼能级差, $c = {c_2}n/2$. 结果如图4所示, 可以看出, 自旋依赖的碰撞相互作用对畴的形成起着非常重要的作用. 图 4$F = 1$旋量凝聚体的自旋畴示意图. 图(b)中, ${m_f} = \pm 1$时凝聚原子会分成三个畴而且有明显的边界, 相互作用会诱导畴边界交叠如图(a)和图(c)所示, 在图(c)中自旋畴已经没有了明显边界[7] Figure4. Spin-domain diagrams for condensates with $F = 1$. The cloud is separated into three domains with distinct boundaries in (b), components ${m_f} = \pm 1$ are miscible as shown in (a), all three components are generally miscible in (c).
由上两式可以看出, 光晶格中旋量原子的哈密顿量和固体物理里的海森伯自旋链的哈密顿量形式非常相似, 所以我们也称这个系统为光晶格中的原子自旋链模型. 和海森伯自旋链相比, 原子自旋链有两个独有的优点: 其一是格点之间的耦合是长程耦合, 充分考虑了非近邻格点间的相互作用; 其二是耦合距离和强度可以调节, 通过调节外场参数, 可以增加或者减弱长程特性, 特别地, 适当调节参数可以实现近邻和次近邻的有效近似. 由于磁偶极相互作用的存在, 张卫平、蒲晗等[11]研究发现铁磁相变和自发磁化可以在一维自旋链中发生, 如图5所示. 图 5 原子自旋链中磁偶极-偶极相互作用诱导的自发磁化[11] 这里纵轴${m_z}$代表$z$方向的自发磁化强度, 横轴${B_\rho }$是$x - y$平面上的外磁场强度, 虚线是平均场近似的结果, 数值模拟所得实线对应的是不同的格点填充数$N = 10,15,20$ Figure5. Spontaneous magnetization of atomic spin chain dominated by magnetic dipole-dipole interaction. ${m_z}$ is the magnetization components in the $z$-axis direction, ${B_\rho }$ is intensity of the external magnetic field. The dashed line represents the mean-field result and the solid lines, from left to right, correspond to the exact numerical results for a two-site lattice with $N = 10,15\;{\rm{ and }}\;20$ atoms.
这个方程描述了自旋激发沿坐标轴方向的传输, 非常类似于量子力学中有效质量为${m^*} = \hbar /{\beta _1}$的粒子的质心运动方程. 和声子定义类似, 这就是光晶格原子自旋链系统中产生的磁振子. 图6给出了原子自旋链中的自旋波产生和传播的示意图. 图 6 原子自旋链中自旋波的激发. 图的上部分是原子自旋链的铁磁基态示意图, 下部分是偶极-偶极相互作用下自旋进动在晶格方向的传播[18] Figure6. Spin waves are excited in atomic spin chain in optical lattice. Top: ferromagnetic ground-state structure of the spinor BEC atomic spin chain. Bottom: spin in each lattice site processes in spin space and spin waves can be excited.
25.2.光晶格中的磁孤子 -->
5.2.光晶格中的磁孤子
凝聚态物理发展过程中, 孤子作为基态元激发的引入对处理非线性问题是巨大的推动, 孤子激发在海森伯自旋链中已经被广泛研究过. 在低温条件下, 这些元激发实际就是我们上面提到的自旋波, 自旋波孤子也是大家比较感兴趣的课题. 只是在固体系统中, 掺杂和缺陷一直存在, 温度的影响也很大, 增加了研究和观测的难度. 这方面, 光晶格系统的优越性很明显: 一方面这里的原子自旋链系统是一个非常纯净的系统, 没有任何杂质; 另一方面, 系统具有很高的可控性, 而且温度的影响变得微乎其微. 在这个意义上, 光晶格可以作为一个非常理想的工具用来模拟固体物理中的许多动力学特征. 基于偶极-偶极的可调性, 原子自旋链中的磁孤子激发被大量地研究. 但是, 针对如何在原子自旋链中实现可观测的孤子的研究很有限. 由于连续近似下自旋波的传播服从方程(17), 通过研究孤子存在的条件, 我们建议通过调节横向场来实现磁孤子的观测. 图7中给出了如何通过控制外部光场的强度和囚禁宽度来实现磁孤子激发. 图 7 通过控制外场实现磁孤子的产生 (a)红失谐光晶格中控制驱动光场和束缚场产生磁孤子, $Q$是驱动光场的强度, $W$是晶格的横向囚禁宽度, 空白的区域对应有磁孤子产生, 反之, 暗的区域不能激发磁孤子; (b)蓝失谐光晶格中调节束缚场来产生磁孤子, 蓝线、绿线和红线分别代表考虑近邻、次近邻和长程的结果, $f(W) > 0$代表有磁孤子激发[19] Figure7. Magnetic soliton are excited by tuning external field: (a) Magnetic soliton are produced by tuning driving light field and trapping potential in red-detuning case, the vertical axis $Q$ stands for the intensity of the modulated laser, and thehorizontal axis $W$ represents the transverse width of the condensate, the blank region corresponds to the existence of solitons; (b) magnetic soliton are produced by tuning trapping potential in blue-detuning case, the three lines correspond to the nearest-neighbor approximation (blue), the next-nearest-neighbor approximation (green),and the continuum limit approximation (red), respectively, magnetic solitons occur in the region $f(W) > 0$.
根据方程(20), 有$\left\langle {\widehat {{E}}(t)} \right\rangle \equiv \left\langle {\widehat {{H}}(t)} \right\rangle \equiv 0$. 但是电磁场的起伏会被放大, 同时产生声子. 使用一个外磁场来驱动囚禁在谐振子势阱中的旋量玻色-爱因斯坦凝聚体, Saito等[24]最先实现了对动力学卡西米尔效应的模拟, 结果如图9所示: 图 9 外磁场驱动下囚禁势中旋量凝聚体的横向和纵向的磁化随时间的演化 红线是横向的磁化${G_{\rm{T}}}$, 蓝线代表纵向的磁化${G_{\rm{L}}}$, 图中插图显示的是横向磁化被放大的过程[23] Figure9. Time evolution of the average squared transverse magnetization ${G_{\rm{T}}}$ (red curve) and longitudinal magnetization ${G_{\rm{L}}}$ (blue curve), the exponential growth of ${G_{\rm{T}}}$ is shown in subgraph.
光晶格自旋链系统中, 磁偶极-偶极相互作用和光诱导的偶极-偶极相互作用能诱导不同的自旋激发, 这和理论上研究有限温度的动力学卡西米尔效应的光学共振腔系统非常相似. 相对弱的磁偶极-偶极相互作用诱导的激发可以作为有限温度激发源, 在外部光场的驱动下, 我们发现磁振子激发会产生指数形式的增长, 如图10所示. 图 10 不同强度的驱动场下自旋起伏的放大倍数随有效温度的变化 图中红色圈、绿色方块和蓝色三角分别代表我们选择的不同的驱动光场强度, 通过适当选择光场强度可以使磁振子激发产生指数形式的增长, 也就是动力学卡西米尔效应[18] Figure10. Amplification factor as a function of the effective temperature under different intensities of the external modulation laser, the the dynamical Casimir effect at finite temperature take place if the proper parameters are selected.