1.National Laboratory of Solid State Microstructures, School of Physics, Nanjing University, Nanjing 210093, China 2.Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, China
Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0301803), the National Natural Science Foundation of China (Grant Nos. 11604103, 91636218, 11474153), and the Natural Science Foundation of Guangdong Province, China (Grant No. 2016A030313436).
Received Date:30 October 2018
Accepted Date:19 January 2019
Available Online:01 February 2019
Published Online:20 February 2019
Abstract:Relativistic wave equations, such as Dirac, Weyl or Maxwell equations, are fundamental equations which we use to describe the dynamics of the microscopic particles. On the other hand, recent experimental and theoretical studies have shown that almost all parameters in cold atomic systems are precisely tunable, so the cold atom systems are considered as an ideal platform to perform quantum simulations. It can be used to study some topics in high energy and condensed matter physics. In this article, we will first introduce the ideas and methods for engineering the Hamiltonian of atoms, mainly related to the theories of laser-assisted tunneling. Based on these methods, one can simulate the equations of motion of relativistic particles and observe some interesting behaviors which are hard to be observed in other systems. The article reviews these recent advances. Keywords:quantum simulations/ optical lattices/ laser-assisted tunneling/ relativistic wave equations
全文HTML
--> --> -->
2.原子哈密顿量的设计简单起见, 本文主要讨论光晶格中无作用的具有$ N $个内态的原子系统, 其哈密顿量可表示为
激光辅助跳跃的基本思路是: 首先抑制原子在近邻格点的自然跳跃(如光晶格势阱较深或近邻格点的能量差较大), 然后通过外加拉曼激光耦合近邻格点中的原子来恢复和调控原子跳跃, 进而产生等效规范势和自旋轨道耦合[8,9]. 这里主要介绍文献[15]中利用激光辅助跳跃产生规范势的方案. 如图1所示, 为实现自旋依赖的光晶格, 可考虑在$ x $方向施加波长处于“反魔数”(“anti-magic”)波段的驻波场, 同时在$ y $方向施加处于“魔数”波段的驻波, 处在两个不同内态$ |g\rangle $和$ |e\rangle $的原子在$ xy $平面内感受到的光晶格势为 图 1 基于激光辅助跳跃实现人工磁场, 黑(灰)色圆分别表示内态为$ |g\rangle $$ (|e\rangle) $的Yb原子 (a)内态被标记为$ |g\rangle $和$ |e\rangle $的原子被囚禁在自旋依赖的光晶格势$ V_g $和$ V_e $中, 其中$ V_g=-V_e $; (b) $ x $方向上的激光辅助跃迁; (c)自旋依赖光晶格示意图. $ y $方向存在自然跳跃, $ x $方向由一束拉曼光$ \varOmega_{\rm R} $诱导跳跃 Figure1. Realization of artificial magnetic field based on laser-assisted tunneling. Gray and black dots represent the Yb atoms correspond to internal states $|g\rangle$ and $|e\rangle$, respectively: (a) The atoms $|g\rangle$ and $|e\rangle$ are trapped in the state-dependent optical lattice potentials $V_g$ and $V_e$, where $V_g=-V_e$; (b) laser-assisted tunneling along $x$ direction; (c) sketch of state-dependent optical lattice. Nature tunneling occurs along the $y$ direction, and the tunneling along $x$ direction is induced by a Raman beam $\varOmega_{\rm R}$.
其中$ \psi_{\pm} $是二分量波矢, 描述具有不同螺旋度(手性)的外尔费米子. 根据螺旋度的定义$ \hat{h}={ {\sigma}}\cdot{{p}}/|{{p}}| $可知, 当粒子自旋与动量平行(反平行)时$ h=+1 $$ \big(h=-1\big) $, 故无质量狄拉克方程实质上可分解为两个螺旋度相反的外尔方程的叠加. 外尔费米子作为预言中的无质量的基本粒子, 在高能物理领域至今仍未被实验发现. 而在凝聚态体系的某些三维晶格的动量空间中, 低能有效哈密顿量由外尔方程描述, 这些系统被称为外尔半金属. 外尔半金属中外尔准费米子激发的发现进一步掀起了在凝聚态系统研究和寻找相对论量子力学描写的准粒子的研究热潮. 冷原子光晶格系统中实现外尔半金属的理论方案已有许多, 如在二维光晶格中引入自旋轨道耦合, 再加一个人工维度可以实现外尔半金属[36]; 通过在两个人工维度中堆垛一维双势阱晶格的拓扑相, 或者是直接将二维的具有交错磁通棋盘结构或蜂巢光晶格堆垛成三维的晶格[37-40], 都可以实现外尔半金属[37]. 在这些方案中, 自旋自由度可选择用两个原子内态或者两个子格子, 对应所需要实现的跳跃项需用到人工自旋轨道耦合和人工磁场. 文献[41]给出了通过堆垛Hofstadter-Harper系统成为一个立方晶格以实现拓扑外尔半金属相的方案. 图3中展示的是沿$ x $和$ z $方向存在激光辅助跳跃的三维晶格示意图. 为实现这样的跳跃, 应先通过在每个格点引入足够大线性倾斜$ \varDelta $以抑制这两个方向的自然跳跃$ (t_x, t_z) $$ \big(t_{x, z}\ll\varDelta\ll E_{\rm {gap}}\big) $, 该线性倾斜可以通过在$ {x}+{z} $方向引入线性的势场(如重力场、磁场等)产生. 正如在2.1节中介绍的方法, 引入两束远失谐频率和动量分别相差$ \delta\omega=\omega_1-\omega_2=\varDelta/\hbar $和$ \delta {{k}}={{k}}_1-{{k}}_2 $的拉曼光可重新诱导这两个方向发生共振跳跃[15,16]. 该三维晶格对应的有效哈密顿量为 图 3 实现外尔半金属的三维立方晶格示意图. 合理设计$x$和$z$方向跳跃, 在动量空间会出现外尔点. 虚线和实线分别表示获得相位${\text{π}}$和0[41] Figure3. Schematic diagram of a three-dimensional cubic lattice of a Weyl semimetal. The Weyl points will be created in the momentum space if the tunneling along $x$ and $z$ directions are well-designed . The dashed and solid lines indicate the phase ${\text{π}}$ and 0, respectively.
$\begin{split}H_{3\rm D}= & -\sum\limits_{m, n, l}(K_x{\rm e}^{-{\rm i}\varPhi_{m, n, l}}a^{\dagger}_{m+1, n, l}a_{m, n, l} \\ &+t_ya^{\dagger}_{m, n+1, l}a_{m, n, l}\\ &+K_z{\rm e}^{-{\rm i}\varPhi_{m, n, l}}a^{\dagger}_{m, n, l+1}a_{m, n, l}+{\rm {h.c.}}), \end{split}$
其中$ a^{\dagger}_{m, n, l} $$ \big(a_{m, n, l}\big) $表示在格点$ (m, n, l) $的产生(湮灭)算符, $ \varPhi_{m, n, l}=\delta{{k}}\cdot{{R}}_{m, n, l}=m\varPhi_x+n\varPhi_y+l\varPhi_z $. 随后选取合适的激光方向使得$ (\varPhi_x, \varPhi_y, \varPhi_z)=$${\text{π}}(1, 1, 2) $, 即$ \varPhi_{m, n, l}=(m+n){\text{π}} $, 如图3(b)所示. 一方面可以将该三维晶格看作是由图3(c)和图3(d)所示的两种不同类型的二维晶格(平行于$ xz $平面)沿$ y $方向的交替叠加, 此时两平面间沿$ y $方向的跳跃是常规的. 从另一个角度来看, 该三维系统可视为图3(a)中磁通为$ \alpha=1/2 $的Hofstader-Harper二维晶格在$ z $方向的堆叠. 此时沿$ z $方向的跳跃会携带相位$ 0 $或$ {\text{π}} $, 分别对应$ m+n $为偶数或奇数的情况. 该系统是空间反演对称破缺的, 对应布洛赫哈密顿量为