自旋-轨道角动量耦合玻色-爱因斯坦凝聚体中基态自旋织构的实验研究

There are many spin textures with different topological property in spinor Bose-Einstein condensates (BEC), which provide an ideal quantum simulation platform for the study of topological structures in condensed matter physics and particle physics. At present, the spin textures prepared in cold atom experiments using spin rotation method are not in the ground state of the system, and the transformation process between spin textures has not been studied. The introduction of spin-orbit coupling into spinor BEC provides a new way to study the evolution and transformation of spin textures at ground-state. There are abundant quantum phases in the spin-orbital-angular-momentum coupling BEC recently realized in our lab, and theory shows that the spin textures of quantum phases in our system have different topological property: one is two-dimensional half-skyrmion and the other one is vortex-antivortex pair. The transformation between half-skyrmion and vortex-antivortex pair can be achieved by precisely controlling the system parameters. In this project, the topological properties of spin textures of these quantum phases will be studied experimentally in our spin-orbital-angular momentum coupled BEC platform, their lifetime and dynamic evolution behavior under perturbation will be measured, the hysteretic characteristic curves in the first-order quantum phase transition process between different spin textures will be observed, and the dependence of spin textures on Raman coupling strength, temperature and other parameters will be studied. This will provide a useful reference for revealing the mechanism of the spin textures in high temperature superconductivity and quantum turbulence.

超冷原子气体的旋量玻色-爱因斯坦凝聚体（BEC）中存在诸多具有不同拓扑属性的自旋织构，可以为凝聚态物理、粒子物理等领域中拓扑结构的研究提供理想的量子模拟平台。目前利用自旋旋转等方法制备的自旋织构不处于系统的基态，且尚未研究自旋织构间的转变过程。在旋量BEC中引入自旋-轨道耦合为研究基态自旋织构的演化和转变提供了新的途径。在我们近期实现的自旋-轨道角动量耦合BEC中存在丰富的量子相，理论表明其自旋织构的拓扑属性分别为二维半斯格明子和涡旋-反涡旋对，通过精确控制实验参数可以实现不同自旋织构间的转换。本项目将在已有自旋-轨道角动量耦合BEC平台上实验确定这些量子相自旋织构的拓扑属性，测量其寿命及在微扰下的动态演化行为，观察自旋织构间一阶量子相变过程中的滞回特性曲线，研究其与耦合强度、温度等参数的依赖关系。这将为揭示以上自旋织构在高温超导、量子湍流等领域中的物理机制提供有益的参考。

本项目是基于87Rb原子玻色-爱因斯坦凝聚态（BEC）实验平台，在已经实现的自旋-轨道角动量耦合体系之中实验测量自旋织构的拓扑结构及其动态演化行为，并进一步观测具有不同拓扑属性的自旋织构间相互转变的量子相变过程。与大多数冷原子领域的SOC实验都是研究自旋和线动量的耦合不同，我们研究的是自旋和轨道角动量的耦合，有助于条纹相等新奇量子态的研究。与通常采用的磁场自旋旋转的方式获得特殊构型的自旋织构的实验不同，我们的实验过程满足绝热条件，在该体系中能获得作为体系基态存在的特殊自旋织构。经过三年的实验研究，本课题在项目的资助下开展了以下一些研究工作：（1）利用液晶涡旋半波片（q板）搭建拉曼光路实现自旋-轨道角动量耦合，从而将体系可控地制备到半斯格明子基态，实验测得的自旋极化率的波动减小，且原子自旋结构的对称性有了明显改善；（2）利用空间光调制器（SLM）制备高阶涡旋拉曼光场，并对相位全息图进行了相应矫正，从而获得了高质量L-G光束；（3）玻色-爱因斯坦凝聚体（BEC）中涡旋物质波的干涉性质的研究可实现类比光学中的Mach-Zehnder 干涉仪的物质波干涉仪；（4）超冷原子长距离高效转移和环形BEC的实现为构建大轨道角动量的自旋-轨道角动量耦合体系，并在其中研究量子条纹相等新奇量子相打下了基础。最后本项目发表了论文一篇，申请专利两个，还有一些工作正在整理，总结来说，项目达到了预期。