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ICQM faculty member Xiong-Jun Liu and the collaborators publish an article in Science Advances reporting a symmetry protected topological quantum state for ultracold atoms
量子材料中心刘雄军组和合作者在Science子刊发表一维对称保护拓扑物态研究

Fig.1Ultracold atoms in an engineered crystal of light.

 

In a recent research,an international team of theoretical and experimental physicists atICQM,Peking University (PKU) and the Hong Kong University of Science and Technology (HKUST) reported the observationof an SPT phase for ultracold atoms using atomic quantum simulation.This work opens the way to expanding the scope of SPT physics with ultracold atoms and studying non-equilibrium quantum dynamics in these exotic systems.Their findings were publishedas a Research Articlein the journal Science Advances on Feb 23, 2018 (doi: 10.1126/sciadv.aao4748).

Symmetry plays a fundamental role in understanding complex quantum matter, particularly in classifying topologicalquantum phases, which have attracted great interests in the recent decade. An outstanding example is the time-reversal invariant topological insulator, relatively new class of material with peculiar electronic properties, that is well understood as a symmetry-protected topological (SPT) material.

As one can classify the shapes of objects based on the mathematical concept called topology, anexotic phase of quantum matter can be understood with underlying topology and symmetry in physical materials. The team created a synthetic crystal for ultracold atoms and for the first time emulates key properties of a 1D topological material beyond the natural condition. The ultracold atoms are billion times more dilute than solids but allow the unique access to the study of complex physics because they are extremely pristine and controllable.

Classification of topological quantum phases hasbrought about a fundamental notion of SPT phases, which are exotic states under the protection of symmetries, and greatly expand our understanding of the fundamental nature of quantum matter. Nevertheless, by far only a small portion of theoretically predicted SPT phases have been discoveredin solid state materials, mainly due to complicated and uncontrollable environment of solid-state materials which causes great challenges in realization.

In the published article, the authors predicted in theory a new type of 1D SPT phase, which isprotected by the hidden magnetic group symmetry and a non-local chiral symmetry, beyond the traditional understanding based on the Artland-Zirnbauer ten-fold classification that a 1D free-fermion topological phase necessitates the protection of particle-hole like symmetries,and observed in experiment such an exotic statein an engineered synthetic crystal with ultracold atoms. This work is indeed the first experimental realization of an SPT phase for ultracold atoms, which opens a great deal of possibilities tosimulate and probenovel SPT physics.In particular, owing to the advantages of the full controllability, it is expected that the present work shall push forward future studies in ultracold atom experiments of interacting SPT phases, which are broadly discussed in theory but very hard to investigate in solid-state materials

Prof. Xiong-Jun Liu from PKU and Prof. Gyu-Boong Jo from HKUST are corresponding authors of the paper. Bo Song, Long Zhang, and Chengdong He are co-first authors. The work is supported by National Key R&D Program of China, NSFC, Thousand-Young-Talent Program of China, Hong Kong Research Grants Council, and the Croucher Foundation.

最近北京大学量子材料中心刘雄军研究小组和香港科大Gyu-Boong Jo研究小组合作完成一项理论和实验结合研究,首次在一维超冷原子光晶格中实现和观测对称保护拓扑物态。该工作以研究长文(Research Article)的形式发表于近期的Science子刊上:Science Advances, 4, eaao4748 (2018)。刘雄军和Gyu-Boong Jo为该工作的共同通讯作者。

拓扑量子物相在过去十年里已发展成为凝聚态物理及超冷原子量子模拟的主流研究方向。相关研究大大加深了对量子物质的基本理解。依据拓扑物态的存在是否依赖于系统的对称特性,可将拓扑相分类成内秉拓扑物态和对称保护拓扑物相。前者不依赖于对称保护,而后者需有对称保护才可稳定存在。其中,在一维量子系统中,已有理论表明拓扑相的存在总是需要相关对称性的保护。对于自由费米子体系,基于熟知的Artland-Zirnbauer十重分类的拓扑理论告诉我们,得到一维拓扑物相需要有类似于粒子-空穴对称的对称保护。在合作小组完成的该篇文章中,他们发现一种新的并非由传统熟知的粒子-空穴或子晶格对称保护的一维拓扑物相。该拓扑态被证明由一种隐藏的滑移镜像对称和非局域手征对称保护。这超出传统的基于Artland-Zirnbauer十重类的拓扑分类理论的范畴。另外,这项工作部分基于刘雄军等人较早期基于拉曼光晶格体系提出的AIII类拓扑物态(2013年PRL工作)。进一步,基于所实现的拓扑体系,他们理论预测、且实验观测到与量子体系拓扑特征相关的两类基本量子动力学行为。这对研究拓扑体系的非平庸量子动力学带来启发。

值得指出,超冷原子中模拟拓扑物相在近三年陆续取得实验进展,而这项工作首次在超冷原子中报道实现由对称保护的拓扑物相,并研究新奇量子动力学行为。这将推动在超冷原子中探索各类对称保护拓扑物性的广泛发展。

刘雄军小组张龙博士和香港科大Bo Song,Chengdong He是工作的共同一作。刘雄军小组博士生潘挺峰亦有实质贡献。该工作获得科技部重点专项,国家自然科学基金委,国家青年千人基金的支持。

 

Fig.1Ultracold atoms in an engineered crystal of light.