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

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, Hong Kong Research Grants Council, and the Croucher Foundation.