ICQM Faculty member Xiong-Jun Liu's group and collaborators publish an article in “Science” about Realization of 2D Spin-orbit Coupling for Bose-Einstein Condensates

Fig. 1：The sketch of realization of 2D SO coupling. The laser induced optical and Raman lattice potentials lead to spin-conserved and spin-flip hoppings along x and y directions, giving 2D SO coupling.

In a recent Article published in Science (Science 354, 83-88 (2016)), the two teams, respectively led by Prof. Xiong-Jun Liu at Peking University and by Prof. Jian-Wei Pan & Prof. Shuai Chen at University of Science and Technology of China, have proposed in theory and realized in experiment the two-dimensional (2D) spin-orbit (SO) coupling and topological bands through a Bose-Einstein Condensate in optical lattice.

The SO interaction of an electron is a relativistic quantum mechanics effect that characterizes the coupling between the motion and spin of the electron. The SO interaction plays an essential role in many prominent effects, with their studies having led to several important research areas, including spintronics, topological insulators, and topological superconductors. On the other hand, there has been considerable interest in emulating SO effects and topological phases with cold atoms, driven by the fact that cold atoms can offer extremely clean platforms with full controllability to explore such exotic physics. Nevertheless, in the past five years, only the 1D SO interaction, which corresponds to an Abelian gauge potential, has been realized for ultracold atoms. Realizing higher dimensional SO couplings, which correspond to non-Abelian gauge potentials, is however much more important, and indeed necessary for the study of broader range of nontrivial topological phases. As a result, to realize a 2D SO interaction became a foremost outstanding goal in the field of ultracold atoms.

In the published article, the PKU and USTC teams propose in theory and realize in experiment two-dimensional (2D) SO coupling and topological bands for a rubidium-87 degenerate gas through an optical Raman lattice, without phase-locking or fine-tuning of optical potentials. A controllable crossover between 2D and 1D SO couplings is studied, and the SO effects and nontrivial band topology are observed by measuring the atomic cloud distribution and spin texture in momentum space. The Hamiltonian realized here describes a minimal quantum anomalous Hall model driven by SO interaction. Our realization of 2D SO coupling exhibits a couple of essential advantages, including the small heating and topological stability, and can open a broad avenue in cold atoms to study exotic quantum phases, including topological superfluids. Prof. Xiong-Jun Liu at PKU, Prof. Jian-Wei Pan and Prof. Shuai Chen at USTC are corresponding authors of the article. The work is supported by NSFC, MOST, CAS.