Simulation of higher-order topological phases and related topological phase transitions in a superconducting qubit

利用超导量子比特模拟高阶拓扑相及相关拓扑相变

Higher-order topological phases give rise to new bulk and boundary physics,as well as new classes of topological phase transitions.While the realization of higher-order topological phases has been confirmed in many platforms by detecting the existence of gapless boundary modes,a direct determination of the higher-order topology and related topological phase transitions through the bulk in experiments has still been lacking.To bridge the gap,in this work we carry out the simulation of a twodimensional second-order topological phase in a superconducting qubit.Owing to the great flexibility and controllability of the quantum simulator,we observe the realization of higher-order topology directly through the measurement of the pseudo-spin texture in momentum space of the bulk for the first time,in sharp contrast to previous experiments based on the detection of gapless boundary modes in real space.Also through the measurement of the evolution of pseudo-spin texture with parameters,we further observe novel topological phase transitions from the second-order topological phase to the trivial phase,as well as to the first-order topological phase with nonzero Chern number.Our work sheds new light on the study of higher-order topological phases and topological phase transitions.

高阶拓扑相的出现带来了新的体态物理和边界态物理,以及新的拓扑相变类型.尽管研究者们通过探测边界态的方式,已在许多平台上证实了高阶拓扑相的存在,但通过测量体态来直接确定高阶拓扑相及相应的拓扑相变还缺乏实验研究.为了填补这一空白,作者利用超导量子比特模拟研究了二维体系的二阶拓扑相及其拓扑相变.得益于量子模拟器的灵活性和可控性,作者首次通过探测体态动量空间中的赝自旋构型,直接确定了高阶拓扑的实现,与以往基于实空间中无能隙边界态的探测实验形成鲜明对比.通过测量赝自旋构型随参数的演化,作者观察到新的拓扑相变类型,包括从二阶拓扑相到拓扑平庸态、以及向陈数不为0的一阶拓扑相的相变.该工作为高阶拓扑相和拓扑相变的研究提供了新的思路.