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中国物理学会期刊

双链超导量子电路中的拓扑非平庸节点

CSTR: 32037.14.aps.72.20230152

Topological nonmediocre nodes on two-leg superconducting quantum circuits

CSTR: 32037.14.aps.72.20230152
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  • 拓扑无能隙系统作为不同量子相的连接, 目前已经成为备受关注的前沿科学. 超导量子电路作为一个重要的全固态量子器件是宏观调控量子效应的优秀平台. 本文在超导量子电路中构建了双链的Su-Schrieffer-Heeger (SSH)模型并发现了拓扑非平庸的节点. 首先设计了电容耦合的双链transmon比特, 之后用两个交流微波驱动每一个transmon比特, 从而实现比特间耦合强度的独立调控, 最后通过选择比特间合适的耦合参数实现交错的双链SSH模型. 接下来探索了交错双链SSH模型的拓扑性质, 首先计算了k空间中双链SSH模型的本征能量, 并发现了两种类型的相边界. 之后在参数空间中画出了拓扑相图, 发现了两类拓扑绝缘相, 其拓扑数分别为1和–1, 对应有两类边界态. 拓扑相图也进一步给出了两类相边界的分布以及它们两侧拓扑数的值. 最后分析了两类相边界的拓扑性质, 发现其中一类拓扑相边界对应的能带有两个拓扑非平庸的节点. 本文的工作为探索链条型物理系统、拓扑物态以及节点型拓扑半金属提供了新的途径.

     

    Topological gapless systems, as the connection of the different topological quantum phases, have received much attention. Topological nonmediocre nodes are typically observed in two- or three-dimensional gapless systems. In this paper, we demonstrate that the topological nonmediocre nodes are existent in a model that lies between one dimension and two dimensions. Superconducting circuits, as essential all-solid state quantum devices, have offered a promising platform for studying the macro-controlling quantum effects. Recently, experimental achievements have enabled the realization of tunable coupling strengths between transmon qubits and the implementation of a one-dimensional Su-Schrieffer-Heeger (SSH) model Li X et al. 2018 Phys. Rev. Appl. 10 054009. According to this work, herein we present a two-leg SSH model implemented in superconducting circuits and demonstrate the existence of topological nonmediocre nodes. Firstly, two-leg superconducting circuit with transmon qubits which are coupled with their nearest-neighbor sites by capacitors is designed. To construct the two-leg SSH model, we introduce two alternating-current magnetic fluxes to drive each transmon qubit. We discover two types of phase boundaries in the SSH model and obtain the corresponding energy spectra and phase diagram. We identify two distinct topological insulating phases characterized by winding number ±1, and the corresponding edge states exhibit distinct characteristics. Moreover, we discuss the topological properties of the two phase boundaries. By representing the Bloch states as a vector field in k space, we demonstrate the existence of two kinks of nonmediocre nodes with first-type phase boundaries. These two nonmediocrenodes possess distinct topological charges of 1 and –1, respectively. On the other hand, the nonmediocre nodes with the second-type phase boundaries are topologically trivial. These results open the way for exploring novel topological states, ladder physical systems, and nodal point topological semimetals.

     

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