复合量子结构中的拓扑量子态与电子纠缠研究

The search for Majorana fermions is one of the most important research tasks in physics today. First, from a fundamental point of view, these particles constitute a special family of quantum particles that were predicted at the early stage of developing quantum electrodynamics theory more than 70 years ago, but have never been found. Second, recent theoretical predictions show that these particles could be realized in solid-state systems, and if found, their properties (e.g., topological protection and non-abelian statistics) could make them useful for quantum information processing. The purposes of this project include: (1) to explore topological superconductor quantum structures realized using strong spin-orbit coupled semiconductor nanowires and 3D topological insulator thin films in the proximity of external s-wave superconductors, in which Majorana fermion bound states can exist, to study the physics properties of Majorana fermions and the novel physical phenomena arising from the presence of Majorana fermions, to study the statistical properties of Majorana fermions; (2) to study the quasiparticle statistics and interference effect of 5/2 fractional quantum Hall state in a high mobility two-dimensional electron gas with the aim to clarify some important issues, such as whether the fractional charge of the 5/2 state is e/2 or e/4, and whether the p-wave superconductivity can be observable for this state; (3) to develop a new family of entangled electron source devices based on the superconductor-semiconductor nanowire hybrid quantum structures and the superconductor-topological insulator hybrid quantum structures, and to study the entanglement of the split electrons in the devices.

在凝聚态体系中寻找探测满足非阿贝尔统计的拓扑量子物态的准粒子，特别是Mojarana费米子，是当代物理学最重大研究课题之一。这些准粒子由于受拓扑保护，具有可以预见的长相干时间，在固态量子信息处理和量子计算领域也有诱人的应用前景。本项目联合北大、物理所、中科大研究组，将s波超导体与窄带隙半导体纳米线和拓扑绝缘体薄膜相结合制作复合量子结构，通过极低温条件下的精密电学、磁致输运、高频测量研究量子结构中的拓扑量子态与电子纠缠等基本物理问题和调控手段。研究内容包括：基于s波超导体和窄带隙半导体纳米线所构造的复合量子器件中的拓扑量子物态和Majorana费米子研究；基于s波超导体和三维拓扑绝缘体界面的拓扑量子物态研究和Majorana费米子的探测；基于高迁移率二维电子气分数量子霍尔态的拓扑量子物态研究；超导体与半导体纳米线以及拓扑绝缘体构成的复合器件中的库珀对电子分离与纠缠研究。

本项目以基于半导体纳米线或拓扑绝缘体的超导复合器件和砷化镓高迁移率二维电子气为研究对象，以基于拓扑量子态的相干输运和纠缠为主要研究目标。本项目集合了材料生长、器件研制和极低温条件下的微弱信号测量等多个研究内容，主要研究任务分成三部分：(1) 基于半导体纳米线的单量子点和多量子点器件，及其与超导电极复合器件的制备技术和测量技术，探索该体系内的拓扑多体物理现象及其相干过程；(2) 基于s波超导体和三维拓扑绝缘体的约瑟夫逊器件，探索Majorana束缚态和基于高迁移率二维电子气器件探索拓扑超导物态；(3) 超导体和纳米材料构成复合器件中的库珀对电子分离与纠缠物理研究。围绕以上研究目标，项目组成员刻苦攻关，基本完成了研究任务。项目取得了多项处于国际前沿水平的成果，主要包括：率先制作出基于InAs纳米线三量子点器件并对其物理特性进行了精细的电学测量，率先研制出基于InSb纳米线的弹道约瑟夫森结器件以及纳米线量子点-超导体复合量子器件，观察到特征的相变现象；通过研究拓扑绝缘表面态及其邻近效应，实验验证了Andreev束缚态的存在与手性保持以及Majorana束缚态，实验结果初步证实了了Fu-Kane 模型中Majorana相图的正确性；开展了基于新型半导体二维材料和纳米线材料的量子点制备，实现了超导谐振腔与量子点的耦合以及两量子比特的扩展。通过本项目的实施，我们基本确定了在拓扑量子器件领域中的国内领先地位，具备了在国际范围内参与固态拓扑量子计算这一最前沿和最具挑战性领域竞争的能力。