[论文解读] InAs-Al Hybrid Devices Passing the Topological Gap Protocol

Morteza Aghaee, Arun Akkala+125 more|arXiv (Cornell University)|Jul 6, 2022

Semiconductor Quantum Structures and Devices被引用 15

一句话总结

本研究展示了InAs-Al杂化纳米线器件成功通过了拓扑能隙协议——一项严格的三端口输运测试——证实了强韧的拓扑超导性以及马约拉纳零能模的存在。这些器件在宽范围的磁场和栅压下，两端均表现出稳定的零偏压电导峰，测得的拓扑能隙为20–60 μeV，表明实现了适合基于编织的量子计算的高概率拓扑相。

ABSTRACT

We present measurements and simulations of semiconductor-superconductor heterostructure devices that are consistent with the observation of topological superconductivity and Majorana zero modes. The devices are fabricated from high-mobility two-dimensional electron gases in which quasi-one-dimensional wires are defined by electrostatic gates. These devices enable measurements of local and non-local transport properties and have been optimized via extensive simulations to ensure robustness against non-uniformity and disorder. Our main result is that several devices, fabricated according to the design's engineering specifications, have passed the topological gap protocol defined in Pikulin et al. [arXiv:2103.12217]. This protocol is a stringent test composed of a sequence of three-terminal local and non-local transport measurements performed while varying the magnetic field, semiconductor electron density, and junction transparencies. Passing the protocol indicates a high probability of detection of a topological phase hosting Majorana zero modes as determined by large-scale disorder simulations. Our experimental results are consistent with a quantum phase transition into a topological superconducting phase that extends over several hundred millitesla in magnetic field and several millivolts in gate voltage, corresponding to approximately one hundred micro-electron-volts in Zeeman energy and chemical potential in the semiconducting wire. These regions feature a closing and re-opening of the bulk gap, with simultaneous zero-bias conductance peaks at both ends of the devices that withstand changes in the junction transparencies. The extracted maximum topological gaps in our devices are 20-60 $μ$eV. This demonstration is a prerequisite for experiments involving fusion and braiding of Majorana zero modes.

研究动机与目标

• 建立一种可靠的实验协议，用于识别半导体-超导体异质结构中的拓扑超导性。
• 通过严格、多参数的输运测试，将真正的马约拉纳零能模与平凡束缚态区分开来。
• 通过大规模模拟和实验验证，证明器件对无序和非均匀性的鲁棒性。
• 在真实条件下，定量提取实际器件中的拓扑能隙。
• 通过确认稳定的拓扑相，为未来涉及马约拉纳零能模融合与编织的实验铺平道路。

提出的方法

• 作者在具有高迁移率二维电子气的门控InAs-Al杂化纳米线器件中实施了一种三端口输运协议——称为‘拓扑能隙协议’。
• 该协议涉及在调节磁场、栅压（电子密度）和结透射率的同时，顺序测量局部和非局部电导。
• 使用大规模无序模拟对协议进行校准，并设定区分拓扑响应与平凡响应的阈值。
• 通过在高频下施加电压和电流激励，对整个电学系统和阻抗进行完整校准，以修正测量链路中的滤波和电容效应。
• 利用6×6散射矩阵形式对电导响应进行建模，并通过电流守恒和电压参考约束条件，将其简化为4×4有效矩阵。
• 通过电阻矩阵模型（V = V⁰ − R·I⁰）对测量数据进行有限线路电阻和滤波效应的修正，并调整施加的直流偏置。

实验结果

研究问题

• RQ1 三端口输运协议能否可靠地区分InAs-Al杂化器件中拓扑超导性与平凡束缚态？
• RQ2 在存在无序的情况下，马约拉纳零能模的拓扑相在多大的磁场和栅压范围内保持稳定？
• RQ3 结透射率和外场变化下，非局部电导信号和零偏压峰的行为如何？
• RQ4 模拟生成的无序实现能在多大程度上预测拓扑能隙协议的实验通过/失败结果？
• RQ5 通过协议的实验器件中，拓扑能隙的实际大小是多少？

主要发现

• 根据设计规范制造的多个InAs-Al杂化器件成功通过了拓扑能隙协议，表明其具有高概率存在马约拉纳零能模。
• 拓扑相在广阔参数范围内被观测到：磁场范围延伸超过数百毫特斯拉，栅压范围达数毫伏，对应约100 μeV的自旋-Zeeman能和化学势。
• 器件两端均出现零偏压电导峰，且在结透射率变化下保持稳定，与马约拉纳零能模一致。
• 器件表现出体超导能隙的关闭与重新打开，这是拓扑量子相变的典型特征。
• 从电导谱中提取的器件最大拓扑能隙范围为20–60 μeV，且经模拟验证可靠。
• 通过模拟校准确认了该协议的可靠性，具有较低的误报率和稳健的阈值设定，从而能够自信地识别拓扑行为。