Zero energy modes in vortex cores: Spectroscopy and Majorana carousel braiding

摘要：量子计算机有望在信息技术、量子化学、安全通信、材料建模和许多其他应用中实现变革性的进步。铝基超导量子比特显示出作为量子计算机构建模块的有前途的特性。然而，即使最近在这一领域取得了令人印象深刻的进展，构建一个功能齐全、实用的量子计算机仍然遥不可及，因为现有的量子位非常迅速地松散了量子信息。如果理论预测的所谓非阿贝尔激励能够在固态纳米级器件中实现，这个问题就可以解决。这种假设的非阿贝尔激励有一个有用而又非常违反直觉的性质。即使它们是相同的，如果其中两个交换位置，它们就会共同进化成新的量子态。该特性可用于执行无噪声量子计算，从而免受环境干扰。PI和研究小组正在开发纳米级电子设备，可以在其中展示非阿贝尔现象。该方法的动机是最近的理论结果，表明这些激励如何有效地从环境引起的扰动中分离出来。研究小组使用传统的超导量子比特、微波技术和量子隧道接触来进行能量测量，并揭示了假设的非阿贝尔马约拉纳模式。一种特殊的柯比诺盘几何利用了包含这些奇异激发的涡旋的量子力学行为。这项研究为研究生和本科生提供了现代物理训练，并为下一代科学家和工程师提供了广泛的指导。技术摘要：利用马约拉纳零模式进行拓扑保护量子计算，已经提出了许多理论模型。然而，它们的关键性质，即非阿贝尔物理，尚未得到实验证明。研究马约拉纳模式的传统方法是将半导体纳米线耦合到超导体上。本项目探索了一条不同的路线。PI和他的团队研究拓扑超导体超导涡旋核心中的马约拉纳模式。目标是将马约拉纳模式从普通电子状态中分离出来，并证明假设的非阿贝尔效应，如果发现这种效应，将有助于解决量子信息技术的关键问题。PI和研究小组进行微波和隧道实验来测量拓扑涡旋核的离散光谱。这类实验的一个重要障碍是在涡旋中存在大量的低能量非拓扑激励。这样的电子可以与马约拉纳模式混合，使非阿贝尔物理无法检测到。研究小组使用最新的理论模型来设计和研究新的装置，在这种装置中，涡流被困在一个超导样品中，该样品有一个圆形孔，底部有一个拓扑超导体或拓扑绝缘体。在这种装置中，马约拉纳能隙被预测为足够大，可以将马约拉纳模与非拓扑激励隔离开来。该研究小组还在开发下一代这样的设备，其中漩涡可以在corbino几何装置中沿着圆形轨迹移动。这种圆周运动被用来交换马约拉纳模式的位置，并引起系统量子态的非平凡变化。马约拉纳模式是用普通量子比特探测和控制的。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical Abstract: Quantum computers are expected to enable transformative advances in information technology, quantum chemistry, secure communications, material modeling, and in many other applications. Aluminum-based superconducting qubits demonstrate promising characteristics as building blocks for quantum computers. Yet, even with the recent impressive advances in this field, building a fully functional and practically useful quantum computer is still beyond reach, since existing qubits loose quantum information very quickly. The problem can be solved if the theoretically predicted, so-called non-Abelian excitations can be realized in solid-state nanoscale devices. Such hypothetical non-Abelian excitations have a useful and very counterintuitive property. Even though they are identical, if two of them exchange places, they collectively evolve into new quantum states. This property can be used to perform noise-proof quantum computation, which is protected from environmental interference. The PI and the research team are developing nanoscale electronic devices in which non-Abelian phenomena can be demonstrated. The approach is motivated by recent theoretical results suggesting how these excitations can be effectively isolated from perturbations caused the environment. The research team uses traditional superconducting qubits, microwave technology, and quantum tunneling contacts to perform energy measurements and to uncover the hypothetical non-Abelian Majorana modes. A special Corbino disk geometry exploits quantum mechanical behavior of vortices containing these exotic excitations. This research offers modern physics training for graduate and undergraduate students and provides extensive mentoring to the next generation of scientists and engineers.Technical Abstract: Many theoretical models have been suggested in which Majorana zero modes are used for topologically protected quantum computation. Nevertheless, their key property, namely the non-Abelian physics, has not yet been demonstrated experimentally. The traditional approach to study Majorana modes is to employ semiconductor nanowires coupled to superconductors. The present project explores a different route. The PI and his team study Majorana modes in the cores of superconducting vortices in topological superconductors. The goal is to isolate Majorana modes from ordinary electronic states and to demonstrate the hypothetical non-Abelian effects, which, if found, will help to solve key problems of quantum information technology. The PI and the research team perform microwave and tunneling experiments to measure the discrete spectrum of the topological vortex cores. An important obstacle in such experiments is the large number of low-energy non-topological excitations present within vortices. Such electrons can mix with the Majorana modes, making non-Abelian physics undetectable. The research group uses recent theoretical models to design and study novel devices in which the vortex is trapped within a superconducting sample having a circular hole with a topological superconductor or a topological insulator at the bottom. In such devices, the Majorana energy gap is predicted to be sufficiently large to isolate the Majorana modes from non-topological excitations. The research team is also developing the next generation of such devices, in which vortices can move following circular trajectories in Corbino-geometry devices. Such circular motion is used to exchange positions of Majorana modes and to induce nontrivial changes in the quantum state of the system. The Majorana modes are probed and controlled using ordinary qubits.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.