Topological Graphene Multiterminal Josephson Junctions

量子系统的拓扑性质近年来引起了理论和实验的广泛关注。非平凡的拓扑性质通常来源于材料的固有性质，例如，具有强自旋轨道相互作用的材料中的拓扑带结构。另一种可能性是在混合结构中结合不同的材料来创建拓扑带结构。这些实现需要复杂的材料，通常难以合成。近年来，人们预测由传统材料制成的多终端约瑟夫森结在超导终端宏观相控制的人工带结构中具有拓扑非平凡态。这些系统提供了通过外部参数原位调整拓扑特性的可能性，因此为拓扑研究提供了相当大的灵活性。这个项目的目的是联合实验和理论努力来实现和理解多终端约瑟夫森结的人工拓扑带结构。我们的项目选择的材料是石墨烯。由于其二维特性，石墨烯提供了充分的设计灵活性，而其固有的高质量和栅极可调性允许将弹道输运控制到单个量子通道。因此，石墨烯是研究拓扑多终端约瑟夫森结的主要候选材料。我们计划通过量子化非局域电导和微波光谱在最小系统（四端约瑟夫森结）中明确地识别和表征拓扑安德烈夫态。长期目标是使用石墨烯多端结来模拟更高有效维度的拓扑结构。

Topological properties of quantum systems have recently attracted considerable theoretical and experimental attention. Nontrivial topological properties usually follow from intrinsic material properties, e.g., topological band structures in materials with strong spin-orbit interaction. Another possibility is the combination of different materials in hybrid structures to create topological band structures. These implementations require complex materials which are often difficult to synthesize.Recently, it has been predicted that multiterminal Josephson junctions made of conventional materials host topologically nontrivial states in an artificial band structure controlled by the macroscopic phases of the superconducting terminals. These systems offer the possibility to tune topological properties in situ by external parameters, and therefore allow considerable flexibility for the investigation of topology.The aim of this project is a joint experimental and theoretical effort to implement and understand artificial topological band structures in multiterminal Josephson junctions. The material of choice for our project is graphene. Due to its two-dimensional nature, graphene offers full flexibility of design, while its intrinsic high quality and gate tunability allows control of ballistic transport down to single quantum channels. Therefore, graphene is a prime candidate for the investigation of topological multiterminal Josephson junctions. We plan to unambiguously identify and characterize topological Andreev states via quantized nonlocal conductance and microwave spectroscopy in a minimal system, a four-terminal Josephson junction. A long-term goal is to use graphene multiterminal junctions to simulate topology in higher effective dimensions.