Topological Insulator-based Josephson Quantum Devices

Due to its non-Abelian statistics,Majorana zero-energy modes（MZMs）have attracted much attention in condensed matter physics.Theorists have proposed a variety of schemes to search for MZMs and using the MZMs to build topological quantum computers.The scheme based on Josephson trijunction on topological insulators,which has the advantages in controllability and scalability while constructing multiple qubits,has previously been studied experimentally and proven to be feasible in some extent.In this thesis work,I focus on to explore the possible coupling effects between two Josephson trijunctions.Some other efforts have also been paid to identify the anomalous Josephson effect,and to explore using topological insulator-based Josephson junction to construct superconducting qubit.This thesis is divided into the following three parts:In the first part,I give a brief introduction on multi-terminal Josephson junctions based on topological insulator.Since a single trijunction is expected to be capable to accommodate MZM,we fabricated devices containing two trijunctions on the surface of topological insulator Bi₂Se₃,to study possible interaction between the MZMs.By controlling the phase differences of the two trijunction,we observed complete minigap closing in each trijunction.Then,we drive both trijunctions into the minigap closing region,expecting to observe minigap re-opening due to the hybridization of the MZMs.We find that,with the variation of the distance between the two trijunctions,the minigap re-opening phenomenon is not found.Further experimental study and data analysis are needed.The second part of the thesis is about the observation of the anomalous Josephson effect in topological insulator-based Josephson trijunction.In the first part,when in-plane magnetic field is applied,the anomalous phase shift accumulates in the junctions,allowing us to observe an anomalous Josephson effect in the device.In order to better study and analyze this effect,we fabricated"T"-type Josephson trijunction.We found that this effect exhibits strong anisotropy with the direction of magnetic field.It unambiguously modifies the Majorana phase diagram of the trijunctions,from which the existence of anomalous Josephson effect can be identified.In the third part we present our explorative experimental work on constructing superconducting qubit based on topological insulators.In the field of quantum computing,superconducting qubit is currently a relatively prevail route.It can be used to help us to verify the existence of MZM in topological insulator-related systems and realizing topological quantum computing.We started with fabricating and measuring traditional Al-Al₂O₃-Al Josephson junction superconducting qubits.Then,we replaced the Al-Al₂O₃-Al junctions with topological insulator-based junctions,and having the devices characterized by high-frequency measurements.Interesting preliminary results were obtained.It indicated that the quality of Josephson junction based on topological insulators needs to be further improved.We wish that the three works mentioned above could improve our knowledges on Josephson trijunction,on the anomalous Josephson effect,and on using these devices and effects to search for Majorana zero mode and to build topological qubits.