Quantum Transport Of Josephson Devices Based On Topological Insulators

Since Feynman first proposed the concept of quantum computer in early 1980's,the research has remained in theory for quite a long time.About one or two decades ago,stimulated by information explosion and the gradual failing of the Moore's law,people began the experimental study on various forms of qubits,such as quantum dot-based spin qubits,Josephson junction-based superconducting qubits,cold atom-based qubits,diamond NV center-based qubits,etc.Thanks to its macroscopic quantum nature,the superconducting qubits are superior to other forms of qubits in terms of better scalability and high fidelity.However,their decoherence time and robustness still need to be improved.In order to completely solve the decoherence problem and increase the robustness,Kitaev proposed the concept of topological quantum computing based on non-Abelian states.Along this direction,the most urgent task is to find/construct Majorana zero modes that contain non-local degenerate states and obey non-Abelian statistics up on exchange operations.The information in a topological qubit is stored non-locally on two degenerate states,and the quantum computing processes are realized via exchange operations of these non-local states.Therefore,quantum computing is insensitive to local perturbations.In this thesis,I will present our recent research progresses in searching for Majorana zero modes.The main contents of this thesis are as follows:In chapter 1,we will first introduce the concept of Majorana zero modes,then review the recent research progresses of the field on Majorana zero modes.In condensed matter physics,by introducing the concept of topology,we can understand better the general physics of edge states.In 2001,Kitaev studied a one-dimensional chain of spin-free p-wave superconductor,and proved that there is a Majorana zero mode at each ends of the chain.Due to the non-Abelian statistics nature of Majorana zero modes,which is promising to be used for building topological quantum computers,this research field quickly became a hotspot.On the route of searching for Majorana zero modes,people have discovered a series of experimental signatures or evidences,such as quantized zero-bias conductance peaks,single electron behavior of superconducting islands,fractional Josephson effect,minigap closure in Josephson junctions,etc.Due to the existence of quasiparticle poisoning which kills the Majorana zero mode,recently people began to perform fast?i.e.,high-frequency?measurements,by which hoping to detect the parity change associated with non-Abelian operations of the Majorana zero modes.In chapter 2,we will describe the growth and transfer technique of the samples.We use the Bridgman method to grow high-quality Bi₂Te₃ bulk crystals for mechanical exfoliation,and use the vapor-liquid-solid?VLS?method to grow(Bi_1-xSb_x)₂Te₃ nanowires.We will also present the technique of transferring exfoliated flakes or nanowires to specific locations on substrates.This technique provides the ground for our next-step device fabrications.In chapter 3,we will present a contact-resistance-measurement method to measure the minigap of Josephson junctions constructed on topological insulators.Because it is difficult to find p_x+ip_y superconductors in the real world,Fu and Kane proposed in 2008 to induce p_x+ip_y-like superconductivity in hybrid structures composed of s-wave superconductors and topological insulators.In their effective model and with low-energy approximation,the minigap of proximitized Josephson junctions in such hybrid structures can be described as the mutual coupling of two Majorana states.When the phase difference across the junction is?,the two Majorana states are decoupled,so that the minigap of the Josephson junction is zero.We constructed Josephson junctions on topological insulators and measured the contact resistance of normal-metal electrodes deposited in the junction areas.Our results verified Fu and Kane's theoretical prediction,directly proving the correctness of the low-energy Hamiltonian and indirectly proving the existence of Majorana state in the junctions.When the phase difference between the two superconductors is?,the Josephson junction reaches a free Majorana state which is not local.In order to study local Majorana zero modes,we further fabricated Josephson trijunctions on topological insulators,and then investigated the minigap closure behavior at the center of the trijunctions.We obtained a phase diagram of minigap closure which is the same as Fu and Kane's prediction for Josephson trijunction.Therefore,our results proved that there is a local Majorana zero mode at the center of the trijunction.In chapter 4,we will present our experimental investigations on superconducting qubits based on topological insulators as well as on those of superconducting qubits based on ordinary superconductors for comparison.In order to quickly measure the parity of a system containing Majorana zero modes and to test the non-Abelian statistical behavior,we fabricated superconducting qubits based on topological insulator and performed high-frequency measurements on them.Our primary experimental efforts pave the way for further investigating the parity of the systems via fast read-out technique.Chapter 5:Summary and Prospect of this thesis.