Superconducting Devices And Hybrid Superconducting Qubits Based On Topological Insulator Nanowires

With the development of nanofabrication technology,the size of nowadays integrated circuits is gradually approaching to the limit where quantum phenomenon rules and the Moore’s Law fails.To meet the explosively growing requirements of data processing in the future,quantum computing technology comes in to the attention of scientists.The research on superconducting quantum computing has been advanced rapidly in recent years.Due to the balanced advantages in scalability and control,superconducting quantum chip has been scaled up to hundreds of qubits,making it one of the most promising candidates for realizing universal quantum computation.However,superconducting qubits are also highly susceptible to the electromagnetic noise in the environment,resulting in the difficulty of improving their coherence time and fidelity in quantum gate operations.This problem becomes particularly destructive as the number of qubits increases.It poses significant challenges in chip design,nanofabrication,low-temperature operations and measurement,if future universal superconducting quantum computation requires a large number of physical qubits resources for error correction.To overcome this challenge,topological quantum computing has been proposed.In this scheme,the qubits are composed of topologically protected non-local Majorana zero modes（MZMs）,and the computing processes are via the braiding operations of the MZMs.It is expected that the intrinsic fault-tolerant nature of the topological qubits will help achieving long coherence time and high-fidelity quantum gates.However,the implementation of topological qubits also faces many challenges,such as in improving the device quality and in avoiding quasiparticle poisoning.A feasible way to address the problem of quasiparticle poisoning is to integrate topological qubits into a circuit quantum electrodynamics（cQED）system,which enables fast manipulation and readout within a limited time window characterized by the quasiparticle poisoning time.For improving the device quality,one can try to construct artificial Kitaev chains with more controllable degrees of freedom.In this thesis,we will present the results of our explorations in the two aspects mentioned above.The main content of this thesis is as follows:In Chapter 1,we will give a brief introduction to the concept and the related development of quantum computing.In particular,we will provide a systematic introduction to two physical implementation systems:topological quantum computing and superconducting quantum computing,including the detailed descriptions of Majorana zero modes and the cQED technology.In Chapter 2,the related experimental techniques will be introduced,including the vapor-liquid-solid（VLS）synthesis of topological insulator nanowires,the simulation and design of superconducting qubits,the fabrication process of hybrid qubits,and our optimization on the ultra-low temperature measurement and control system.In Chapter 3,we will present our theoretical and experimental studies on hybrid superconducting qubits.Theoretically,we proposed that the hybrid split transmon coupled to Majorana zero modes can be used to simultaneously detect the parity mixing and the 4π-period Josephson effect induced by Majorana coupling,which can be used to verify the existence of Majorana zero modes.Experimentally,we fabricated split transmon qubits based on Bi₂Se₃ topological insulator nanowires,demonstrated their flux-tunable spectrum and coherent Rabi oscillations with a characterized qubit lifetime T₁～0.5μs,and estimated the lower bound of quasiparticle poisoning time in our device to be 1μs.This time scale indicates the feasibility of using the currently achievable rf techniques to braid MZMs.Our result also demonstrates the feasibility of integrating topological insulator nanowires into cQED systems,to form a platform for studying the rich physics in topological materials through cQED techniques.In Chapter 4,we will firstly introduce the theoretical and experimental progresses in the field in constructing artificial Kitaev chains.Then,we will present our results of transport measurements on Andreev molecules based on(Bi_xSb_1-x)₂Te₃ nanowires.Our preliminary results indicate that a strong coupling can be achieved between the two Andreev bound states in two neighboring Josephson junctions,when the two junctions constructed on a(Bi_xSb_1-x)₂Te₃ nanowire are separated by about 200 nm.This would lay the foundation for future constructing artificial Kitaev chains.In Chapter 5,we will summarize the above contents and provide an outlook on future research in the field.