Quantum Control And Many-body Physics In Superconducting Circuits

Quantum computers are built upon two-level systems obeying the quantum mechanical laws.Due to the quantum behaviors like quantum superposition and quantum entanglement,quantum computers have great advantages over classical computers in solving specific prob-lems,such as database searching,prime factoring and machine learning,which has attracted a lot of attention and investment from scientists,companies,and the government.During the past few decades,scientists have developed many physical platforms for realizing quantum com-puters,among which superconducting quantum computing based on Josephson junctions has become the leading candidate for its high control accuracy,good scalability and long coherence time.Recently,great achievements have been made in superconducting quantum computing.Fidelities of universal quantum logic gates have reached the fault-tolerance threshold for sur-face code error correction;large multiqubit systems with more than 50 superconducting qubits have been demonstrated.These achievements make it a good platform eminently suitable for demonstrating noisy intermediate-scale quantum computing,which provides us a new tool to solve many useful problems.Despite all the achievements,the general consensus is that the ultimate goal of realizing a fault-tolerant universal quantum computer still requires long-term efforts.In this context,two major research directions,improving the quality of qubits(e.g.,co-herence time and control fidelity)and exploring new applications for current quantum devices,are pursued in parallel.In this thesis,we will discuss our experimental achievements in the two research directions,the dephasing-insensitive quantum information processing and quantum simulation of many-body mobility edge.The thesis is structured as follows.We start with an overview of quantum computing in chapter 1,including the history,background and basic concepts,which will give readers a whole picture of the field.Then,chapter 2 focuses on the transmon superconducting qubit,where its structure,control,interaction,measurement and scalability are discussed in de-tail guided by DiVincenzo criteria.Based on the previous two chapters,our research progress on the suppression of qubit dephasing and two-qubit gate with inherent dynamical decoupling is presented in chapter 3.We propose and demonstrate a dephasing-insensitive quantum com-putation scheme in superconducting circuits with all-to-all connectivity,which prolongs qubit dephasing time by an order of magnitude during the storage and processing of qunatum informa-tion.This scheme has been used in a series of subsequent experiments.Chapter 4 describes our research progress on quantum simulation.We investigate a theoretically debated phenomenon-many-body mobility edge with 19 superconducting qubits.We observe that the onset of many-body localization depends on energy,which opens an avenue for the experimental exploration of this debate with quantum simulators.In the last chapter,we conclude the thesis with a sum-mary of my research work,where my recent work-quantum simulation of Stark many-body localization is also included,and a brief outlook on future research.