Propagation And Localization Of Collective Excitations Of Superconducting Qubits

With the gradual failure of Moore's Law,the development of classical computers gradually reached the bottleneck.Due to the superposition of quantum mechanics,the speed of quantum computing in certain algorithms will be much faster than that of classical computers in theory.Google has achieved quantum supremacy in 2019,futher proving the superiority of quantum computing.As a potential alternative to classical computing,quantum computing has attracted more and more attention from scientists.It has broad application prospects in cryptography,cloud computing,biopharmacy,financial analysis and other fields.At present,the main possible schemes to realize gate-model quantum computation are superconducting quantum computation,ion trap quantum computation,optical quantum computation,quantum dot quantum computation and so on.Among them,superconducting quantum computing is considered to be one of the most promising schemes to realize universal quantum computing due to its superior properties such as high precision control,long decoherence time and tunable coupling strength.There are three research areas of quantum computation:quantum annealing,quantum simulation,and gate-model quantum computation.Quantum annealing is mainly used to search the combinatorial optimization problem and local optimal value problem of discrete space.Quantum simulation mainly simulates the evolution of quantum many-body system with time and studies the physical process of quantum many-body system.The goal of universal quantum computing is to realize arbitrarily quantum algorithm,which is the ultimate goal of quantum computing and also the most difficult one.There are many algorithms to implement general quantum computing,such as surface code,quantum random walks,etc.Among them,surface code is considered to be one of the most worthy schemes.In surface code,per-step fidelity threshold is about 99%to realize error correction.The higher the fidelity of per-step is,the less qubits needed is needed.Thus,how to improve the fidelity of single-qubit gate,two-qubit gate and readout of the qubits has become a research hotspot.In recent years,scientists have achieved single-qubit gate of 99.9%fidelity and two-qubit gate of 99.4%fidelity in superconducting quantum computing area,which place Josephson quantum computing at the fault-tolerant threshold for surface code error correction.However,fixed coupling strength qubits design adapted has some principle problems,like residual zz coupling,which becomes worse when quantity gets more,and the frequency crowding becomes an another problem.These two problems will decrease fidelity of two-qubit gate.The first part of this thesis demonstrates we get high fidelity of single-qubit gate and two-qubit gate(99.8%)through tunable coupling strength qubit design,meanwhile we can decrease the influcnce of the residual zz coupling and frequency crowding when the system expands.This thesis will promote the development of gate-model quantum computing based on surface code.In quantum simulation experiments,the dynamics of quantum many-body systems is a research hotspot,including quantum random walks,quantum localization phenomenon,etc.The Bose-Hubbard model,one of the most prominent models in condensed matter physics,embraces rich underlying physics of strong correlated systems,and has been investigated experimentally in optical lattices and circuit quantum electrodynamics.Many novel dynamical phenomena of the Bose-Hubbard model have been observed in 1D and 2D systems,such as the dynamical behaviors of quantum phase transitions between the superfluid and Mott insulators,the localization with disordered local potentials,and the stabilization of Mott insulators.Different from the 1D case,the Bose-Hubbard ladder can show unique emergent effects,especially,topological effects,which have been studied both theoretically and experimentally.This thesis would do research on whether there are any special dynamical properties in the ladder model,which are distinct to the 1D case.The second part of this thesis demonstrates the implement of many-body quantum system simulation of the Bose-Hubbard ladder model in an integrated 24-qubit superconducting process,through multi qubits exciation,frequency align and other high precision control of more than 20 qubits.We observe completely different unique properties of the single excitation and double excitation modes.This shows the superconducting quantum chip as a powerful quantum simulation platform,and has important guide to statistical research of strong correlation many-body systems,also lays a solid foundation for the study of many-body physical systems by using many-body quantum qubits system.This is the first time that more than 20 qubits with high-precision quantum coherence control has been achieved in a solid-state quantum computing system in the world.