Quantum Control And Simulation Based On Superconducting Qubits

Quantum computers hold the potential of outperforming any classical supercomputers at prac-tical problems such as factorization,optimization,and quantum simulation.Despite their incredible power,the physical realization remains a great technical challenge.However,recent progresses in coherence,scalability,and controllability for quantum computing based on superconducting cir-cuits have shed light on the research and attracted considerable attention from many research insti-tutes and technology companies all over the world.Within a few years,superconducting quantum devices are likely to scale up to 50 qubits or more,which might be powerful enough to solve certain problems that are intractable on classical supercomputers.This thesis covers all author's experimental activities related to the precise control of super-conducting qubits.The first three chapters review the basic knowledge on quantum computing and superconducting qubits.Chapters 4 and 5 focus on the author's own experiments on two types of qubits,the phase qubits and Xmon qubits.As phase qubits have relatively short coherence times,the author and coworkers have found a way of combining two known decoherence suppression methods,quantum uncollapsing and dynamical decoupling,based on which both energy relaxation and dephasing processes can be suppressed effectively.In another work done by the author and coworkers,correlations between the environmental noises of different superconducting qubits on the same chip have been investigated,leading to the conclusion that the state transfer along an array of physical qubits can suppress dephasing.Later on after switching to Xmon qubits which exhibit excellent coherence performance,the author and coworkers have taken a big step toward build-ing large-scale quantum processors.Using a superconducting chip with high connectivity between qubits,they have produced and characterized the genuinely entangled Greenberger-Horne-Zeilinger state with up to ten qubits,achieving the largest entanglement created so far in solid-state architec-tures.The fidelity obtained by 10-qubit quantum state tomography is about 66.8%.Furthermore,they have utilized this quantum processor to simulate the many-body localization effect in con-densed matter physics,providing the direct experimental evidence of the long-time logarithmic growth of entanglement entropy for the first time.Chapter 6 is the conclusion and perspective.Coherence and scalability have always been the central issues of realizing large-scale quantum computing.The author and coworkers' experiments provide valuable experience and technical support in dealing with these key issues.Furthermore,they have presented a successful experiment of emulating many-body localization with a 10-qubit superconducting processor,which indicates that it is promising to investigate quantum many-body physics on the platform of superconducting qubits.