Dephasing And Quantum-state Manipulation In Superconducting Multilevel And Multiqubit Systems

Quantum computing is based upon the principles of quantum mechanics,which has tremendous advantages in solving many specific problems as compared to its classical counterpart.Superconducting quantum circuit is one of the most competitive platforms for quantum computing with low dissipation,flexible control and manipulation,and good scalability and integration,which has attracted much attention and has advanced rapidly in recent years.In this thesis,the design,characterization,and quantum-state control and manipulation of superconducting qubits are systematically studied.The main results and achievements can be summarized as follows.(1)Design,simulation,and fabrication of superconducting qubits.Circuit quantum electrodynamics based Xmon-type superconducting multiqubit devices are designed and their fundamental parameters are simulated by finite element analysis.The device parameters are optimized and achieved in practical micro-nanofabrication processes,which are suitable for the quantum-state control,manipulation,and measurement.(2)A superconducting multiqubit measurement system including both hardware and software parts is developed and constructed.The hardware part of the system includes a cryogen-free dilution refrigerator,various signal sources,signal-measuring instruments,microwave mixing modules,signal circuits and filters,isolators,and amplifiers.A software package named Qu Lab is developed for multiqubit measurement based on python language with independent intellectual property right.(3)Dephasing induced by coherent-state photons in superconducting qubits are studied.Dephasing in qubit is generally considered to be caused by the fluctuation of qubit frequency due to external noise,which is equivalent to the random walk of the phase of qubit state.The low-frequency part of noise can be filtered out by dynamic decoupling thus dephasing can be suppressed effectively.However,it is found that dephasing caused by coherent-state photons in the readout resonator is not suppressed by dynamic decoupling,which therefore cannot be described by the random phase walk model.We have successfully put forward a new explanation for this phenomenon.(4)Quantum optical phenomena are studied in the superconducting multilevel systems.Superconducting qubit devices are natural quantum multilevel systems.We have experimentally demonstrated the stimulated Raman adiabatic passage in Xmon devices for the first time and discussed its promising application in quantum gate operation.Autler-Townes splitting in superconducting three-level system are also studied.Furthermore,scattering of one-dimensional light field by superconducting three-level system is discussed.(5)Finally,the control,manipulation,and measurement of the superconducting multiqubit devices are performed based upon the achievements described above.The multiqubit operation and readout are implemented,and the two-qubit i SWAP and b SWAP gates are realized successfully.