超导量子计算以其优异的性能被认为是最可能实现量子计算机的方案之一，基于约瑟夫森结的超导量子比特是组成超导量子计算机的基础单元，为此对单量子比特性能的研究以及多量子比特集成的研究是当前研究的重点。本论文以超导传输子量子比特以及超导Xmon量子比特为研究对象，对量子比特能级结构和时域特性进行系统的研究，同时对影响门操作保真度以及量子态保真度的因素进行分析。 论文首先对超导量子比特制备工艺进行介绍，列举光刻、溅射、电子束曝光、双角度蒸发以及剥离等关键步骤工艺参数，对影响样品稳定性和成品率的因素进行分析，设计“T”型结构，释放悬梁结构处的应力，改善版图设计。接着对低温测试系统进行介绍，包括稀释制冷机降温流程以及测试线路连接，着重强调各级温区间的滤波和衰减以及电磁屏蔽，防止外界热噪声和电磁噪声对样品退相干特性的影响。基于该测试系统对超导传输子量子比特的能谱结构以及时域特性进行表征，频域测试主要包括谐振腔色散偏移、真空拉比劈裂、量子比特能级结构和多光子过程；时域测试主要包括拉比振荡实验、能量退相干时间和相位退相干时间（拉姆齐条纹和自旋回波测试）测试。我们利用量子态层析对密度矩阵进行重构，对量子态误差进行表征，分析其主要误差来源。此外我们还对量子比特进行单发测量。为分析影响单比特门操作保真度的因素，我们以主方程理论为基础，对驱动脉冲失谐量、幅度误差和长度误差等可能的影响因素进行数值计算和仿真。最后我们对多比特近邻固定耦合Xmon芯片进行能谱和退性干特性的测试，分析z控制线接地问题对能谱稳定性和相位退相干的影响，以及邻近量子比特间相互串扰问题。
In comparison to other candidate quantum computing systems, superconducting circuit is a parameter controllable macroscopic quantum system, which makes the connectivity and control easier. In addition, superconducting circuit allows for scalable quantum integrated circuits. The heard of superconducting quantum computer is superconducting qubit. Realization of quantum computing requires that coherence time must be longer than the length of logic gate operations. However, due to the thermal noise, electromagnetic noise and materials defects, the coherent time of superconducting qubit is limited. It is necessary to isolate the qubit from environment noise and maintain a high level of coherence. Compared to other proposal of superconducting qubit, three-dimensional Transmon qubit and Xmon qubit have longer coherent time. Therefore, this thesis will focus on the simulation, fabrication and measurement of high quality Transmon qubit and Xmon qubit. First, we introduce the fabrication process and experimental setup. The critical steps in fabrication are UV-lithography, sputtering, electron-beam lithography and dual-angle evaporation. In order to make sure the structural integrity of Josephson junction, we use ZEP and copolymer resist bilayer to build the Dolan bridge and add “T” structure to relief the stress. Constructing the low temperature measurement system, we must pay attention to shield electromagnetic noise and reduce the thermal noise. A series of attenuators and low pass filters are applied to the control signal lines and probe signal lines. In the spectrum domain, we measure the cavity shift, vacuum Rabi splitting, single qubit spectroscopy and multi-photon process. Then in the time domain, we characterize the coherence property by Rabi oscillation experiment, energy relaxation experiment, Ramsey fringe experiment and spin echo experiment. Besides, we do the experiment of quantum state tomography to reconstruct the density matrix of the quantum state. Based on the retrieved density matrix, we analysis the possible reasons that influences the state fidelity. Continuing, we do the study of quantum gate fidelity. To physically realize the quantum computer, not only the coherent qubit state is required, logical gate operation is also significant. We simulate the potential factors that might have effects on the quantum gate fidelity and give suggestion to the experiment. Finaly, we test the coherence property on three-bit nearest neighbor coupling Xmon qubit. From the spectroscopy, we find the phenomenon that the connection method of z control line will induce the flux crosstalk then the spectrum would not stable. Moreover, due to the close distance between two neighbor qubits, when we control one qubit, the quantum state of another qubit will also be changed, which should be forbidden.
