Superconducting Coplanar Waveguide Cavity On The High Resistivity Silicon Substrate

Superconducting quantum bit (qubit), one of the physical systems to realize the quantum computation, has been paid extensive attention. Superconducting qubits are solid circuits which are scalable and compatible with the fabrication process of semiconductor. In order to couple the qubits, it is important to design and fabricate superconducting coplanar waveguide (SCPW) cavities with high quality factor (Q). Such kind of cavities not only constitute the quantum computation circuits with strong coupling but also provide a new way in the research of quantum electrodynamics (QED). At the same time, it is necessary to fabricate tunable cavities for simplifying measurement of the coupled qubits.The planar transmission line cavities are usually used for coupling and filtering. This thesis mainly introduces the half wavelength-open circuit-SCPW cavities. It could be used to couple qubits with two ports. Here I will introduce my studies in the following aspects:1、The design and measurement of300nm Nb cavitiesThe fundamental resonance frequency (fo) of cavities with four different length are calculated with the resonance frequency formula and checked with the simulation software. A microwave circuit PCB is designed for impedance matching, which will transmit most of the power into the cavity. We fabricate Nb cavities. The experimental results in the liquid helium are fitted with the theoretical formula. Then we calculate the effective permittivity for precise design of cavity. Five kinds of planar coupling capacitors (including IDC) are fabricated to get the accurate Q because the calibration of VNA is operated at room temperature.2、Analying and measuring the dependence of the cavity characteristics on the film thickness, material and temperatureCavities with Nb and NbN are fabricated with four different thicknesses. We measure fo and Q with different material and thicknesses. Then the kinetic inductance theory is applied to analyze these results. And also, the300nm Nb cavity is measured with varying temperature, which is analyzed with kinetic inductance, normal electron resistance in two-fluid model and loss angle tangent in substrate.3、The design and measurement of the tunable cavityThe DC-SQUID could be equivalent to a tunable inductance. So we fabricate a cavity with a DC-SQUID, which is used to tune the cavity with the external magnetic flux. According to experimental result, we know that the DC-SQUID has been destroyed or become normal state. We discuss the further research plan. We also fabricate a tunable cavity based on graphene whose resistance could be tuned.