Tunable Half-wavelength Superconducting Coplanar Waveguide Resonator

Quantum computers, which are powerful in storing and dealing with information, are promising to replace classical computers in many fields. Superconducting quantum qubits have been proved to be the best candidate to realize quantum computers. The superconducting coplanar waveguide resonator has the advantage of low loss and high Q. It can be used to measure the states of qubits and/or coupling qubits. In order to achieve flexible control of the coupling between superconducting qubits, it is urgent to develop a resonator whose resonant frequency can be in situ adjusted. Here we present a tunable half-wavelength superconducting coplanar waveguide resonator (SCPWR) in which one or three dc-SQUIDs are embedded in the center conductor to tune the resonant frequency. One by one we introduce the research background of the tunable SCPWR, relevant basic theory, design, fabrication, experimental results and theoretical analysis.The first chapter introduces superconducting qubits, quantum computer, the property and RSJ model of a Josephson junction, which is closely related to superconducting circuits. The relationship between the current in superconducting quantum interference device and the applied magnetic flux is described. Then, the research background and development of superconducting resonators are briefly reviewed. The applications and basic structure of superconducting coplanar waveguide resonator are also introduced. Tunable SCPWR, the topic of this thesis, is outlined including three solutions to realize the tuning of resonant frequency.The second chapter describes the structure of coplanar waveguide and the method of calculating basic parameters. Because the half-wavelength resonator with open load in this thesis has the property of parallel resonant circuit, the basic characteristics of the parallel resonant circuit and the half-wavelength open transmission line are discussed. Then the capacitive coupling, impedance matching and the principle of vector network analyzer are introduced, which are necessary for the design and measurement of the resonator.The third chapter introduces two kinds of software, i.e., HFSS and ADS, which are used to design and simulate the resonators. Then the fabrication including DUV exposure and electron beam evaporation is described in detail. At last, the working principles of Triton 400 dilution refrigerator, which provides cryogenic environment, and the diagrams of measuring circuit, are discussed.The fourth chapter gives the results of the fabricated resonator without dc-SQUID and IV characteristics of the Josephson junction, which show good performance. Then the resonant frequency of SCPWR with one or three dc-SQUIDs inserted with different external flux bias is measured through a vector network analyzer. The dependence of Q factor on the external magnetic flux is extracted. The experimental results show good agreement with the theoretical ones.