Master's thesis presentation, Emil Rehnman

Examiner: Jonas Bylander
Supervisor: Janka Biznárová and Anita Fadavi Roudsari
Opponent: Nelson Rebelo

Abstract: In superconducting circuits, coplanar waveguide (CPW) resonators play an important role in qubit readout and control. They also function as a probe for loss in qubits. Loss in CPWs is mainly caused by two-level systems (TLS), radiation and quasi-particles. These competing loss sources are all affected by the resonator size. By measuring the internal quality factor power dependence of resonators with varying geometry we find that for small resonators, TLS loss is limiting low-power Q and radiation is the main source of high-power loss. Large resonators show a weaker Qi power dependence which suggests a smaller contribution from TLS loss. Changing the thin film deposition method greatly improved Qi for these resonators indicating that quasi-particle loss was limiting Qi.

Another parameter that affects TLS loss is the gap between the CPW centre conductor and ground plane. Using a large gap, we measured single-photon level Qi of 1.5 million. The larger gap reduces the TLS loss by diluting the electric field in the TLS hosting regions. We also investigate the loss in so called trenched resonators. By removing the silicon substrate in the resonator gap, one of the TLS hosting regions can be moved away from the areas with highest electric field strengths. A trenching recipe using deep reactive ion etching was developed and trenched resonators were fabricated and measured. The low-power quality factor in the trenched resonators was significantly higher compared to the standard resonator and we achieved a single-photon level Qi of 1.9 million.