Master thesis defense by Malte Wettergren Andreasen

Microwave Characterization of Components in Superconducting Transmon Devices

Improvement of superconducting qubits is instrumental for fault tolerant quantum computing to become a reality. Characterization of materials and device properties play a central role in the feedback-loop that enables better design and fabrication of future quantum processors. By operating in a cryogenic setting using hardware and software tools for resonator characterization and multiplexed quantum control and readout of qubits, this thesis aims to describe the components in qubit devices, as well as discuss the decoherence mechanisms responsible for quantum information loss in the devices. The intrinsic quality of the resonators was shown to vary drastically across the chip, and a systematic deviation in resonance frequencies was observed, proposed to stem from smaller coupling capacitance than expected. Key metrics from the qubit analysis, including energy relaxation and phase coherence of two qubits, were tracked over time, revealing significant variability in coherence times, particularly in energy relaxation T₁. Furthermore, qubit coherence was notably impacted by interference from low-frequency noise. These observations underline the importance of frequent qubit calibration and isolation of qubits from environmental noise.