Nanohybrid Quantum Circuits Based on Superconducting Coplanar Waveguide Resonators

Abstract

The Majorana transmon is a proposed quantum circuit expected to host Majorana zero modes (MZMs) in topological Josephson junctions. These systems have been predicted as a building block for fault-tolerant quantum computing due to topological protection of quantum states. However, realizing these devices requires field-resilient superconducting resonators and long coherence gatemon qubit. This thesis addresses these challenges by developing field-resilient superconducting resonators and demonstrating topological material- based gatemon qubits. In the first part, we investigate superconducting coplanar waveguide resonators using Nb, NbTi, and NbTiN thin films with various thicknesses for magnetic field resilience. When in-plane fields are applied to these films, unavoidable misalignment creates out-of-plane field that induce Abrikosov vortices, leading dissipation. Among the materials, 45-nm-thick NbTi resonators achieve internal quality factor (Q_i) exceeding 10^4 at 0.4T, demonstrating field resilience while maintaining high Q_i. In the second part, two-level system (TLS) loss is characterized in NbTi resonators on Si/SiO_2 substrates, motivated by the requirements of gatemon. TLS loss originates from defects in amorphous dielectric materials and at interfaces. Power- and temperature-dependent measurements indicate TLS-limited behavior with Q_TLS,0 = (5.14 - 9.21) × 10^4, consistent with values for quantum circuit applications. In the final part, we report a gatemon qubit based on Three-dimensional Dirac semimetal Cd_3As_2 Josephson junction. By tuning the Josephson energy via gate voltage, the qubit frequency is modulated, which shifts the resonator frequency through qubit-resonator coupling. Time-domain measurements show coherent Rabi oscillations with a Rabi decay time of 8.8 ns, providing a foundation for Majorana-based qubits.