Research Projects

Superconducting quantum circuits show great promise for quantum computing due to improved gate fidelity and scalability. However, surface and interface defects—especially oxide layers—host two-level systems (TLS) that cause dielectric loss, limiting device performance by inducing decoherence. This project aims to enhance coherence times by identifying and mitigating loss mechanisms in superconducting quantum circuits. To achieve this, we propose a comprehensive microwave measurement study of three distinct 2D superconducting resonator geometries: lumped-element resonators, broad-frequency-span multimode resonators, and tripole-stripline resonators in a 3D cavity architecture. Measuring their quality factor at single-photon power levels will enable the identification of TLS-related dielectric losses arising from different interfaces. Furthermore, the geometric energy participation ratio model will be used to quantify bulk- and surface-dielectric losses across interfaces, such as metal-air, metal-substrate, and substrate-air regions. The experimental study involves four key steps: a) preparation of the substrates, b) growth and characterization of the superconducting materials, c) fabrication of the device and d) microwave measurements of the devices at the single-photon power regime. Choice of materials is crucial for this particular study. We will use Sapphire and Silicon as substrates, and Tantalum and Niobium-titanium-nitride for the superconducting metals. UHV DC-magnetron sputtering tool will be used for the metal growth and photolithography technique will be used for patterning the design. The metal deposition will follow the characterization process using X-ray diffraction spectroscopy, energy-dispersive X-ray spectroscopy and X-ray reflectivity to optimize the growth quality and achieve the desired thickness of the metal on the substrate. The fabricated devices will be characterized by hard X-ray photoelectron spectroscopy to get the valuable information of the oxide layers formed at the surface of the metal, substrate and their interface. After completing the first three steps of device preparation, it will be cooled down to milli-Kelvin temperature range to eliminate the thermal noise and microwave measurement will be performed at single-photon power level to obtain the quality factor of the device. The power dependent Q-factor measurements will determine the TLS-related dielectric loss in the device. The Q-factor measurements, combined with the participation-ratio model, will optimize the proposed resonator geometries to minimize microwave losses and achieve high-Q on the order of 10⁶ at single-photon power, comparable to the best reported qubits. These high-Q resonator designs and surface treatments developed during fabrication process will be applied to superconducting qubit devices to achieve longer qubit coherence time in the order of half a millisecond.