PhD defense: Michaela Eichinger

Novel Methods and Materials for Superconducting Qubits and Circuits

The development of superconducting quantum processors faces significant hardware challenges related to achieving reproducibility, scalability, and novel device architectures. Identifying and developing materials and nanofabrication techniques is essential to improve coherence and enable the realization of new qubit types with superior operation schemes or insensitivity to local environmental fluctuations. To date, the dominant nanofabrication techniques for realizing modern superconducting qubit architectures are centered on implementing and refining semiconductor technology processes, primarily through the use of organic resist masks. While these methods have been instrumental in the experimental realization of superconducting qubits, it is imperative to explore alternative methods for creating circuits that can approach the ideal of defect-free quantum devices and limit the impact of processing effects.

The thesis explores such alternatives and presents new techniques that unlock the fully in-situ fabrication of superconducting resonator and qubit devices using novel inorganic stencil lithography methods and a design technique called the Shadow Mask Polarizer. The latter simplifies the realization of quantum architectures by eliminating the need for repeated switching between nanofabrication tools and laborious alignment of material layers. Moreover, the technique lifts geometric design limitations, and eliminates parasitic shadow structures which are common loss sources in superconducting circuit devices. The compatibility of the developed stencil masks with ultra-high vacuum and molecular beam epitaxy systems represents a critical advancement in the ability to realize qubit circuits and other quantum devices with highly crystalline films and interfaces.   

In addition, the thesis includes work on realizing gate-tunable qubits based on selective area grown indium arsenide nanowires on silicon, emphasizing the material challenges of superconductor-semiconductor hybrid systems and their integration into qubit architectures. Finally, we conclude with ideas on using the developed stencil lithography techniques as an alternative route to realize superconductor-semiconductor hybrid systems and gate-tunable qubits accordingly.