UC Berkeley

A central discourse in the field of superconducting quantum computing surrounds the question of whether the transmon qubit is the correct unit cell for scaling to fault-tolerant quantum computation. Since the transmon was first introduced, there has been a rapid development of more complex qubit designs that promise better noise protection and higher fidelity control. Despite these innovations, all of the largest superconducting qubit processors that exist today rely on the transmon qubit as the fundamental unit cell, and many use the flux-tunable variant to manage many of the difficult scaling constraints. However, the simplicity of the fixed-frequency, single-junction transmon remains highly appealing for building robust, large-scale systems. In this thesis, we address several key scaling challenges that have historically limited fixed-frequency transmon architectures. First, we present an analysis of the frequency-crowding problem for fixed-frequency architectures. We model the statistical distribution of qubit frequencies resulting from fabrication variance and propose strategies for mitigating spectral collisions. Next, we explore the use of parametrically modulated couplers to engineer fast, high-fidelity entanglement between highly-detuned transmons, which circumvents both frequency crowding and gate-speed limitations imposed by fixed-coupling architectures. Finally, we address the challenge of system instability caused by interactions with two-level system (TLS) defects. Since fixed-frequency transmons cannot be tuned away from these parasitic resonances, we introduce a resource-efficient method to integrate electric-field tuning of TLS defects into a scalable architecture. By addressing these key challenges, this work provides a pathway for scaling fixed-frequency transmons toward fault-tolerant quantum computation.