Scaling Superconducting Quantum Processors: Coherence, Frequency Targeting and Crosstalk

Doctoral thesis, 2024

The advancement of quantum computing hinges on the scalability and performance of quantum processors. Superconducting qubits require precision engineering to achieve long coherence times and high gate fidelities. However, their performance remains constrained by challenges such as fabrication uncertainty, imperfections in materials, and unintended signal crosstalk, all of which impose significant limitations on scalability and reliability.

A significant portion of this work investigates the reproducibility of Josephson junctions, essential components of superconducting qubits. Variability in these junctions leads to deviations in qubit frequencies, degrading gate fidelity. A streamlined fabrication process using Patch-Integrated Cross-Type (PICT) junctions reduced the steps for junction fabrication while maintaining reproducibility and qubit coherence. Further improvements were achieved by optimizing the fabrication process and using slightly larger junction sizes, leading to a qubit frequency reproducibility of 40 MHz (1%) on a planar chip.

Qubit coherence, essential for maintaining quantum states and enabling error-free operations, is investigated from a material perspective. Two-level systems (TLS) at material interfaces are identified as dominant sources of decoherence. Through TLS spectroscopy, we probe the density of defects in the vicinity of the qubit and their presence within the junction's tunnel barrier. Additionally, we evaluate the impact of fabrication steps on superconducting circuit quality factors and improve the average qubit quality factor to 5 million.

Crosstalk, arising from unintended drive-line interactions, degrades gate fidelity and coherence. On the first-generation 25-qubit flip-chip quantum processing unit (QPU), the average drive-line crosstalk was benchmarked at 40 dB. The second-generation QPU enabled studies on parameter targeting, including the influence of chip-to-chip spacing on qubit frequencies and crosstalk’s impact on gate fidelity. Advanced frequency allocation strategies were introduced to account for fabrication uncertainties while maximizing frequency separation between neighboring qubits. While these approaches effectively mitigate gate collisions, our results emphasize the need for further suppression of crosstalk and active mitigation techniques to achieve higher fidelities in large-scale quantum processors.