Scaling Superconducting Quantum Processors: Coherence, Frequency Targeting and Crosstalk
Doktorsavhandling, 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.

3D integration

Josephson junction reproduciblity

quantum computing

Superconducting qubits

quantum processor

TLS spectroscopy

Kollektorn, Kemivägen 9, Chalmers.
Opponent: Dr. Tobias Lindström, National Physical Laboratory, Great Britain

Författare

Amr Osman

Chalmers, Mikroteknologi och nanovetenskap, Kvantteknologi

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npj Quantum Information,;Vol. 9(2023)

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Quantum computers have the potential to revolutionize technology, offering solutions to problems beyond the reach of classical computers.
From advancing weather forecasting to accelerating material and drug discovery, these advanced machines promise to transform our world.
However, building a quantum computer is no simple task.
Superconducting qubits, one of the most promising quantum computing technologies, require exceptional precision in design and fabrication to perform reliably.

This thesis focuses on overcoming key obstacles in scaling superconducting quantum processors, including improving their coherence, achieving precise frequency control, and reducing unwanted interference, known as crosstalk.
Superconducting qubits rely on tiny components called Josephson junctions.
Variations in the size or quality of these junctions can compromise processor performance.
By optimizing fabrication processes and introducing streamlined techniques, this work achieves enhanced control over the reproducibility of qubit properties.

Another important aspect of this work is qubit coherence, which determines how long a qubit can reliably hold information.
The thesis explores the role of defects in materials—known as two-level systems (TLS)—that interfere with the qubit’s quantum state.
Through detailed analysis and improvements in fabrication techniques, the quality of superconducting circuits is significantly enhanced, extending qubit coherence times.

The work also investigates crosstalk, a challenge that arises when signals intended for one qubit inadvertently affect its neighbors. Using a flip-chip architecture, this research implements strategies to minimize these interactions and improve the overall fidelity of quantum operations. While significant progress has been made, the findings emphasize the need for active mitigation techniques to further suppress crosstalk.

This thesis contributes valuable insights and solutions for advancing superconducting quantum processors, paving the way for scalable quantum computers in the future.

An Open Superconducting Quantum Computer (OpenSuperQ)

Europeiska kommissionen (EU) (EC/H2020/820363), 2018-10-01 -- 2021-09-30.

Open Superconducting Quantum Computers (OpenSuperQPlus)

Europeiska kommissionen (EU) (EC/HE/101113946), 2023-03-01 -- 2026-08-31.

Styrkeområden

Informations- och kommunikationsteknik

Nanovetenskap och nanoteknik

Livsvetenskaper och teknik (2010-2018)

Materialvetenskap

Ämneskategorier

Materialteknik

Elektroteknik och elektronik

Nanoteknik

Infrastruktur

Nanotekniklaboratoriet

ISBN

978-91-8103-139-3

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5597

Utgivare

Chalmers

Kollektorn, Kemivägen 9, Chalmers.

Opponent: Dr. Tobias Lindström, National Physical Laboratory, Great Britain

Mer information

Senast uppdaterat

2024-11-22