A scalable semiconductor quantum computation platform based on Ge hole spin-qubits in rhombic quadruple quantum dots in strained Ge/SiGe

Project Information

DEMETER

Grant agreement ID: 101066761

DOI

10.3030/101066761

Project closed

EC signature date 29 May 2022

Start date 1 January 2023

End date 31 December 2024

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 215 534,40

Coordinated by

UNIVERSITY COLLEGE CORK - NATIONAL UNIVERSITY OF IRELAND, CORK
Ireland

Periodic Reporting for period 1 - DEMETER (A scalable semiconductor quantum computation platform based on Ge hole spin-qubits in rhombic quadruple quantum dots in strained Ge/SiGe)

Reporting period: 2023-01-01 to 2024-12-31

Summary of the context and overall objectives of the project

A quantum computer works fundamentally differently to a classical computer since quantum superposition takes advantage of the exciting ability of quantum particles to exist in more than one state at the same time. Because of quantum particle behaviour, some tasks such as superdense coding and teleporting unknown states, which are impossible to realize with classical resources, become possible. To achieve an advantage beyond the capabilities of any classical computer, quantum researchers and major national and international programmes such as the EU Quantum Technologies Flagship are focusing on speeding up common computational tasks including unsorted database searches and factorization.

There are several proposed platforms for qubit realization, and quantum computation. Broadly they can be based on ion-traps, superconducting junctions, photonic circuits and semiconductor quantum dots, each of which can reach different clock speed and gate fidelity. While there has been great scientific progress and proof-of-concept demonstrations on all of these platforms, to address the challenge of scalability it makes sense to use all the machinery of traditional semiconductor integrated circuits which also opens the possibility of integrating classical computing with quantum accelerators. Semiconductor-based qubit platforms have gained significant traction and unlike superconducting qubits, which require highly complex control mechanisms, they offer natural scalability and compatibility with complementary metal-oxide-semiconductor (CMOS) technology.
To this end, DEMETER employed a semiconductor materials platform and aimed at exploring qubits in electrostatically defined quantum dots by focusing on their fabrication, characterization, and integration with high-speed readout methodologies. The DEMETER project was conceived to push the boundaries of semiconductor-based quantum computing by exploring hole-spin qubits in germanium (Ge) quantum wells and its tin alloy (GeSn). The motivation behind this research stems from the need for scalable quantum computing architectures that leverage the mature fabrication techniques of semiconductor industries. The central idea was to develop rhombic quadruple quantum dot (RQQD) systems using strained Ge/SiGe heterostructures, providing a robust platform for high-fidelity qubit manipulation.

Compared to spin qubits in silicon (Si), where unfavourable band alignment prevents strain engineering, low-disorder reproducible quantum wells for high-mobility holes can be realized in strained Ge/SiGe heterostructures. The exact relation between the electrically-tuneable spin-orbit coupling in Ge and the operation temperature is not well known but due to the low effective mass and high mobility it may be expected that holes in strained Ge/SiGe can operate at a temperature higher than 1.5K which is state-of-the art for Si. This would allow integration of the driving cryo-electronics closer to the quantum circuit, further avoiding any sources of noise.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The overarching goal of DEMETER has been to develop rhombic quadruple quantum dot (RQQD) systems using strained Ge/SiGe heterostructures, providing a robust platform for high-fidelity qubit manipulation.

This core objective was underpinned by with the following targets:
1. High-Precision Fabrication of Quantum Dot Devices:
• Development of Ge-based RQQD systems with precisely defined gate structures.
• Advancement of electron beam lithography (EBL) techniques to enhance resolution and pattern fidelity.
• Implementation of titanium nitride (TiN) gate technology, known for its superconducting properties and compatibility with quantum applications.

2. Characterization and Quantum Readout Implementation:
• Electrical and structural validation of fabricated quantum dot devices.
• Explore coherence time (T2) optimization to exceed 1 μs, ensuring prolonged quantum information retention.
• Achieve single-qubit gate operations exceeding 140 MHz, enabling faster computation.
• Implementing RF-based fast qubit readout exceeding 6 MHz, which is crucial for scalable architectures.

During the fellowship, I successfully designed and fabricated multiple batches of quantum devices, including RQQDs and Hall-bar structures. The full process—mesa etch, ALD, ohmic contact formation, fine-line gate patterning—was carried out using high-resolution EBL and TiN deposition techniques. I also developed and optimized HSQ-based EBL processes, which led the foundation for showcasing high-fidelity multi-gate TiN patterning applicable to both planar and fin-type quantum structures.

The fabricated devices were tested at room temperature and then shipped to our collaborators for low-temperature quantum transport measurements. While final benchmarking is ongoing, all device processing steps, and planned deliverables were completed. The groundwork is in place for deeper characterization and further publications.

While there is still progress to be made on quantum measurements, the project has made significant strides on the fabrication and characterization of RQQD systems using strained Ge/SiGe heterostructures. By leveraging advanced nanofabrication techniques, one of the core achievements of the project has been the successful fabrication of high-precision quantum dot devices, with particular emphasis on electron beam lithography (EBL) and titanium nitride (TiN) gate technology. These advancements can be used for multigate prototype architectures of two types of spin-qubit devices, yielding higher fidelity and reproducibility of quantum gate structures than the commonly used lift-off process, thereby paving the way for more reliable qubit operations in a scalable architecture. Additionally, the integration of RF-based fast readout methodologies aims to enhance qubit measurement speed, a crucial step toward practical quantum computing implementations.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The DEMETER project has successfully demonstrated the feasibility of Ge-based quantum devices, bridging the gap between fundamental research and practical implementation. By leveraging advanced nanofabrication techniques, TiN gate technology, and high-speed qubit readout methodologies, this research has opened new avenues for scalable quantum computing architectures. DEMETER laid the groundwork for future breakthroughs in semiconductor quantum computing by enhancing our knowledge of quantum device fabrication and pushing the boundaries in EBL processing techniques. While some experimental characterizations remain ongoing, the project has successfully laid a foundation for future advancements in quantum device engineering, material science, and scalable quantum computing architectures. The nanofabrication processes will be used in other projects at the Host institution enhancing their quantum technologies portfolio.

Moving forward, the continued refinement of Ge-based quantum systems and the expansion of collaborative research efforts will play a pivotal role in advancing the feasibility of fault-tolerant quantum computing. To this end, the following directions will be prioritized:
• Completion of Qubit Characterization: Once measurements at Basel are finalized, results will be analyzed to refine qubit performance metrics.
• Scaling Up Device Architectures: Further research will explore the integration of multi-qubit arrays using Ge-based platforms.
• Industry Engagement: Potential collaborations with semiconductor companies will be sought to explore commercial applications of the developed quantum device architectures.

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