Scalable Silicon Spin Qubits Achieve Over 99% Fidelity for Quantum Computing with CMOS Technology

Insider Brief:

• Scientists from University of New South Wales, Diraq, imec and KU Leuven successfully fabricated high-fidelity silicon spin qubits using 300mm CMOS foundry technology, achieving over 99% fidelity in all operations.

• The study demonstrates the scalability of silicon spin qubits for industrial production, leveraging well-established semiconductor manufacturing processes.

• Improvements upon previous achievements include addressing noise issues, particularly nuclear spin noise, through isotopic purification, resulting in longer coherence times.

Silicon spin qubits, due to their compatibility with existing semiconductor manufacturing techniques, are considered to have high potential for scalable and commercially viable quantum processors. A recent arXiv preprint led by a team of scientists from the University of New South Wales and Diraq, in collaboration with imec and KU Leuven, provides an update on the the fabrication of these qubits on a 300mm semiconductor wafer. According to the study, this approach achieved over 99% fidelity in all qubit operations.

We’ve Been Here Before

For decades, the semiconductor industry has been fine-tuning the manufacturing process to support everything from smartphones to supercomputers. Silicon spin qubits are considered a strong candidate for scalable quantum computing as they can leverage these well-established precision engineering processes. However, moving from academic prototypes to industrial-scale production has raised concerns about whether the performance observed in controlled environments can be maintained at scale. The study demonstrates that by using 300mm wafer-scale complementary metal-oxide-semiconductor (CMOS) foundries, researchers can successfully fabricate and operate a two-qubit device, preserving the high performance required for practical applications.

The recent study published by the team improves upon a previous Nature Physics publication, which focused on high-fidelity two-qubit gates in silicon quantum dots.​ While the earlier research achieved over 99% fidelity for two-qubit gates, the latest results demonstrate even greater scalability and reliability, emphasizing the transition to large-scale, industrial-grade fabrication of qubits using 300mm wafers​.

Most notably, according to the team, is the use of 300mm CMOS foundry technology to fabricate the spin qubits, a shift that allows for the mass production of qubits while preserving the high fidelities necessary for fault-tolerant quantum computing. In contrast, the previous work still relied on more controlled, smaller-scale processes. This transition answers a key question that arose from earlier research: whether the high-fidelity operations observed in laboratory environments could be scaled up using industrial techniques​.

Additionally, while the earlier research identified noise sources such as charge noise and nuclear spin noise as the primary barriers to improving fidelity, the new study focuses on resolving these issues through isotopic purification and improved device engineering​. This results in even longer coherence times, including a T1 spin lifetime of 6.3 seconds. Furthermore, the use of gate set tomography is a common feature between both studies, but the new research demonstrates even more detailed error analysis and error correction strategies, pushing closer to the 99.9% fidelity required for practical quantum computing at scale.

A Closer Look at Overcoming Challenges with Noise and Coherence

The reliability of qubit operations is often challenged by environmental noise, particularly charge noise and nuclear spin noise. Charge noise stems from interactions between the electron spins and electric fields in the surrounding environment. However, the study highlights that the dominant source of operational errors in this device was nuclear spin noise from residual isotopes in the silicon substrate, specifically the 29Si isotope. This is actually good news, as nuclear spin noise can be mitigated through further isotopic purification, a process that has already been successfully demonstrated in other research contexts.

On the other hand, charge noise is deemed less of a problem–the researchers attribute this to the device’s design, which uses quantum dots in a traditional CMOS architecture, effectively suppressing charge noise and resulting in long coherence times. According to the study, the spin coherence times in the device were measured at T1 = 6.3 seconds and T2 = 803 microseconds for one of the qubits, both of which are improvements over previous silicon-based qubit systems.

Device Fabrication: Transitioning from Research to Industrial-Scale Production

A key achievement of this study is the successful integration of qubit fabrication into established semiconductor manufacturing processes. The device was designed by Diraq and fabricated at imec using 300mm wafer-scale processes—which testifies to the possibility of scaling up production. This industrial process combines mature CMOS technology with rapid fabrication cycles and design flexibility, ultimately creating a cost-effective path forward.

For a more in depth understanding of the architecture, the study describes the device as comprising a double quantum dot structure, with single-electron transistors for spin readout. These quantum dots, formed under plunger gate electrodes, capture electrons whose spins can be manipulated using microwave signals–meaning, the ability precisely control qubit states which is essential for reliable quantum operations. The device’s architecture also includes a triple-layer overlapping polysilicon gate stack, which is separated by high-temperature oxides to further add to the precise control.

To accurately assess the qubits’ performance, the research team used GST, a method that provides a comprehensive view of the sources of error in the system. The study reports that all operations exceeded 99% fidelity, a necessary threshold for fault-tolerant quantum computing. However, the researchers also note areas for further improvement include addressing the residual nuclear spin noise and improving the isotopic purity in the silicon substrate.

Demonstrating Scalable, High-Fidelity Qubit Production

Despite the ever-present need to continuously fine-tune processes that increase qubit control and mitigate noise, by demonstrating high-fidelity silicon spin qubits fabricated in a 300mm CMOS foundry, the research team has shown that large-scale qubit production is possible and may result in hardware resilient enough to meet the performance requirements for fault-tolerant quantum systems.

Contributing authors on the study include Paul Steinacker, Nard Dumoulin Stuyck, Han Lim, Tuomo Tanttu, MengKe Feng, Andreas Nickl, Santiago Serrano, Marco Candido, Jesus D. Cifuentes, Fay E. Hudson, Kok Wai Chan, Stefan Kubicek, Julien Jussot, Yann Canvel, Sofie Beyne, Yosuke Shimura, Roger Loo, Clement Godfrin, Bart Raes, Sylvain Baudot, Danny Wan, Arne Laucht, Chih Hwan Yang, Andre Saraiva, Christopher C. Escott, Kristiaan De Greve, and Andrew S. Dzurak..