Pioneering scalable hole-spin qubits for quantum computing

A team led by UNSW Sydney, in collaboration with Australian start-up Diraq, has reached a major milestone by demonstrating hole-spin qubits using industry standard silicon manufacturing processes.  

Hole-spins – positively charged particles in semiconductors – have the potential to revolutionise quantum computers by offering rapid operation speeds and seamless integration with existing silicon technology.  

The work, published in Nature Communications, opens the door for quantum computers to be manufactured in existing silicon chip factories, with the potential to significantly speed up their development.  

“This is an important step towards a new generation of high-speed silicon quantum bits, and it’s notable that the team included researchers at all levels from undergraduate students to experienced research scientists,” says Dr Scott Liles, a postdoctoral fellow from UNSW Physics and lead author on the latest paper.

International search for the best qubit 

Right now, there is a huge international effort to develop a quantum computer – an advanced machine that harnesses quantum mechanics to solve problems far beyond the reach of today's supercomputers.  

Their ability to process vast amounts of data and perform calculations at extraordinary speeds will allow them to provide answers to some of the toughest questions in science and technology today, including optimising drug design, composing new materials and even machine learning.  

The basic building block of a quantum computer is a qubit, or a “quantum bit”, made of semiconductors. In quantum computing, a chip is a physical platform that houses qubits and other quantum components. 

While great progress has been made, a key question in quantum computing remains: what is the best and most suitable way to make qubits?  

Two of the key factors that will make the best qubit are how reliably the qubits can be operated, and how easily a single qubit prototype can be scaled up to the millions of qubits needed to build a functional quantum computer. 

One potential solution is the development of hole qubits. Holes – essentially positively charged electrons in semiconductors – have been predicted to have promising properties for qubits in a quantum computer, including a rapid control speed and all electrical operation. 

A gap in the research 

In particular, silicon based qubits are attracting interest, partly because  they can be integrated into the manufacturing processes that have been optimised by the multi-trillion dollar semiconductor industry. 

This means that designing qubits that are compatible with this existing manufacturing technology could allow large scale quantum computers to be produced in the same factories that make ordinary silicon chips.  

While holes are used in almost all conventional silicon chips – such as those used in computers and phones – most research into semiconductor qubits has instead concentrated on electrons. Additionally, challenges in fabricating silicon hole-based qubit devices, and in developing accurate theories describing their operation, have posed significant challenges to realising hole qubits.  

Due to the gap in the research, the techniques to fabricate hole-based qubits in silicon have not been as well developed as their electron-based counterparts. So, while theory has suggested holes would make good qubits, the challenge of fabrication and control have limited experimental studies of hole qubits. 

Latest research into silicon hole qubits 

Addressing the challenge of making the hole qubit device has been a research focus for Professor Alex Hamilton and Dr Liles for over a decade. Working in collaboration with Diraq founder Professor Andrew Dzurak, Dr Liles and Prof. Hamilton have made several key breakthroughs in the field of planar hole qubits. 

Step-by-step, the team have collaborated to progress the research into silicon hole qubits. From their first demonstrations of planar silicon hole devices using CMOS (Complementary Metal-Oxide-Semiconductor) technology, to optimising and improving the model and developing a device that uses both holes and electrons.   

In their latest paper, the multidisciplinary team presents the first demonstration of a hole-based qubit on a planar MOS (metal-oxide-semiconductor) silicon structure.  

Despite this success, the challenge of controlling and measuring the new type of qubit remained. This hurdle was addressed by undergraduate student Daniel Halverson as part of a Sydney Quantum Academy scholarship program. Mr Halverson developed a unique software code – available to other researchers to download – that is able to perform real time simulation of the complex hole spin dynamics. 

By comparing the experimental and simulated results the team were able to control the qubit and demonstrated qubit operation speeds as fast as a few nanoseconds, which is several orders of magnitude faster than equivalent electron qubits.  

“This collaboration supports Diraq’s goal of creating the world’s first commercial quantum computer using silicon-chip based technology, a platform that is cost efficient, energy efficient, and due to its compact size has a small infrastructure footprint,” says Prof. Dzurak. 

“Production of these semiconductor chips in commercial foundries, such as IMEC is where the economies of scale really make sense - as you can make many millions of transistors on a square centimetre of silicon, ensuring the power of quantum will be accessible, sustainable and a viable option for commercial applications.”

Notably, these qubits can be controlled up to 1000 times faster than electron-based qubits, offering major performance improvements, highlighting the potential of hole-spin qubits for faster and more efficient quantum computing applications. 

What’s next?

This work successfully demonstrates a hole-spin qubit in a planer silicon device, paving the way for new approaches to building quantum computing hardware. This advancement opens promising possibilities for integrating quantum devices into existing silicon technology, a crucial step toward scalable and practical quantum computers. 

The detailed simulations carried out in this latest paper also revealed key parameters for the fabricated hole-spin qubit that will form the basis for the ongoing development of new types of hole spin based qubits.  

“We are already building on this work, and with 2025 being the international year of quantum science and technology we look forward to sharing further exciting developments in silicon quantum technologies,” says team leader Prof. Hamilton. 

The team are currently working on a collaboration with commercial partner Diraq and industrial partner IMEC to look at producing these qubits using standard industry processes.

The work was a collaboration between researchers including Dr. S.Liles, Mr. D. Halverson and Prof. Alexander Hamilton at UNSW, R. Eggli at the University of Basel, and members of the Australian company Diraq led by Prof. Andrew Dzurak.