Researchers at the University of New South Wales and IMEC have achieved single-qubit gate fidelities up to 99.8% using hole spin qubits fabricated in natural silicon. This result, representing the highest performance reported to date in this material, advances silicon-based quantum computing by leveraging existing semiconductor manufacturing processes. The work demonstrates fast qubit control and exchange pulsing within a device with near-identical geometries and fabrication procedures used in recent high-fidelity electron spin qubit measurements. With future optimizations, these hole spin qubits are expected to enable new capabilities for quantum CMOS architectures.
Hole Spin Qubit Performance: 99.8% Accuracy in Natural Silicon
A single-qubit gate fidelity of up to 99.8% has been demonstrated using hole spin qubits fabricated on a standard silicon platform, a result that significantly elevates the potential of this approach to scalable quantum computing. The work circumvents the need for costly and complex isotopic purification typically required in other quantum computing architectures. Researchers utilized a 300mm CMOS process, the same technology underpinning modern microchips, to create the quantum dots confining the hole spins.
These dots, formed beneath precisely patterned gates, allow for all-electrical control of the qubits, a key advantage for integration and scalability. The device architecture closely mirrors those already used for high-performance electron spin qubits, streamlining potential future co-integration of both types of qubits on a single chip.
Fast qubit control, enabled by exchange pulsing, and industrial-grade fabrication techniques were central to achieving the reported fidelity. The team measured a two-qubit gate quality factor of 240, indicating a physical fidelity limit of 99.7%. This indicates that the observed errors are approaching the fundamental limits imposed by the physical system itself, rather than being dominated by technical imperfections. The ability to operate qubits with such high accuracy in natural silicon is particularly noteworthy, as it avoids the expense and complexity associated with using isotopically purified silicon, a common practice in other qubit modalities.
Detailed analysis of the device revealed that the confined holes form quantum dots, approximately 10 nanometers in the plane of the silicon chip. This unique confinement lifts the degeneracy of light and heavy hole bands, ensuring that only heavy hole states are occupied.
The team demonstrated full control over the absolute number of holes within each quantum dot, a critical step for reliable qubit operation. They achieved this by tracking dot occupation using a charge sensor integrated with radio-frequency reflectometry, which boasts a high readout bandwidth. Researchers explain in their published work that knowing the occupation is important because it sets the nature of the ground state, which strongly influences the properties of the qubit.
Electric-Dipole Spin Resonance Enables Ultrafast Control
The ability to manipulate quantum bits with exceptional speed and precision has advanced with the demonstration of electric-dipole spin resonance (EDSR) control in hole spin qubits fabricated on standard silicon. This advancement relies on leveraging EDSR, a technique that uses electric fields to control spin, offering a potentially faster alternative to traditional magnetic control methods. This precise control, combined with the fast response of EDSR, enables exceptionally rapid qubit manipulation.
300mm CMOS Fabrication for Scalable Hole Spin Qubit Arrays
This achievement bypasses the need for specialized materials or fabrication methods that often hinder the widespread production of quantum processors, positioning silicon as a viable foundation for future quantum technologies. The team’s work, detailed in recent findings, leverages existing industrial infrastructure to create qubits with exceptional performance characteristics.
The fabrication process resulted in single-qubit gate fidelities up to 99.8%, a result that surpasses previous benchmarks for natural silicon and approaches the levels achieved with isotopically purified materials. The use of natural silicon, containing 4.7% spinful 29Si nuclei, further distinguishes this work from many other quantum computing approaches that rely on costly and complex isotopic purification.
Suppression of Nuclear Noise via Hole’s p-wave Nature
This level of precision, reported in Nature Electronics, surpasses previous benchmarks for qubits constructed from this material and signals a potential pathway toward scalable quantum computing leveraging existing semiconductor manufacturing processes. Unlike many quantum computing efforts reliant on exotic materials, this work utilizes natural silicon containing 4.7% spinful 29Si nuclei, a significant advantage for cost and manufacturability. A key innovation lies in the suppression of nuclear noise, a persistent challenge in silicon-based quantum dots.
The researchers found that the p-wave nature of holes dramatically suppresses unwanted nuclear hyperfine noise. Importantly, these high-fidelity results were achieved using devices with near-identical geometries and fabrication procedures used in recent high-fidelity electron spin qubit measurements, suggesting a seamless transition for existing fabrication infrastructure.
Devices were constructed on a 10 nm SiO2 oxide on an undoped natural silicon substrate, utilizing electron beam lithography and subtractive patterning to define the quantum dots. This compatibility with industrial techniques positions hole spin qubits as a strong contender for integration into complex quantum CMOS architectures.
DiVincenzo Criteria Met in Silicon Hole Spin Qubits
Isaac Vorreiter of the University of New South Wales, along with colleagues, report single-qubit gate fidelities up to 99.8% in hole spin qubits, a performance level previously unseen in natural silicon and a critical step toward practical quantum processors. This success hinges on leveraging existing semiconductor manufacturing processes, specifically a 300 mm CMOS platform, rather than requiring entirely new infrastructure. This allows for potential integration of both electron and hole-based qubits onto a single chip, opening possibilities for hybrid quantum architectures.
High-Fidelity Gate Operation Above Surface Code Threshold
Silicon-based qubits achieved single-qubit gate fidelities up to 99.8%, a performance level exceeding previously reported results in natural silicon and approaching the threshold needed for practical quantum error correction. This achievement bypasses a significant hurdle in quantum computing, as many approaches rely on expensive isotopic enrichment to minimize noise-inducing silicon-29 isotopes. This compatibility streamlines development and potentially reduces manufacturing costs, a critical factor for widespread adoption of quantum technologies. The use of a p-type device, combined with fast qubit control and exchange pulsing, enabled the observed high fidelities.
Single-Shot Readout with RF Reflectometry and Pauli Spin Blockade
This approach allows for rapid and accurate determination of qubit states within a device fabricated using standard, industrial CMOS processes, a key step toward practical quantum technologies. Central to this readout method is the use of an adjacent single-hole transistor functioning as a sensitive charge detector. By monitoring changes in the reflected radio frequency signal, the team can determine the spin state of the qubit without directly measuring it, minimizing disturbance and maximizing readout fidelity. Numerical calculations confirmed this geometry, crucial for understanding the qubit’s behavior and optimizing control parameters.
This precise control is further enhanced by the ability to fully control the absolute number of holes in each dot, a capability demonstrated by tracking dot occupation down to the last hole. As described in the paper, the pulse sequence consists of three steps: initialization, control, and measurement. This sequence begins by initializing the spins in a singlet state before applying microwave pulses for single-qubit control, culminating in a projective spin readout.
The resulting charge sensor signal, analyzed as a function of measure time, reveals a maximized readout fidelity when the system is properly configured. The researchers found that the g-factors of the two qubits were 0.37 and 0.64.
This single-shot readout capability is not merely a technical demonstration; it is integral to the overall high performance achieved by the group. The ability to accurately and rapidly determine qubit states is essential for implementing complex quantum algorithms and scaling up the number of qubits in a quantum processor.
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