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New silicon-based arrays pave the way for scalable quantum computers
(Nanowerk Spotlight) The vision of quantum computing has captivated scientists with the potential to revolutionize technology by solving problems far beyond the reach of classical computers. Despite the allure, progress has often been hindered by the sheer complexity of controlling quantum states.
The challenge lies not only in creating qubits – quantum bits that can exist in multiple states simultaneously – but also in scaling these systems to build practical, large-scale quantum computers. Each new approach has pushed the boundaries, yet significant obstacles remain.
One particularly promising avenue focuses on semiconductor spin qubits, which offer a pathway to integrating quantum systems with the existing infrastructure of semiconductor manufacturing. This could bridge the gap between theoretical potential and practical implementation, transforming quantum computing from a laboratory curiosity into a scalable technology.
Recent progress in quantum technology is making scalable quantum computing more feasible, particularly through innovations in atomic arrays for spin-based quantum computers in silicon. Researchers have now developed methods to integrate ion-implanted donor spins – types of qubits known for their long coherence times and compatibility with industry-standard metal-oxide-semiconductor (MOS) processes – into these arrays. This development opens new possibilities for constructing large-scale quantum computers that can be reliably manufactured using existing semiconductor technologies.
Their study, published in Advanced Materials ("Scalable Atomic Arrays for Spin-Based Quantum Computers in Silicon"), makes substantial strides in overcoming the major obstacles to scaling quantum systems. By combining precise techniques for placing donor atoms within silicon and incorporating high-dimensional qudits – quantum bits that can reliably distinguish between and operate on multiple basis states, as opposed to traditional qubits which typically utilize two basis states – the researchers have developed innovative methods that enhance both the accuracy of qubit placement and the overall stability and performance of the quantum computing system.
The heart of this approach lies in the use of donor atoms implanted into silicon – a method that combines the benefits of long coherence times with the robustness of semiconductor technology. Donor spins, particularly those based on phosphorus, antimony, and bismuth, have shown remarkable potential as qubits due to their long-lasting quantum states and high gate fidelities. These attributes make them ideal candidates for constructing large-scale quantum computers.
To achieve the level of precision necessary for scalable quantum computing, the researchers employed a technique known as deterministic single-ion implantation. This method involves using a highly controlled ion beam to implant individual donor atoms into a silicon substrate with nanometer-scale accuracy. The ability to place donor atoms with such precision is critical for the construction of quantum devices that require regular arrays of qubits, which must be spaced at specific intervals to function correctly.
Ion implantation
Ion implantation configuration: An atomic-force microscope (AFM) cantilever with an aperture dwells over an implantation site on the silicon substrate configured with biased, charge-sensitive detector electrodes. The substrate is passivated with a 5 nm thin gate oxide. Implanted ions dissipate kinetic energy and create free electron–hole pairs that induce a signal at the detector electrodes. The signal amplitude is proportional to the number of electron–hole pairs and can be used to trigger a step-and-repeat sequence for the deterministic engineering of donor arrays. (Image: Reproduced from DOI:10.1002/adma.202405006, CC BY)
One of the key innovations in this research is the use of molecular ions, such as 31PF2, which consist of a phosphorus atom bonded to two fluorine atoms. These molecular ions offer a significant advantage over single atoms by increasing the detection confidence during implantation. The fluorine atoms, which are rapidly diffused out of the active region during thermal annealing, provide a boost in the signal detected during implantation. This allows for the precise placement of phosphorus atoms at the desired depth within the silicon substrate, significantly improving the accuracy and reliability of qubit formation.
The researchers also explored the use of heavier donor atoms, such as antimony (123Sb) and bismuth (209Bi), which offer even greater potential for scalability. These atoms, due to their larger nuclear spins, can be used to create qudits. The ability to encode information in higher dimensions without increasing the physical size of the quantum system is a powerful tool for quantum computing, potentially allowing for more complex computations with fewer qubits.
The combination of these approaches – using molecular ions for precise placement and heavy donor atoms for increased qubit capacity – forms a comprehensive strategy for building scalable quantum computers. The researchers demonstrated this by creating regular arrays of donor atoms with a spacing of approximately 300 nanometers, a configuration suitable for the operation of dipole-coupled "flip-flop" qubits. These qubits, which leverage the interaction between nuclear spins and electrons, are a promising architecture for building robust quantum systems.
Beyond the technical achievements, the significance of this research lies in its potential to make quantum computing more practical and scalable. By integrating these advanced techniques with existing semiconductor manufacturing processes, the team has laid the groundwork for constructing quantum computers that could one day operate on the same scale as today's classical computers. This work represents not just an incremental step, but a meaningful advance toward realizing the full potential of quantum computing.
The development of scalable atomic arrays for spin-based quantum computers in silicon is not just a technical achievement but a pivotal step toward the future of computing. By integrating advanced quantum technologies with conventional semiconductor manufacturing, this research provides a pathway for developing quantum devices that are both powerful and practical.
The ability to create precise, large-scale qubit arrays using donor atoms and molecular ions, along with the potential to employ high-dimensional qudits, opens new possibilities for quantum information processing. These advancements bring us closer to realizing quantum computers that can solve problems currently beyond the reach of classical systems, potentially transforming fields such as cryptography, materials science, and complex system modeling.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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