Breakthrough in Quantum Computing: An 11-Qubit Atom Processor in Silicon

Quantum computing has taken a significant leap forward with the development of an 11-qubit atom processor in silicon, as detailed in a groundbreaking study published in Nature. This achievement represents a crucial milestone in the quest for practical quantum computers, demonstrating that silicon—the foundation of classical computing—can also serve as a powerful platform for quantum information processing. The research, conducted by a team at Silicon Quantum Computing Pty Ltd, showcases exceptional performance metrics including gate fidelities exceeding 99% and the ability to generate entanglement across multiple qubits.

The 14|15 Platform: Silicon-Based Quantum Computing

The processor operates on what researchers call the "14|15 platform," referring to the periodic table positions of silicon (14) and phosphorus (15). This approach leverages the exceptional properties of phosphorus atoms embedded in silicon, where nuclear spins exhibit coherence times extending to seconds—a critical requirement for quantum computation. Unlike other quantum computing platforms that face challenges with manufacturing and control-system miniaturization, silicon quantum processors offer compatibility with existing semiconductor manufacturing infrastructure, potentially enabling more straightforward scaling.

What makes this platform particularly promising is its use of precision manufacturing techniques to place individual phosphorus atoms within nanometers of each other. When these atoms are positioned in close proximity (approximately 3 nanometers apart), they couple through hyperfine interaction to a shared electron. This configuration creates nuclear spin registers that enable high-fidelity multi-qubit control and the execution of quantum algorithms. The shared electron naturally serves as an ancilla qubit, facilitating quantum non-demolition readout of nuclear spins and enabling native multi-qubit gates.

Processor Architecture and Connectivity

The 11-qubit atom processor consists of two multi-nuclear spin registers connected through electron exchange interaction. One register contains four phosphorus atoms (n1-n4) coupled to electron e1, while the other contains five phosphorus atoms (n5-n9) coupled to electron e2. The critical innovation enabling this architecture is the exchange coupling between the two electrons, which provides a fast and efficient quantum link between the registers. This connectivity scheme allows for both local operations within registers and non-local operations across registers, creating an all-to-all connectivity pattern essential for complex quantum algorithms.

The distance between the nuclear spin registers—approximately 13 nanometers center-to-center—was atomically engineered to enable optimal exchange coupling. This precise placement is achieved through scanning tunneling microscope lithography, allowing researchers to position individual atoms with atomic-scale accuracy. The exchange coupling strength (J ≈ 1.55 MHz) operates in a regime that minimizes susceptibility to charge noise while maintaining high operational fidelity.

Performance Metrics and Gate Fidelities

The research team achieved remarkable performance across multiple metrics. Single-qubit randomized benchmarking revealed gate fidelities ranging from 99.10% to 99.99%, with most qubits operating above 99.90%. These exceptional values stem from the long coherence times observed in the system: nuclear spin phase coherence times (T₂*) ranged from 1 to 46 milliseconds, extendable to 3-660 milliseconds with Hahn echo refocusing techniques. Electron spins demonstrated phase coherence times of approximately 20 microseconds (T₂*) and 350 microseconds (T₂^Hahn).

Two-qubit operations showed equally impressive performance. The electron-electron controlled rotation (CROT) gate achieved a fidelity of 99.64(8)%, while nuclear-nuclear controlled-Z (CZ) gates reached fidelities as high as 99.90(4)%. These values represent state-of-the-art performance for semiconductor-based quantum processors and approach the thresholds required for fault-tolerant quantum error correction. The researchers attribute this high performance to systematic investigations of qubit stability, contextual errors, and crosstalk, which informed the development of scalable calibration and control protocols.

Entanglement Generation and Multi-Qubit States

A key demonstration of the processor's capabilities was the generation of Bell states—maximally entangled two-qubit states—across various qubit pairs. Local Bell states (within the same register) achieved fidelities ranging from 91.4(5)% to 99.5(1)%, with the peak value representing the highest Bell-state fidelity reported in semiconductor devices to date. Non-local Bell states (across registers) showed fidelities from 87.0(4)% to 97.0(2)%, demonstrating effective connectivity between the two spin registers.

The researchers further demonstrated the processor's all-to-all connectivity by generating Greenberger-Horne-Zeilinger (GHZ) states with increasing numbers of qubits. They successfully created entanglement across up to eight nuclear spins, with three-qubit GHZ states achieving fidelities of 90.8(3)% and eight-qubit entanglement maintaining fidelity above the 50% threshold required to witness genuine multi-qubit entanglement. This scalability in entanglement generation represents a significant step toward implementing quantum error correction protocols that require connectivity across multiple qubits.

Scalability and Future Implications

Beyond the impressive qubit count and performance metrics, the research addresses critical challenges in scaling quantum processors. The team developed efficient recalibration protocols that scale linearly with the number of coupled spin registers—a crucial advancement for building larger quantum systems. By characterizing electron spin resonance frequencies for a single reference configuration, researchers can infer all other frequency positions within a register, dramatically reducing calibration overhead as systems scale.

The success of this 11-qubit processor validates silicon as a viable platform for quantum computing and demonstrates a path toward fault-tolerant quantum computation. Future work will focus on benchmarking performance with arbitrary spectator qubit states, characterizing error and leakage channels, and implementing control optimizations through pulse shaping and parallelized drive execution. Researchers also aim to atomically engineer registers to optimize hyperfine couplings, potentially increasing gate speeds and overall performance.

This breakthrough represents more than just another qubit count milestone—it demonstrates a comprehensive approach to quantum processor design that addresses both performance and scalability. By maintaining high-fidelity operations while increasing qubit count and connectivity, the research provides a blueprint for future quantum processors that could eventually surpass classical computing capabilities for specific applications. As quantum computing continues to advance, silicon-based approaches like the 14|15 platform offer a promising path toward practical, scalable quantum information processing that leverages decades of semiconductor manufacturing expertise.