Silicon spin qubits offer a promising route to developing scalable quantum computers because they are compatible with the semiconductor manufacturing techniques used to make modern computer chips. However, they face a significant challenge: their accuracy can decline when multiple qubits are controlled simultaneously. For quantum computers to become practical, they must be able to operate large numbers of qubits in parallel without reducing the precision of their operations.
In a study published in Nature Physics, researchers from the RIKEN Center for Emergent Matter Science and their collaborators have tackled this problem by demonstrating high-fidelity control of multiple silicon spin qubits operating simultaneously. Using an array of five silicon qubits controlled through a single shared microwave line, the team identified a major source of errors during parallel operations and developed a method to reduce their impact. Leon Camenzind, a research scientist at RIKEN and co-first author who supervised the experiments, said, “This shows that high-fidelity control can be maintained even when several qubits are operated at the same time and represents an important step toward scaling up silicon-based quantum processors.”
Identifying and Correcting Errors in Parallel Qubit Operations
Quantum computers process information using quantum bits, or qubits, which can represent combinations of the conventional binary states 0 and 1. In silicon spin qubits, information is stored in the spin of an electron confined within a silicon substrate. Precise control over these spins is essential for performing quantum operations accurately. However, when microwave signals are used to control several qubits simultaneously, unwanted errors can arise and compromise computational precision.
The researchers began their work by finding that the major source of this degradation is a phenomenon known as the AC Stark shift, which is induced by the microwave control signals. This effect changes the effective energy levels of a qubit, shifting the phase of its quantum state and making its operation less precise. When several qubits are driven simultaneously, these shifts can interfere with the intended control of each qubit.
To address this problem, the team developed a scalable mitigation protocol based on phase calibrations. “Rather than requiring an entirely separate calibration procedure for every possible combination of qubits, our method uses calibrations between pairs of qubits to compensate for the unwanted phase shifts,” explains Yi-Hsien Wu, the first author who carried out the experiments as part of his PhD research.
Using an optimized device design and carefully calibrated microwave control pulses, the team achieved single-qubit gate fidelities well above 99.99% on each of the five qubits when operated individually, with the best approaching 99.999%. Fidelity measures how accurately a quantum operation is performed. Importantly, by using the protocol, they were able to achieve fidelities above 99.99% when up to three qubits were operated simultaneously. During parallel operation of all five qubits, the fidelities remained at approximately 99.9%, a level close to the practical requirements of fault-tolerant quantum computing.
A Step Toward Scalable, Fault-Tolerant Quantum Computers
The remaining degradation was attributed to drive-induced decoherence caused by the increased microwave power needed for simultaneous operation. Unlike the phase shifts addressed by the new calibration protocol, this effect involves a loss of quantum coherence. According to Camenzind, “This is a limitation, but further improvements to device design could help mitigate this source of error.”
The results demonstrate that high-fidelity control need not be sacrificed when silicon spin qubits operate in parallel. “By addressing a key source of control errors through pairwise calibration,” says Camenzind, “our study supports the use of shared control lines as a practical approach to scaling up quantum processors. Combined with silicon’s compatibility with established semiconductor manufacturing, we believe that this advance could help pave the way toward larger quantum processors capable of performing increasingly complex computations.”
“Our long-term goal is to build a fault-tolerant quantum computer based on silicon spin qubits, and reliable parallel control is an essential step toward that goal,” adds Seigo Tarucha, project manager of a silicon quantum computing project under JST’s Moonshot Goal 6.
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