RIKEN overcomes hidden error source in quantum computers

Summary

RIKEN researchers demonstrated that five silicon spin qubits on a shared microwave control line can be operated simultaneously with approximately 99.9% fidelity. The team identified AC Stark shift as a hidden error source and compensated for this effect using a pairwise calibration method.

Why it matters

Scaling quantum processors requires the parallel control of many qubits without loss of accuracy. This study shows that shared control lines, a practical necessity for large arrays, do not require a significant trade-off in fidelity. Since silicon spin qubits are compatible with existing semiconductor manufacturing processes, this result carries direct industrial importance for developing scalable and manufacturable quantum hardware.

Highlights

• AC Stark shift identified as a hidden phase error source in parallel qubit control
• Pairwise calibration prevents the exponential increase in calibration complexity as the number of qubits grows
• ~99.9% simultaneous fidelity across all five qubits; individual gates exceed 99.99%

Critical take

While the 99.9% simultaneous fidelity achieved for five qubits is a meaningful step, it falls below the ~99.99+ gate fidelity generally considered necessary for practical fault-tolerant quantum computing. Additionally, while driver-induced residual decoherence is acknowledged, it remains unresolved. Furthermore, five qubits on a single shared line represents a proof-of-concept scale. Whether the pairwise calibration strategy remains valid for hundreds or thousands of qubits remains an open question not addressed in the paper.