Researchers at the RIKEN Center for Emergent Matter Science have achieved single-qubit gate fidelities exceeding 99.99 percent in a five-qubit silicon spin quantum array operated through a single shared microwave line, addressing a central scaling challenge in quantum processor design according to a study published on October 15, 2026, in Nature Physics.
Scaling up quantum processors requires shifting from sequential operations to the simultaneous control of multiple qubits. In silicon spin qubits, coherence properties are characterized by an average Hahn echo decay time of approximately 60 microseconds and an average dephasing time near 10.3 microseconds. This stark difference makes sequential control impractical because idle qubits dephase much faster than driven ones. Parallel control, however, typically degrades fidelities due to crosstalk, a hurdle that has challenged both semiconductor spin systems and superconducting architectures.
To overcome this, researchers demonstrated parallel control across a five-quantum-dot array fabricated in a silicon substrate. The device, operated by applying microwave signals via a shared screening gate electrode, manipulates all five spin states simultaneously through electric dipole spin resonance. According to Nature reporting, the system exhibits a Zeeman energy difference of approximately 150 megahertz between neighboring qubits, alongside an average dephasing time of 10.3 microseconds and an average Hahn echo decay time of 60 microseconds.
Mitigating AC Stark Shifts via Phase Calibrations
The primary driver of control errors during simultaneous operations is the AC Stark shift induced by microwave control signals. These signals alter a qubit’s effective energy levels, shifting the phase of its quantum state and reducing operational precision. To combat this without requiring exhaustive calibrations for every possible qubit combination, the team devised a scalable mitigation protocol.
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.
Yi-Hsien Wu, Ph.D. researcher and first author
Single-qubit randomized benchmarking experiments utilized Kaiser-window control pulses. Phys.org notes that these tailored pulses feature excitation spectra engineered to suppress direct microwave-drive-induced crosstalk caused by spectral leakage, a necessity when operating multi-qubit arrays through a shared control line.
Sustaining High Fidelities Across Five Qubits
When operated individually, each of the five qubits achieved single-qubit gate fidelities well above 99.99 percent, with top performances nearing 99.999 percent. By applying their phase calibration protocol, the researchers maintained fidelities above 99.99 percent when driving up to three qubits simultaneously. When scaling up to full five-qubit parallel operation, fidelities remained at or above 99.9 percent, hitting the practical threshold required for fault-tolerant quantum error correction protocols such as the surface code.
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.
Leon Camenzind, RIKEN research scientist and co-first author
During simultaneous operation of all five qubits, minor remaining fidelity degradation was linked to drive-induced decoherence stemming from the higher microwave power demands of multi-qubit driving. Seigo Tarucha, project manager of a silicon quantum computing project under JST’s Moonshot Goal 6, emphasized that reliable parallel control marks an essential step toward building a fault-tolerant silicon quantum computer.