A fidelity milestone in silicon
A team led by researchers at RIKEN has demonstrated single-qubit gate fidelities above 99.99 percent across all five qubits in a silicon spin qubit array, with some measurements approaching 99.999 percent. The results were published in Nature Physics.
Fidelity is a measure of how often a quantum operation produces the intended result. Error rates below one in ten thousand put silicon devices in the range where error correction schemes can start to work, and this is the highest single-qubit performance reported so far for silicon spin qubits.
Silicon spin qubits store information in the spin of a single electron. They are roughly a thousand times smaller than the superconducting transmons used by most quantum computers today, and they can be made with the same CMOS fabrication tools that produce ordinary computer chips, which is why foundries study them closely.
Running qubits side by side
The harder problem is parallel control. When several qubits share one microwave control line, signals bleed into each other and fidelity drops. The team traced the main source of that degradation in a five-qubit silicon array and designed a mitigation protocol based only on pairwise phase calibrations.
Using tailored Kaiser-window control pulses on a shared line, the researchers kept fidelities above 99.99 percent for each qubit individually and maintained that level while operating up to three qubits at the same time. When all five ran in parallel, fidelity stayed at 99.9 percent or above, the practical fault-tolerant threshold.
Why it matters for scaling
Quantum computers need thousands of qubits to tackle useful problems, and each qubit needs control electronics. If every qubit required its own dedicated line, the wiring alone would become unmanageable. Sharing control lines is a route around that bottleneck.
Seigo Tarucha, who manages a silicon quantum computing project under a Japanese Moonshot programme, said reliable parallel control is an essential step toward a fault-tolerant machine built on silicon spin qubits. The paper suggests the control approach should extend to larger arrays, based on the power scaling the team observed. Independent labs will now try to reproduce the numbers, which is the usual test for claims this precise.
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