Silicon Qubits Run Together at About 99.9% Accuracy After Fix for Hidden Error Source

Researchers at the RIKEN Center for Emergent Matter Science and their collaborators report that they have operated several silicon spin qubits at the same time with very high accuracy, addressing a hidden obstacle to building larger quantum computers. The study was published in Nature Physics.

Quantum computers use quantum bits, or qubits, which can exist in combinations of 0 and 1, unlike the bits in conventional machines. In silicon spin qubits, information is stored in the spin of individual electrons confined within silicon. Reliable calculations depend on extremely precise control of those spins, which is delivered with microwave signals.

The trouble comes when many qubits are driven at once. The researchers say operating multiple qubits simultaneously can create unwanted effects that interfere with accuracy, and they traced much of the problem to a phenomenon called the AC Stark shift. This occurs when microwave control signals alter a qubit's effective energy levels, changing the phase of its quantum state. Those phase shifts can disrupt the intended operation of individual qubits, especially when several are controlled together.

The team's experiments used an array of five qubits connected to a shared microwave control line. To counter the Stark-shift problem, they developed a calibration method that compensates for the unwanted phase shifts. Instead of calibrating every possible combination of qubits separately, the technique relies on calibrations between pairs of qubits. That makes it more practical for systems with larger numbers of qubits. "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," said Yi-Hsien Wu, the first author, who carried out the experiments as part of his PhD research.

The researchers combined the calibration approach with an improved device design and carefully adjusted microwave control pulses. They measured fidelity, which describes how closely a quantum operation matches its intended result. When the five qubits were tested individually, each achieved single-qubit gate fidelity well above 99.99%, with the best approaching 99.999%.

Performance held up when qubits worked together. With as many as three qubits operating simultaneously, fidelity stayed above 99.99%. With all five controlled at once, accuracy remained at approximately 99.9%, which the researchers describe as approaching the levels needed for practical fault-tolerant quantum computing.

The method did not remove all errors. The researchers found that the remaining loss of accuracy was linked to drive-induced decoherence, an effect caused by the greater microwave power needed to run multiple qubits at once. Unlike the phase shifts the new technique corrects, decoherence involves the loss of the delicate quantum coherence that reliable operations require. Leon Camenzind, a RIKEN research scientist and co-first author who supervised the experiments, called this "a limitation," adding that "further improvements to device design could help mitigate this source of error."

Why does this matter? Giving each qubit its own independent control system would make increasingly complex devices hard to manage. Shared control lines could offer a more practical way to expand these systems while maintaining precision. Camenzind said the study "supports the use of shared control lines as a practical approach to scaling up quantum processors," and noted that silicon's compatibility with established semiconductor manufacturing could help pave the way toward larger processors capable of more complex computations.

The broader aim is fault-tolerant quantum computers, machines that can calculate reliably despite the errors and disturbances that affect quantum systems. Seigo Tarucha, project manager of a silicon quantum computing project under JST's Moonshot Goal 6, said the long-term goal is a fault-tolerant quantum computer based on silicon spin qubits, and that reliable parallel control is an essential step toward it.

The demonstration involved five qubits, far fewer than a large processor would need, and the drive-induced decoherence remains an open problem. The researchers point to refined device design as the next avenue for reducing it.