Researchers at the RIKEN Center for Emergent Matter Science and their collaborators have overcome a significant obstacle to building larger quantum computers, achieving highly accurate control of several silicon spin qubits operating at the same time. In a study published in the journal Nature Physics, the team traced a major source of simultaneous-operation errors to a microwave effect called the AC Stark shift and showed that a pairwise calibration technique, combined with an improved device design, kept five qubits on a shared control line accurate to about 99.9% even when all five ran at once.
Silicon spin qubits store quantum information in the spin of individual electrons confined within silicon. They are seen as promising building blocks for future quantum computers because they can be produced using the same semiconductor manufacturing methods that make today's computer chips. But when multiple qubits are controlled at the same time, their operations tend to become less accurate, an obstacle to building practical, large quantum machines.
The experiments used an array of five qubits connected to a shared microwave control line. The researchers traced much of the simultaneous-operation problem to the AC Stark shift, an effect in which microwave control signals alter a qubit's effective energy levels and change the phase of its quantum state. Instead of calibrating every possible combination of qubits separately, the team's method relies on calibrations between pairs 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 study's first author, who carried out the experiments as part of his PhD research.
Combined with an improved device design and carefully adjusted microwave pulses, the calibration delivered exceptional accuracy. Tested individually, each of the five qubits achieved single-qubit gate fidelity well above 99.99%, with the highest approaching 99.999%. With three qubits operating simultaneously, fidelity remained above 99.99%, and with all five active at once, accuracy stayed at approximately 99.9%, approaching the levels needed for practical fault-tolerant quantum computing.
"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," said Leon Camenzind, a RIKEN research scientist and co-first author who supervised the experiments.
The technique did not eliminate all errors. The remaining loss of accuracy was linked to drive-induced decoherence, an effect of the greater microwave power required to operate multiple qubits at once, in which the delicate quantum coherence needed for reliable operations is lost. "This is a limitation, but further improvements to device design could help mitigate this source of error," Camenzind said.
The team argues the results support using shared control lines as a practical route to larger processors, since giving every qubit its own independent control system would make increasingly complex devices difficult to manage. "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," said Seigo Tarucha, project manager of a silicon quantum computing project under JST's Moonshot Goal 6.
Analysis
Why This Matters
- Silicon spin qubits can be manufactured with the same processes used for conventional computer chips, so progress in controlling them could make larger quantum processors more practical to build.
- Parallel control of many qubits at once is essential for real quantum computers; this work shows shared microwave control lines can deliver high accuracy rather than requiring a separate control system for every qubit.
- Accuracy of about 99.9% with all five qubits active approaches the levels the study identifies as needed for fault-tolerant quantum computing, where machines can correct their own errors.
Background
Conventional computers store information as bits that represent either 0 or 1. Quantum computers use qubits, which can exist in combinations of both states, and silicon spin qubits store that information in the spin of individual electrons confined within silicon. Microwave control signals steer the qubits, and performing reliable quantum calculations requires extremely precise control over those electron spins. A long-standing obstacle has been that operating several silicon qubits at once creates unwanted effects that lower accuracy. The RIKEN team's work corrects one major source of those errors, the AC Stark shift, through pairwise calibration, while a second source, drive-induced decoherence linked to the higher microwave power needed for parallel operation, remains for device design to address.
What to Watch
- Whether further improvements to device design can reduce drive-induced decoherence, the remaining source of error identified in the study.
- Whether the pairwise calibration approach stays practical as arrays grow beyond five qubits and more qubits share a single microwave control line.
- Progress toward the team's stated long-term goal of building a fault-tolerant quantum computer based on silicon spin qubits.
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- Partly supportedScientists have cleared a hidden obstacle to scaling up silicon quantum computers.Latest Science News -- ScienceDaily: “The researchers have now made important progress toward solving this problem.”
- Partly supportedThe researchers have overcome a significant obstacle to building larger quantum computers.Latest Science News -- ScienceDaily: “The researchers have now made important progress toward solving this problem.”
