Silicon spin qubits could play a major role in the future of quantum computing because they can be produced using the same semiconductor manufacturing methods that make today’s computer chips. However, researchers have faced a significant challenge. When multiple silicon qubits are controlled at the same time, their operations tend to become less accurate. Building practical, large quantum computers will require scientists to control many qubits simultaneously while keeping errors to a minimum.
Researchers at the RIKEN Center for Emergent Matter Science and their collaborators have now made important progress toward solving this problem. In a study published in Nature Physics, the team demonstrated highly accurate control of several silicon spin qubits operating simultaneously. Their experiments used an array of five qubits connected to a shared microwave control line. By identifying a major cause of errors during simultaneous operation, the scientists developed a technique to reduce those errors and maintain precision.
Leon Camenzind, a RIKEN research scientist and co-first author who supervised the experiments, explains the significance of the results: “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.”
Scientists Identify a Hidden Source of Quantum Computing Errors
Unlike conventional computers, which store information as bits representing either 0 or 1, quantum computers use quantum bits, or qubits, that can exist in combinations of both states. Silicon spin qubits store this information using the spin of individual electrons confined within silicon. Performing reliable quantum calculations requires extremely precise control over these electron spins.
Microwave signals provide that control, but operating multiple qubits simultaneously can create unwanted effects that interfere with their accuracy. The researchers traced much of this problem to a phenomenon called the AC Stark shift.
This effect occurs when microwave control signals alter a qubit’s effective energy levels, producing changes in the phase of its quantum state. Those phase shifts can disrupt the intended operation of individual qubits, particularly when several are being controlled at once.
To counteract the problem, the team created a calibration method that compensates for these unwanted shifts. Rather than separately calibrating every possible combination of qubits, the technique relies on calibrations performed between pairs of qubits. This makes the approach more practical for systems containing larger numbers of quantum bits.
“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,” explains Yi-Hsien Wu, the first author who carried out the experiments as part of his PhD research.
Quantum Qubits Achieve Accuracy Above 99.99%
The researchers combined their calibration technique with an improved device design and carefully adjusted microwave control pulses. Their results showed that exceptionally high accuracy could be maintained even when multiple qubits were operating simultaneously.
When the five qubits were tested individually, each achieved single-qubit gate fidelity well above 99.99%, with the highest approaching 99.999%. Fidelity describes how closely a quantum operation matches its intended result.
The team then tested how well the system performed when several qubits were active together. With as many as three qubits operating simultaneously, fidelity remained above 99.99%. Even when all five qubits were controlled at once, accuracy stayed at approximately 99.9%, approaching the levels needed for practical fault-tolerant quantum computing.
Although the calibration method greatly reduced errors, it did not eliminate them entirely. The researchers found that the remaining loss of accuracy was linked to drive-induced decoherence, an effect caused by the greater microwave power required to operate multiple qubits simultaneously.
Unlike the unwanted phase shifts corrected by the new technique, decoherence involves the loss of the delicate quantum coherence needed for reliable operations.
According to Camenzind, “This is a limitation, but further improvements to device design could help mitigate this source of error.”
A New Path Toward Larger Quantum Processors
The findings show that controlling several silicon spin qubits at the same time does not necessarily require a major sacrifice in accuracy. By correcting a key source of errors, the researchers have demonstrated a promising approach for operating more qubits through shared microwave control lines.
This is especially important for the development of large quantum processors. Giving every qubit its own independent control system would make increasingly complex devices difficult to manage. Shared control lines could offer a more practical way to expand these systems while maintaining the precision needed for quantum calculations.
“By addressing a key source of control errors through pairwise calibration,” says Camenzind. “Our study supports the use of shared control lines as a practical approach to scaling up quantum processors. Combined with silicon’s compatibility with established semiconductor manufacturing, we believe that this advance could help pave the way toward larger quantum processors capable of performing increasingly complex computations.”
The broader objective is to develop quantum computers that can reliably perform calculations despite the errors and disturbances that affect quantum systems. Such machines, known as fault-tolerant quantum computers, will require precise, dependable control over large numbers of qubits.
“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,” adds Seigo Tarucha, project manager of a silicon quantum computing project under JST’s Moonshot Goal 6.