Scientists at the RIKEN Center for Emergent Matter Science and collaborators have made significant progress in controlling multiple silicon spin qubits at the same time while maintaining high accuracy, according to findings published in Nature Physics. Silicon spin qubits are considered promising for future quantum computers because they can be manufactured using existing semiconductor production techniques. However, researchers have struggled with a critical problem: when several qubits are controlled simultaneously, their operational accuracy tends to decline, which poses a major obstacle to building practical large-scale quantum computers.
The research team identified a primary source of errors occurring during simultaneous qubit operation: the AC Stark shift. This phenomenon takes place when microwave control signals alter a qubit’s effective energy levels, causing unintended changes in the phase of its quantum state. These phase shifts can compromise the intended operations of individual qubits, particularly when multiple qubits are being controlled together. To address this issue, the researchers developed a calibration technique that compensates for these unwanted shifts without requiring separate calibration procedures for every possible combination of qubits. Instead, the method relies on calibrations performed between pairs of qubits, making it more practical for larger systems.
When tested individually, each of the five qubits achieved single-qubit gate fidelity well above 99.99%, with the highest reaching approximately 99.999%. When three qubits operated simultaneously, fidelity remained above 99.99%. Even with all five qubits controlled at once, accuracy stayed near 99.9%, approaching levels required for practical fault-tolerant quantum computing. The researchers combined their calibration technique with improved device design and refined microwave control pulses to achieve these results.
Although the new calibration method significantly reduced errors, some accuracy loss remained. The team attributed this residual error to drive-induced decoherence, an effect caused by the higher microwave power needed to operate multiple qubits simultaneously. The findings demonstrate that controlling several silicon spin qubits together does not necessarily require major sacrifices in accuracy. This approach could enable scaling up quantum processors using shared microwave control lines, which would be more practical than providing each qubit with independent control systems as devices become more complex.
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