Scientists at the RIKEN Center for Emergent Matter Science have made significant progress in addressing a critical challenge for quantum computing development. Silicon spin qubits are particularly promising for future quantum processors because they can be manufactured using existing semiconductor production techniques. However, when multiple qubits are controlled at the same time, their operations typically become less accurate, which poses a major barrier to building practical large-scale quantum computers.
The research team identified a major source of errors during simultaneous qubit operation: a phenomenon called the AC Stark shift. This effect occurs when microwave control signals alter a qubit’s energy levels, causing unwanted phase shifts that disrupt individual qubit operations. To address this problem, the researchers developed an innovative calibration technique that compensates for these phase shifts. Rather than creating separate calibration procedures for every possible combination of qubits, their method relies on calibrations performed between pairs of qubits, making it practical for systems with larger numbers of quantum bits.
The team tested their approach on an array of five qubits connected to a shared microwave control line. Individual qubits achieved single-qubit gate fidelity well above 99.99%, with the highest reaching approximately 99.999%. When multiple qubits were tested simultaneously, fidelity remained above 99.99% with up to three qubits operating together, and stayed at approximately 99.9% when all five qubits were controlled at once. These accuracy levels approach what is needed for fault-tolerant quantum computing, though the researchers note that remaining errors stem from drive-induced decoherence caused by increased microwave power requirements.
The findings have important implications for scaling quantum processors. Using shared control lines rather than individual control systems for each qubit offers a more practical way to expand these systems. Combined with silicon’s compatibility with established semiconductor manufacturing, the advance represents a significant step toward developing larger quantum processors capable of performing increasingly complex calculations.
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