Key Findings
A collaborative team from RIKEN, National Taiwan University, ETH Zurich, and TU Delft has achieved simultaneous single-qubit gate fidelities of ≥99.9% across a five-qubit silicon (28 Si/SiGe) quantum dot array. The researchers successfully utilized a single, shared impedance-matched microwave transmission line to control the qubits, crossing the minimum threshold required for 2D surface code quantum error correction (QEC).
Technical Details
The team identified off-resonant microwave-drive-induced AC-Stark shifts as the primary cause of parallel drive crosstalk, which previously degraded gate fidelities from >99.9% down to 99.5%. To resolve this, they implemented an N(N-1) pairwise calibration protocol. Because the resulting phase shifts (Δφ ij) sum linearly, the crosstalk can be compensated by measuring pairwise phase accumulation between individual qubit pairs.
- Performance: Up to three qubits operated simultaneously at 99.99% accuracy; all five qubits maintained ≥99.9% fidelity.
- Residual Exchange: Decoupled CZ sequence measurements confirmed that residual exchange interactions (J off) between neighboring quantum dots remained below 16.6 kHz during single-qubit driving.
Background and Context
Simultaneous multi-qubit operations are essential for cyclic QEC protocols because idle silicon spin qubits dephase significantly faster (T 2 * ≈ 10 μs) than driven qubits (T 2,R ≈ 100 μs). Previous attempts to drive multiple qubits in parallel suffered from interference that compromised accuracy.
Significance and Outlook
The use of a single shared control line reduces physical wiring density and thermal loads, enabling modular silicon spin-qubit unit cells of approximately 10 qubits per line. The researchers indicate that replacing surface micromagnets with integrated back-end-of-line (BEOL) nanomagnets could double the local field gradient. This modification would allow for faster gate operations (2–3 MHz) at lower microwave power, providing a pathway toward 10-qubit parallel control modules in commercial silicon manufacturing.
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