High-fidelity flopping-mode single-spin operation via triple-dot orbital-band engineering

Abstract Fast electrical spin rotations in quantum dots require strong spin-charge hybridization, but the same hybridization can increase sensitivity to detuning noise. In a double-quantum-dot (DQD) flopping-mode qubit, the sweet-spot width and the Rabi enhancement are both tied to the same orbital splitting, limiting post-fabrication optimization of the speed-coherence trade-off. Here, we demonstrate a GaAs triple-quantum-dot (TQD) flopping-mode qubit in which the detuning of the third dot, ϵ 34 , provides an additional electrical tuning parameter for orbital-band flattening. Tuning ϵ 34 flattens the qubit-frequency dependence on the detuning of the target DQD while maintaining strong spin-charge hybridization, enabling electrically driven Rabi frequencies above 100 MHz. We then use feedforward-neural-network (FNN) feedback to track slow qubit-frequency drift and connect this engineered operating point to high-fidelity gates. This feedback suppresses slow qubit-frequency fluctuations, reducing their standard deviation to 388 kHz, and yields a π /2 gate fidelity of 99.56% with a 4.05 ns gate time. These results identify TQD orbital-band engineering as an electrical route for tuning the speed-coherence trade-off, complementary to material-intrinsic spin-orbit coupling and fixed micromagnet geometry.