Kavli Affiliate: Giordano Scappucci
| Summary:
Singlet-triplet qubits offer an attractive encoding for semiconductor quantum computing, combining ancilla-free readout, reduced sensitivity to common-mode noise, and baseband voltage control. However, the Zeeman energy difference $ΔE_mathrmZ$ is typically fixed by local magnetic field gradients or $g$-factor inhomogeneities, leaving the exchange interaction $J$ as the only dynamically tunable parameter. This always-on $ΔE_mathrmZ$ precludes orthogonal control of the qubit’s rotation axes and introduces unwanted state rotations during idling. Here we demonstrate all-electrical orthogonal control of a degenerate singlet-triplet (DST) qubit formed by two hole spins in a germanium double quantum dot. Exploiting the electrically tunable anisotropic $g$-factors of the two spins, we identify a regime where both $ΔE_mathrmZ$ and $J$ vanish, making the $S$ and $T_0$ states degenerate at the idle point. By applying only baseband voltage pulses, we independently control both $J$ and $ΔE_mathrmZ$, enabling fully orthogonal $Z$- and $X$-axis rotations. Randomized benchmarking yields an average physical single-qubit gate fidelity of 99.53% for a gate duration of approximately 100 ns. Finally, we electrically tune the degenerate point across a wide range of magnetic field orientations, enabling operation in a regime of enhanced coherence time and offering a route towards multi-qubit scaling under a shared global magnetic field.
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