Curator's Take
AI Commentary
This article demonstrates the first all‑electrical orthogonal control of a degenerate singlet‑triplet qubit in germanium hole spins, eliminating the always‑on Zeeman gradient that has limited idle‑time fidelity in previous ST qubits. By exploiting tunable anisotropic g‑factors to bring both exchange J and ΔE_Z to zero at the idle point, the authors achieve fully independent X‑ and Z‑axis rotations with a 99.53 % single‑qubit gate fidelity in ~100 ns—on par with the best semiconductor spin qubits today. The ability to shift the degenerate operating point electrically across magnetic‑field orientations paves the way for multi‑qubit scaling under a shared global field, reducing the need for local micromagnets. Maintaining the precise g‑factor balance in larger arrays will be a key engineering challenge moving forward.
— Mark Eatherly
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 $ΔE_\mathrm{Z}$ 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 $ΔE_\mathrm{Z}$ 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 $ΔE_\mathrm{Z}$ 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 $ΔE_\mathrm{Z}$, 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.