Speaker: Jann H. Ungerer (Harvard University)

Title: Orthogonal Baseband-Control of a Germanium Hole-Spin Qubit

Baseband control of hole-spin qubits in germanium [1,2] is an attractive route to scalable qubit operation, eliminating the need for microwave hardware while leveraging the platform's long coherence times [3], flexible connectivity [4], and strong spin-orbit coupling [5]. A fundamental obstacle, however, is that baseband pulse sequences are generically non-orthogonal, which imposes significant overhead on universal single-qubit control.

We overcome this by engineering a singlet-triplet qubit in germanium with a degenerate idle point, where both the Zeeman energy gradient and the exchange energy vanish, rendering the two control axes naturally orthogonal [6]. This enables universal, all-electrical single-qubit control with nanosecond baseband pulses, without the pulse-engineering overhead required by non-orthogonal control.

Looking ahead, the orthogonal control axes provide direct, independent access to variations in the underlying Hamiltonian parameters during gate operations, offering a natural handle for real-time noise characterization and correction via online neural-network inference and learning [7,8]. This opens a path toward adaptive, real-time qubit control.

[1] Jirovec, D, et al. Nature Materials 20.8 (2021): 1106-1112.

[2] Wang, C., et al. Science 385.6707 (2024): 447-452.

[3] Hendrickx, N. W., et al. Nature Materials 23.7 (2024): 920-927

[4] Tidjani, H., …, Ungerer, J.H., et al. arXiv:2512.01634 (2025)

[5] Bulaev, D. V., and Loss, D. Physical review letters 98.9 (2007): 097202

[6] Nguyen, P.X.*, Tsoukalas, K.*, Ungerer, J.H.*, et al. in preparation (2026), *equal contributions

[7] Reuer, K., et al. Nature Communications 14.1 (2023): 7138

[8] Hoang, D., et al. arXiv:2602.02056 (2026)