TITLE: Exploiting Spin–Orbit Physics in Germanium for Next-Generation Spin Qubits
ABSTRACT: Quantum computing based on electron and hole spins in semiconductors offers a compelling route toward scalable quantum hardware, building on decades of advances in microelectronics. A key challenge, however, is to achieve spin qubits that can be controlled rapidly and electrically, while maintaining long coherence times. In recent years, hole spins confined in strained germanium quantum wells have emerged as a highly promising platform, combining strong spin–orbit interactions, weak hyperfine coupling, and the absence of valley degeneracies.
In this colloquium, I will introduce the basic physics of spin qubits in germanium and explain why spin–orbit coupling—often viewed as a source of decoherence—can instead be turned into a powerful resource for qubit control. I will then present experiments on a gate-defined hole singlet–triplet qubit, where we observe that the qubit frequency can be tuned by nearly an order of magnitude using only millivolt-scale changes in a single gate voltage. This striking effect corresponds to a strong, electrically driven modulation of the effective g-factor and enables fast, all-electrical qubit control.
To understand the physical origin of this sensitivity, I will show how realistic quantum-dot devices naturally host inhomogeneous strain fields arising from lattice mismatch and thermal contraction. By combining nanoscale strain measurements with finite-element simulations and a Luttinger–Kohn description of hole states, we find that small gate-induced displacements of the quantum dots through these strain landscapes can strongly modify the spin–orbit interaction and the g-tensor. These results highlight an intimate connection between strain, spin–orbit physics, and qubit control in germanium.
I will conclude by discussing how this understanding opens new opportunities for strain and device engineering, transforming an apparent source of variability into a design knob for fast, tunable, and potentially higher-temperature spin qubits compatible with silicon-based quantum technologies
