Fully Tunable Strong Spin-Orbit Interactions in Light Hole Germanium Quantum Channels

Spin-orbit interaction (SOI) is a fundamental component for electrically driven spin qubits and hybrid superconducting-semiconducting systems. In particular, Rashba SOI (RSOI) is a key mechanism enabling all-electrical spin manipulation schemes. However, in common planar systems, RSOI is weak because of the small mixing between heavy holes (HH) and light holes (LH), and instead relies on complex strain and interface phenomena that are hard to reliably harness in experiment. Here, MOS-like epitaxial Ge on relaxed \GeSn{} is introduced and shown to exhibit an inherently large, highly gate-tunable RSOI that is compatible with both spin qubits and hybrid devices. This large RSOI is a consequence of the LH-like ground state in Ge. Notably, the built-in asymmetry of the device causes the RSOI to completely vanish at specific gate fields, effectively acting as an on/off SOI switch. The LH $g$-tensor is less anisotropic than that of state-of-the-art HH qubits, alleviating precise magnetic field orientation requirements. The large in-plane $g$-factor also facilitates the integration of superconductors. Moreover, the out-of-plane $g$-factor is strongly gate-tunable and completely vanishes at specific gate fields. Thus, this material system combines the large RSOI with the scalability of planar devices, paving the way towards robust spin qubit applications and enabling access to new regimes of complex spin physics.