Currently, both in academia and industry, different material platforms are investigated for the realization of quantum processors. For implementations in the solid state domain, superconducting compounds and group IV semiconductor elements are among the most promising ones. Superconducting circuits have proven to overcome classical computation on specific tasks while semiconductor qubits are gaining interest due to the compatibility with industrial fabrication techniques and long coherence times. The possibility of sharing the same material for both platforms would be a great technological advancement. Planar Ge offers unique features in this respect. On the one hand, the Fermi pinning allows the realization of proximity induced superconductivity. On the other hand, high fidelity spin qubits in Ge quantum dots have been demonstrated and are competing with their Si counterparts. However, the Ge hole spin qubits reported so far consist of a single isolated spin, which requires a large Zeeman splitting and therefore magnetic field. This prohibits the co-integration with Al-based superconducting technology.
In this talk I will discuss the first hole singlet-triplet qubit, realized in planar Ge. By exploiting the effects of spin orbit coupling, two-axis, in-situ tunable, electrical control is achieved, without the need of dynamical nuclear polarization or micromagnets. Due to the large g-factor differences between the two quantum dots, they can be operated at out-of-plane fields well below 10mT, i.e. the critical field of Aluminium. By varying the magnetic field direction we can furthermore investigate the hole spin orbit physics. By performing Landau-Zener sweeps we can disentangle the Zeeman mixing effect from the cubic Rashba spin-orbit induced coupling between singlet and triplet states. Our results emphasize the need for a complete knowledge of the energy landscape when working with hole spin orbit qubits with large g-factor differences.