  Spin qubits are considered to be among the most promising candidates for
building a quantum processor. GroupIV hole spin qubits have moved into the
focus of interest due to the ease of operation and compatibility with Si
technology. In addition, Ge offers the option for monolithic
superconductor-semiconductor integration. Here we demonstrate a hole spin qubit
operating at fields below 10 mT, the critical field of Al, by exploiting the
large out-of-plane hole g-factors in planar Ge and by encoding the qubit into
the singlet-triplet states of a double quantum dot. We observe electrically
controlled g-factor-difference-driven and exchange-driven rotations with
tunable frequencies exceeding 100 MHz and dephasing times of 1 $\mu$s which we
extend beyond 150 $\mu$s with echo techniques. These results demonstrate that
Ge hole singlet-triplet qubits are competing with state-of-the art GaAs and Si
singlet-triplet qubits. In addition, their rotation frequencies and coherence
are on par with Ge single spin qubits, but they can be operated at much lower
fields underlining their potential for on chip integration with superconducting
technologies.
