  Gatemon qubits are the electrically tunable cousins of superconducting
transmon qubits. In this work, we demonstrate the full coherent control of a
gatemon qubit based on hole carriers in a Ge/Si core/shell nanowire, with the
longest coherence times in group IV material gatemons to date. The key to these
results is a high-quality Josephson junction obtained in a straightforward and
reproducible annealing technique. We demonstrate that the transport through the
narrow junctions is dominated by only two quantum channels, with transparencies
up to unity. This novel qubit platform holds great promise for quantum
information applications, not only because it incorporates technologically
relevant materials, but also because it provides new opportunities, like an
ultrastrong spin-orbit coupling in the few-channel regime of Josephson
junctions.
