  Hole spin qubits based on germanium (Ge) have strong tunable spin orbit
interaction (SOI) and ultrafast qubit operation speed. Here we report that the
Rabi frequency (f_Rabi) of a hole spin qubit in a Ge hut wire (HW) double
quantum dot (DQD) is electrically tuned through the detuning energy and middle
gate voltage (V_M). f_Rabi gradually decreases with increasing detuning energy;
on the contrary, f_Rabi is positively correlated with V_M. We attribute our
results to the change of electric field on SOI and the contribution of the
excited state in quantum dots to f_Rabi. We further demonstrate an ultrafast
f_Rabi exceeding 1.2 GHz, which evidences the strong SOI in our device. The
discovery of an ultrafast and electrically tunable f_Rabi in a hole spin qubit
has potential applications in semiconductor quantum computing.
