  Strong, direct Rashba spin-orbit coupling in Si, Ge, and the Ge/Si core/shell
nanowire quantum dot (QD) allows for all electrical manipulation of the hole
spin qubit. Motivated by this fact, we analyze different fabrication-dependent
properties of nanowires, such as orientation, cross section, and the presence
of strain, with the goal being to find the material and geometry that enables
the fastest qubit manipulation, whose speed can be identified using the Rabi
frequency. We show that QD in nanowires with a circular cross section (cNWs)
enables much weaker driving of the hole spin qubit than QDs embedded in square
profile nanowires (sNWs). Assuming the orientation of the Si nanowire that
maximizes the spin-orbit effects, our calculations predict that the Rabi
frequencies of the hole spin qubits inside Ge and Si sNW QD have comparable
strengths for weak electric fields. The global maximum of the Rabi frequency is
found in Si sNW QD for strong electric fields, putting this setup ahead of
others in creating the hole spin qubit. Finally, we demonstrate that strain in
the Si/Ge core/shell nanowire QD decreases the Rabi frequency. In cNW QD, this
effect is weak; in sNW QD, it is possible to optimize the impact of strain with
the appropriate tuning of the electric field strength.
