  We present angle-dependent measurements of the effective g-factor g* in a
Ge-Si core-shell nanowire quantum dot. g* is found to be maximum when the
magnetic field is pointing perpendicular to both the nanowire and the electric
field induced by local gates. Alignment of the magnetic field with the electric
field reduces g* significantly. g* is almost completely quenched when the
magnetic field is aligned with the nanowire axis. These findings confirm recent
calculations, where the obtained anisotropy is attributed to a Rashba-type
spin-orbit interaction induced by heavy-hole light-hole mixing. In principle,
this facilitates manipulation of spin-orbit qubits by means of a continuous
high-frequency electric field.
