  Two-level quantum systems with strong spin-orbit coupling allow for
all-electrical qubit control and long-distance qubit coupling via microwave and
phonon cavities, making them of particular interest for scalable quantum
information technologies. In silicon, a strong spin-orbit coupling exists
within the spin-3/2 system of acceptor atoms and their energy levels and
properties are expected to be highly tunable. Here we show the influence of
local symmetry tuning on the acceptor spin-dynamics, measured in the
single-atom regime. Spin-selective tunneling between two coupled boron atoms in
a commercial CMOS transistor is utilised for spin-readout, which allows for the
probing of the two-hole spin relaxation mechanisms. A relaxation-hotspot is
measured and explained by the mixing of acceptor heavy and light hole states.
Furthermore, excited state spectroscopy indicates a magnetic field controlled
rotation of the quantization axes of the atoms. These observations demonstrate
the tunability of the spin-orbit states and dynamics of this spin-3/2 system.
