  The states of a boron acceptor near a Si/SiO2 interface, which bind two
low-energy Kramers pairs, have exceptional properties for encoding quantum
information and, with the aid of strain, both heavy hole and light hole-based
spin qubits can be designed. Whereas a light-hole spin qubit was introduced
recently [Phys. Rev. Lett. 116, 246801 (2016)], here we present analytical and
numerical results proving that a heavy-hole spin qubit can be reliably
initialised, rotated and entangled by electrical means alone. This is due to
strong Rashba-like spin-orbit interaction terms enabled by the interface
inversion asymmetry. Single qubit rotations rely on electric-dipole spin
resonance (EDSR), which is strongly enhanced by interface-induced spin-orbit
terms. Entanglement can be accomplished by Coulomb exchange, coupling to a
resonator, or spin-orbit induced dipole-dipole interactions. By analysing the
qubit sensitivity to charge noise, we demonstrate that interface-induced
spin-orbit terms are responsible for sweet spots in the dephasing time T2* as a
function of the top gate electric field, which are close to maxima in the EDSR
strength, where the EDSR gate has high fidelity. We show that both qubits can
be described using the same starting Hamiltonian, and by comparing their
properties we show that the complex interplay of bulk and interface-induced
spin-orbit terms allows a high degree of electrical control and makes acceptors
potential candidates for scalable quantum computation in Si.
