  Full electrical control of quantum bits could enable fast, low-power,
scalable quantum computation. Although electric dipoles are highly attractive
to couple spin qubits electrically over long distances, mechanisms identified
to control two-qubit couplings do not permit single-qubit operations while
two-qubit couplings are off. Here we identify a mechanism to modulate
electrical coupling of spin qubits that overcomes this drawback for hole spin
qubits in acceptors,that is based on the electrical tuning of the direction of
the spin-dependent electric dipole by a gate. In this way, inter-qubit coupling
can be turned off electrically by tuning to a "magic angle" of vanishing
electric dipole-dipole interactions, while retaining the ability to manipulate
the individual qubits. This effect stems from the interplay of the $\rm T_d$
symmetry of the acceptor state in the Si lattice with the magnetic field
orientation, and the spin-3/2 characteristic of hole systems. Magnetic field
direction also allows to greatly suppress spin relaxation by phonons that limit
single qubit performance, while retaining sweet spots where the qubits are
insensitive to charge noise. Our findings can be directly applied to
state-of-the-art acceptor based architectures, for which we propose suitable
protocols to practically achieve full electrical tunability of entanglement and
the realization of a decoherence-free subspace.
