  Paramagnetic defects and nuclear spins are the major sources of magnetic
field-dependent spin relaxation in point defect quantum bits. The detection of
related optical signals has led to the development of advanced relaxometry
applications with high spatial resolution. The nearly degenerate quartet ground
state of the silicon vacancy qubit in silicon carbide (SiC) is of special
interest in this respect, as it gives rise to relaxation rate extrema at
vanishing magnetic field values and emits in the first near-infra-red
transmission window of biological tissues, providing an opportunity for
developing novel sensing applications for medicine and biology. However, the
relaxation dynamics of the silicon vacancy center in SiC have not yet been
fully explored. In this paper, we present results from a comprehensive
theoretical investigation of the dipolar spin relaxation of the quartet spin
states in various local spin environments. We discuss the underlying physics
and quantify the magnetic field and spin bath dependent relaxation time $T_1$.
Using these findings we demonstrate that the silicon vacancy qubit in SiC can
implement microwave-free low magnetic field quantum sensors of great potential.
