  Silicon-vacancy qubits in silicon carbide (SiC) are emerging tools in quantum
technology applications due to their excellent optical and spin properties. In
this paper, we explore the effect of temperature and strain on these properties
by focusing on the two silicon-vacancy qubits, V1 and V2, in 4H SiC. We apply
density functional theory beyond the Born-Oppenheimer approximation to describe
the temperature dependent mixing of electronic excited states assisted by
phonons. We obtain polaronic gap around 5 and 22~meV for V1 and V2 centers,
respectively, that results in significant difference in the temperature
dependent dephasing and zero-field splitting of the excited states, which
explains recent experimental findings. We also compute how crystal deformations
affect the zero-phonon-line of these emitters. Our predictions are important
ingredients in any quantum applications of these qubits sensitive to these
effects.
