  Quantum information processing (QIP) with solid state spin qubits strongly
depends on the efficient initialisation of the qubit's desired charge state.
While the negatively charged tin-vacancy ($\text{SnV}^{-}$) centre in diamond
has emerged as an excellent platform for realising QIP protocols due to long
spin coherence times at liquid helium temperature and lifetime limited optical
transitions, its usefulness is severely limited by termination of the
fluorescence under resonant excitation [1,2,3]. Here, we unveil the underlying
charge cycle of group IV-vacancy (G4V) centres and exploit it to demonstrate
highly efficient initialisation of the desired negative charge state of single
SnV centres while preserving long term stable optical resonances. We
furthermore all-optically probe the coherence of the ground state spins by
means of coherent population trapping and find a spin dephasing time of
5$\mu$s. Additionally, we demonstrate proof-of-principle single shot spin state
readout without the necessity of a magnetic field aligned to the symmetry axis
of the defect.
