Atomic defects in solids offer access to atom-like quantum properties without complex trapping methods while displaying a rich physics due to interactions with their solid-state environment. Such properties have made them an advantageous building block for quantum information processing, in particular to construct a quantum network, where information would be encoded in spins and transferred between nodes through photons. Among defects in solids, the negatively charged silicon-vacancy centre in diamond (SiV$\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$) has attracted attention for its very promising optical properties for such a network. In this thesis, we investigate the spin properties of the silicon-vacancy centre as a potential spin-photon interface. First, we use resonant excitation of an SiV$\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ centre in an external magnetic field to selectively address different electronic states and analyse the resulting fluorescence. We find evidence of selection rules in the optical transitions revealing that the centre possesses an electronic spin S = 1/2. Making use of the dependence of such selection rules on the applied magnetic field orientation, we resonantly drive two optical transitions forming a $\Lambda$-scheme. In the double resonance condition, we achieve coherent population trapping, whereby the SiV$\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ is pumped into a dark state corresponding to a superposition of the two addressed ground states of opposite spin. This technique allows us to evaluate the coherence time of the dark state and hence of the spin, while demonstrating the possibility of all-optical control of the spin when a $\Lambda$-scheme is available. We then use resonant optical pulses to initialise and read out the spin state of a single SiV$\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$. By tuning a microwave pulse into resonance between two ground states of opposite spin, we demonstrate optically detected magnetic resonance. Subsequently, by varying the duration of a resonant microwave pulse, we achieve coherent control of a single SiV$\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ electronic spin. Through Ramsey interferometry, we measure a spin dephasing time of 115 $\pm$ 9 ns. We then investigate interactions of the SiV$\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ with its environment. We analyse the hyperfine interaction of the SiV$\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ spin with the nuclear spin of $\mathrm{<mrow\; data-mjx-texclass="ORD">29</mrow>}$Si, with a view to taking advantage of the long-lived nuclear spin in the future. We show that single-phonon-mediated excitations between electronic states of the SiV$\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ are the dominant spin dephasing and population decay mechanism and evaluate how external strain alters optical selection rules and can be used to improve the coherence time of the spin.