  Colour centres in diamond have emerged as versatile tools for solid-state
quantum technologies ranging from quantum information to metrology, where the
nitrogen-vacancy centre is the most studied to-date. Recently, this toolbox has
expanded to include different materials for their nanofabrication
opportunities, and novel colour centres to realize more efficient spin-photon
quantum interfaces. Of these, the silicon-vacancy centre stands out with
ultrabright single photon emission predominantly into the desirable zero-phonon
line. The challenge for utilizing this centre is to realise the hitherto
elusive optical access to its electronic spin. Here, we report spin-tagged
resonance fluorescence from the negatively charged silicon-vacancy centre. In
low-strain bulk diamond spin-selective excitation under finite magnetic field
reveals a spin-state purity approaching unity in the excited state. We also
investigate the effect of strain on the centres in nanodiamonds and discuss how
spin selectivity in the excited state remains accessible in this regime.
