Optical signatures of silicon-vacancy spins in diamond.

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 novel colour centres to realize more efficient spin-photon quantum interfaces. Of these, the silicon-vacancy centre stands out with highly desirable photonic properties. The challenge for utilizing this centre is to realize the hitherto elusive optical access to its electronic spin. Here we report spin-tagged resonance fluorescence from the negatively charged silicon-vacancy centre. Our measurements reveal a spin-state purity approaching unity in the excited state, highlighting the potential of the centre as an efficient spin-photon quantum interface.