The negatively charged silicon vacancy (SiV) color center in diamond has recently proven its suitability for bright and stable single photon emission. However, its electronic structure so far has remained elusive. We here explore the electronic structure by exposing single SiV defects to a magnetic field where the Zeeman effect lifts the degeneracy of magnetic sublevels. The similar response of single centers and a SiV ensemble in a low strain reference sample proves our ability to fabricate almost perfect single SiVs, revealing the true nature of the defect's electronic properties. We model the electronic states using a group-theoretical approach yielding a good agreement with the experimental observations. Furthermore, the model correctly predicts polarization measurements on single SiV centers and explains recently discovered spin selective excitation of SiV defects. Negatively charged silicon vacancy (SiV − ) color centers in diamond show a typical room-temperature zero phonon line (ZPL) at 738 nm which splits into a four line fine structure centered at about 737 nm when cooled down to liquid helium temperature [1][2][3]. The origin of the fine structure splitting is attributed to a split ground and excited state [1]. One mechanism that can account for the level splitting is spin-orbit (SO) coupling, like it is present for the excited state in negatively charged nitrogen-vacancy (NV − ) centers [4]. Alternatively, Clark et al. [1] and Moliver [5] suggest a tunnel splitting whereas Goss et al.[6] assume a Jahn-Teller (JT) effect in addition to SO coupling to lift the orbital degeneracy between the electronic states which account for the presumed optical transition 2 E u → 2 E g . To form doubly degenerate 2 E many-body wave functions, at least a trigonal defect symmetry is required [7,8]. The molecular structure of the SiV center was predicted using density functional theory (DFT) to show a rather unique split vacancy configuration, exhibiting a D 3d symmetry [9]. Yet, polarization [10,11] and uniaxial stress measurements [2] evidenced lower symmetrical point groups such as C 2 or D 2 symmetry. Still, all these experimental evidences were obtained using samples that possess strongly strained environments for the defect centers. In this letter, however, we present evidence for the predicted D 3d symmetry by performing spectroscopy on SiV centers in low-strain samples.Recently published EPR measurements showed that the presumed neutral charge state SiV 0 is a S = 1 system [12]. This suggests that its negative counterpart SiV − is a paramagnetic S = 1/2 system, although this has not been confirmed by independent EPR measurements so far. Very recently, we reported direct spin-selective population of the SiV − excited states under a magnetic field, resulting in a spin-tagged resonance fluorescence pattern [13], suggesting that the SiV − shows effectively S = 1/2. In the present letter, we experimentally explore the electronic states of the SiV center by measuring Zeeman splittings and polarization orientation of t...

We study single silicon vacancy (SiV) centres in chemical vapour deposition (CVD) nanodiamonds on iridium as well as an ensemble of SiV centres in a high-quality, low-stress CVD diamond film by using temperaturedependent luminescence spectroscopy in the temperature range 5-295 K. We investigate in detail the temperature-dependent fine structure of the zero-phonon line (ZPL) of the SiV centres. The ZPL transition is affected by inhomogeneous as well as temperature-dependent homogeneous broadening and blue shifts by about 20 cm −1 upon cooling from room temperature to 5 K. We employ 6

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.

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