Impurity-vacancy complexes in diamond are an attractive family of spin defects since $\mathrm{N}{V}^{–}, \mathrm{Si}{V}^{–}, \mathrm{Ge}{V}^{–}$, and $\mathrm{Sn}{V}^{–}$ have emerged as promising platforms for quantum applications. Although boron is most easily incorporated into diamond, a boron-vacancy complex in the negative charge state $\left(\mathrm{B}{V}^{–}\right)$ has eluded experimental observation. This center was theoretically predicted as another promising spin qubit. In this work, we experimentally observed an electron paramagnetic resonance (EPR) spectrum identified as $\mathrm{B}{V}^{–}$ in synthetic diamonds via a Fermi-level tuning. Fingerprints of $\mathrm{B}{V}^{–}$ such as the spin multiplicity of $S=1, C_{3\mathrm{v}}$ symmetry, and the zero-field splitting ($D=2913$ MHz), in addition to ${}^{10}\mathrm{B}$ and ${}^{11}\mathrm{B}$ hyperfine (HF) interactions, have been confirmed. Moreover, optically pumped spin polarization has been observed with 3.0–3.6 eV excitation. However, unlike the $\mathrm{N}{V}^{–}$ center, the photoluminescence as well as optically detected magnetic resonance from $\mathrm{B}{V}^{–}$ have not been confirmed even at low temperatures. We speculate that the Jahn-Teller instability in the triplet excited states of the $\mathrm{N}{V}^{–}$ and $\mathrm{B}{V}^{–}$ centers results in different optical properties.

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• Color centers
• Wide band gap systems
• Electron paramagnetic resonance