  Point defects in insulators are considered promising candidates for quantum
technologies. In keeping with this, we present an extensive optically-detected
magnetic resonance (ODMR) study at room-temperature on individual TR12 centers
(ZPL at 471nm), which are known in the literature since 1956. In this work we
found TR12 centers to show a strong ODMR signal under optical saturation. These
observed defects were created in high-purity epitaxial layers of diamond by
standard irradiation and annealing processes. From the analysis of the ODMR
spectra along with antibunching measurements and coherent population trapping,
we proposed the energy level structure of TR12 center, consisting of ground
state and excited state singlets complemented by a metastable triplet in
between. Mapping the fluorescence dependence of the center on an external
magnetic field and on the polarization of laser excitation, allows us to
identify twelve inequivalent orientations for TR12 centers. This includes the
exact orientations of the dipole transition and the triplet axes in the diamond
lattice in full agreement with the results of modeling based on the proposed
level structure. Furthermore, a static Jahn-Teller effect was detected through
fluorescence switching between two levels at low optical excitation power,
directly observable in the real-time fluorescence signal for various
polarization of laser excitation. Based on these results we discuss the
prospects of the TR12 center in diamond for quantum sensing and quantum
information processing.
