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(a) Structure of the NV color center in the diamond lattice, consisting of a substitutional nitrogen (N) and a neighboring vacancy (V). (b) Schematic representation of the energy levels for the negatively charged center. It can be optically excited with a laser at 532 nm wavelength and emits a broadband luminescence with a zero phonon line at 637 nm. The ground level is a spin triplet with a zero-field frequency splitting of 2.87 GHz between and states.
(a) Optically detected ESR signal for an ensemble of NV centers, and the influence of a static magnetic field. (b) Variation of the ESR frequencies associated with the four spin orientations as a function of the magnitude of the applied magnetic field oriented along the  crystal axis. The angles between the NV spins and the magnetic field direction are 90° and 35.3°, respectively. The small splitting in the zero-field spectrum is due to the fact that the local axial symmetry is broken because of the strain effects, and the degeneracy between and levels is removed.
(a) Schematic representation of the NV centers' four possible spin orientations in the diamond matrix relative to its  crystal axis. Due to the symmetry, the angle between two possible orientations is 109.4°. The laboratory reference axis (A) is chosen to be the symmetrical axis of two possible orientations marked 1 and 2. is the angle between the axis A and the magnetic field when it is oriented to lie in the (3,4)-plane perpendicular to the -crystal axis. (b) ESR frequencies of spins associated with the corresponding orientations 1, 2, 3, and 4, as a function of the rotation angle . Experimental results are shown in dots, and the curves are the simulation results.
Influence of the applied magnetic field on the NV center average lifetime. The lifetime changes when the external magnetic field is applied, due to the mixing between and states. The magnetic field orientation is parallel (a) or perpendicular (b) to the  axis, and the magnetic field magnitude is varied. In (b), the magnetic field direction corresponds to an -angle of −100°. (c) The magnetic field magnitude is fixed at 15 mT and its orientation is rotated in a plane perpendicular to the  axis.
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