We present an experimental study of the longitudinal electron-spin relaxation of ensembles of negatively charged nitrogen-vacancy (NV -) centers in diamond. The measurements were performed with samples having different NVconcentrations and at different temperatures and magnetic fields. We found that the relaxation rate T1 -1 increases when transition frequencies in NVcenters with different orientations become degenerate and interpret this as cross-relaxation caused by dipole-dipole interaction.

Negatively-charged nitrogen-vacancy (NV − ) centers in diamond have generated much recent interest for their use in sensing. The sensitivity improves when the NV ground-state microwave transitions are narrow, but these transitions suffer from inhomogeneous broadening, especially in high-density NV ensembles. To better understand and remove the sources of broadening, we demonstrate room-temperature spectral "hole burning" of the NV ground-state transitions. We find that hole burning removes the broadening caused by magnetic fields from13 C nuclei and demonstrate that it can be used for magnetic-field-insensitive thermometry. The nitrogen-vacancy (NV) color center in diamond is a defect center consisting of a substitutional nitrogen atom adjacent to a missing carbon atom. When negatively charged (NV − ), its ground state has electronic spin 1 (Fig. 1a), and physical parameters such as magnetic field, electric field, and temperature affect the energies of its magnetic sublevels [1][2][3]. One can measure these parameters by employing optically-detected magnetic resonance (ODMR) techniques [4,5], which use microwave (MW) fields resonant with the NV transitions and detect changes in fluorescence in the presence of excitation light. The NV− ground-state sublevels can be optically accessed and have long spin-relaxation times at room temperature [6], making them useful for sensing. When limited by spin-projection noise, the sensitivity is proportional to Γ/N , where Γ is the ODMR linewidth and N is the number of NV centers probed [1,7,8]. In practice, the transitions are inhomogeneously broadened due to differences in the NV local environments, limiting the ensemble sensitivity. Diamond samples with more paramagnetic impurities also have more inhomogeneous broadening, meaning that larger N often comes with larger Γ. Furthermore, NVs with different Larmor frequencies dephase, which is a limitation in some applications. Although refocusing pulse sequences (such as Hahn echo) can restore the coherence, identifying the sources of ODMR linewidth broadening is essential for NV applications and for understanding the underlying diamond spin-bath and crystal-strain physics.In this work we demonstrate novel use of saturation spectroscopy (or "hole-burning") techniques in an NV ensemble. This is motivated by saturation spectroscopy in atoms, where a spectrally-narrow pump laser selects atoms of a particular velocity class by removing them from their initial state, allowing one to recover narrow absorption lines with a probe laser [9].We present two hole-burning schemes. The analytically simpler scheme ("pulsed hole-burning") is depicted in Fig. 1b. This scheme addresses a two-level subsystem (m s = 0 and +1) and uses a modified pulsed-ODMR sequence (similar to that of Ref.[8]). A spectrally-narrow "hole" π-pulse first shelves some NVs into the m s = +1 state, after which a probe π-pulse reads out its effect on the NV population distribution. Figure 1c shows that using this method can yield hole widths significantly narrower than t...

The ability to create time-dependent magnetic fields of controlled polarization is essential for many experiments with magnetic resonance. We describe a microstrip circuit that allows us to generate strong magnetic field at microwave frequencies with arbitrary adjusted polarization. The circuit performance is demonstrated by applying it to an optically detected magnetic resonance and Rabi nutation experiments in nitrogen-vacancy color centers in diamond. Thanks to high efficiency of the proposed microstrip circuit and degree of circular polarization of 85% it is possible to address the specific spin states of a diamond sample using a low power microwave generator. The circuit may be applied to a wide range of magnetic resonance experiments with a well-controlled polarization of microwaves.