Colour centres in nanodiamonds are an important resource for applications in quantum sensing, biological imaging, and quantum optics. Here we report unprecedented narrow optical transitions for individual colour centres in nanodiamonds smaller than 200 nm. This demonstration has been achieved using the negatively charged silicon vacancy centre, which has recently received considerable attention due to its superb optical properties in bulk diamond. We have measured an ensemble of silicon-vacancy centres across numerous nanodiamonds to have an inhomogeneous distribution of 1.05 nm at 5 K. Individual spectral lines as narrower than 360 MHz were measured in photoluminescence excitation, and correcting for apparent spectral diffusion yielded an homogeneous linewidth of about 200 MHz which is close to the lifetime limit. These results indicate the high crystalline quality achieved in these nanodiamond samples, and advance the applicability of nanodiamond-hosted colour centres for quantum optics applications. Nanodiamonds (NDs) hosting optically active point defects ('colour centres') are an important technical material for applications in quantum sensing [1], biological imaging [2][3][4], and quantum optics [5]. One colour centre which has attracted recent attention is the negatively charged silicon vacancy (SiV − ) defect, which consists of a silicon atom taking the place of two adjacent carbon atoms in the lattice [6]. The SiV − centre in diamond has risen to prominence on the basis of its superb spectral properties, including a strong zero-phonon line (ZPL) at 737 nm which contains 70% of the fluorescence from this colour centre [7]. In low-strain bulk diamond, the SiV − centre has exhibited lifetime-limited spectral linewidths at 4 K with no spectral diffusion [8]. These ideal properties have enabled the efficient production of indistinguishable photons from distinct emitters [9]. Recent studies in bulk diamond have shown that the electronic spin coherence time in the SiV − centre is fundamentally limited by fast phonon-induced orbital relaxation in the ground state [10,11]. Small NDs should impose boundary conditions that prevent the availablilty of phonons at the critical frequency, thereby extending coherence time. This has increased the motivation to find well-behaved SiV − centres in the nanodiamond environment.Although SiV − centres have been observed to fluoresce in NDs as small as molecules (1.6 nm) [12], the ND host has always led to less homogeneous photon emission [13][14][15][16]. Some promising results have been recently reported for larger hybrid nanostructures [17], but the obstacle persists for SiV − applications requiring ND environments. Here we report unprecedented optical properties of SiV − colour centres hosted in nanodiamonds. Individual spectral lines close to the lifetime limit were measured for SiV − centres in OPEN ACCESS RECEIVED

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...