Figure 1

(a) Simplified energy-level structure of the NV defect. The zoom indicates the energy levels and transition rates used for studying the NV defect spin dynamics in the dark. (b) Integrated PL signal as a function of the dark time measured for a single NV center hosted in a 20 nm ND. The solid line is a fit to Eq. (1), which yields $T_1=16\pm 1$ $\mu$s and $T_m=160\pm 30$ ns. The experimental sequence used to measure $T_1$ is shown in inset. Laser pulses with 3 $\mu$s duration are used both for initialization of the NV defect in $m_s=0$ and for spin-state readout by recording the PL signal in a detection window corresponding to the first 300 ns of the optical pulses.Reuse & Permissions

Figure 2

(a) Relaxation curves measured for a single NV defect hosted in a small ND ($d_0=13\pm 3$ nm, blue curve) and a larger one ($d_0=73\pm 10$ nm, red curve). Data fitting with Eq. (1) (solid lines) gives $T_1$ values of $3.6\pm 0.3$ and $802\pm 136$ $\mu$s, respectively. The corresponding AFM images are shown on top of the graph. (b) Longitudinal spin relaxation rate $1/T_1$ of the NV defect electron spin as a function of the ND diameter. We note that no obvious correlations with the size was observed for the other fitting parameters in Eq. (1), which were found to be $C_m=0.084\pm 0.037$, $C_1=0.21\pm 0.12$, and $T_m=198\pm 72$ ns (mean $\pm$ s.d.). (c) The ND is modeled as a sphere with diameter $d_0$ with a bath of randomly fluctuating surface spins with density $\sigma$. In (b), the markers are experimental data while the lines are the results of the calculation using this model for a NV spin located at the center of the sphere (solid line) and 3 nm below the surface (dotted line). The parameters of the calculation are $T_1^{\mathrm{bulk}}=2$ ms and $\sigma =1$ nm${}^{-2}$.Reuse & Permissions

Figure 3

(a) Histograms of the $1/T_1$ relaxation rate obtained from a set of 33 single NV defects hosted in isolated NDs. The measurement is performed before any treatment (top panel), after adding 1 mM of the Gd${}^{3+}$ solution (middle panel) and after further adding 10 mM of solution (bottom panel). (b) Relaxation rate measured after the first (circles) and second (triangles) treatment step as a function of the rate of the bare ND. The solid lines are data fitting with linear functions whose slope indicates the average quenching ratio $\eta =T_{1,\mathrm{bare}}/T_{1,\mathrm{treated}}$. We obtain $\eta =7$ (green line) and $\eta =31$ (orange line). A dashed line of slope $\eta =1$ is plotted for reference.Reuse & Permissions

Figure 4

$\mathcal{I}\left(\tau \right)$ relaxation curves measured after repeated treatments with the Gd${}^{3+}$ solution. Solid lines are fit to Eq. (1). Inset: Effective $T_1$ contrast $C_1^{\mathrm{eff}}$ as a function of $T_1$. The solid line is the result of the calculation (with no fit parameter) using a rate equation model with the parameters of NV centers in bulk diamond.[18]Reuse & Permissions