High-Sensitivity Magnetometry Based on Quantum Beats in Diamond Nitrogen-Vacancy Centers

We demonstrate an absolute magnetometer based on quantum beats in the ground state of nitrogen-vacancy centers in diamond. We show that, by eliminating the dependence of spin evolution on the zero-field splitting $D$, the magnetometer is immune to temperature fluctuation and strain inhomogeneity. We apply this technique to measure low-frequency magnetic field noise by using a single nitrogen-vacancy center located within 500 nm of the surface of an isotopically pure (99.99% ${}^{12}\mathrm{C}$) diamond. The photon-shot-noise limited sensitivity achieves $38\mathrm{nT}/\sqrt{\mathrm{Hz}}$ for 4.45 s acquisition time, a factor of $\sqrt{2}$ better than the implementation which uses only two spin levels. For long acquisition times ($>10\mathrm{s}$), we realize up to a factor of 15 improvement in magnetic sensitivity, which demonstrates the robustness of our technique against thermal drifts. Applying our technique to nitrogen-vacancy center ensembles, we eliminate dephasing from longitudinal strain inhomogeneity, resulting in a factor of 2.3 improvement in sensitivity.

Figure 1

(a) A single-tone microwave pulse interacts with all NV center ground-state sublevels. (b) Ramsey-type magnetometry using quantum beats between $|B⟩$ and $|D⟩$. (c) Ramsey fringes using the protocol in (b). The fitted decay envelope yields $T_2^{*}=30(1)\mu \mathrm{s}$. Inset: Absolute value of the Fourier transform of the Ramsey fringes. The three peaks correspond to the three hyperfine resonances between $m_s=\pm 1$ levels.

Figure 2

(a) Ramsey fringe of the $\{0,1\}$ basis (left panel) and $\{1,-1\}$ basis (right panel) for different temperatures. (b) Zero-field splitting $D$ dependence on temperature as measured in the $\{0,1\}$ basis (red). The $\{1,-1\}$ basis is immune to changing $D$ (blue). (c) Fluorescence level dependence on temperature for fixed delay time, $18.475\mu \mathrm{s}$ for the $\{0,1\}$ basis (upper panel) and $19.720\mu \mathrm{s}$ for the $\{1,-1\}$ basis (lower panel) [indicated by the arrows on the $\tau$ axis in (a)]. Red curves are measured data. Dashed lines indicate the temperature change. Solid lines are the calculated fluorescence level for the $\{0,1\}$ basis.

Figure 3

(a) Measured noise spectrum using the $\{0,1\}$ and $\{1,-1\}$ bases. Dashed lines are the noise floor near 1 Hz. (b) Allan deviation of the noise.

Figure 4

Fourier transform of $P(\tau )$ in both bases for an ensemble of NV centers. Each peak corresponds to the $m_I=-1$ level. Black curves are Gaussian fits.