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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% 12C) diamond. The photon-shot-noise limited sensitivity achieves 38  nT/sqrt[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  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.

ABSTRACT:We have studied optical and spin properties of near-surface nitrogen-vacancy (NV) centers incorporated during chemical vapor phase growth of isotopically purified 12 C single-crystal diamond layers. The spectral diffusion-limited line width of zero-phonon luminescence from the NV centers is 1.2 ± 0.5 GHz, a considerable improvement over that of NV centers formed by ion implantation and annealing. Enhanced spin dephasing times (T 2 * ≈ 90 μs, T 2 ≈ 1.7 ms) due to the reduction of 13 C nuclear spins persist even for NV centers placed within 100 nm of the surface.KEYWORDS: diamond, nitrogen-vacancy center, chemical vapor deposition, quantum information processing, magnetometry I n recent years, quantum information processing (QIP) based on nitrogen-vacancy (NV) centers in diamond has seen much progress.1,2 The spin-dependent optical properties of NV centers allow optical readout 3 and manipulation 4 of the spin states, coherent population trapping, 5 and entanglement between photons and the spin qubits.6 Since the NV center exhibits a long electron-spin coherence time, 7 it is also a promising system for applications in quantum memory 8−10 and magnetometry. 11−15For diamond-based QIP and magnetometry, it is often desirable for NV centers to be located within nanometers of the diamond surface. For QIP, near-surface NV centers enable efficient optical coupling to on-chip waveguides and cavities.16−20 For nanoscale magnetometry, near-surface NV centers will reduce the sensor-sample distance, thereby improving sensitivity. The most well-studied way to create near-surface NV centers is a procedure based on ion implantation and annealing.21−23 However, these NV centers suffer from large optical spectral diffusion prohibiting efficient spin readout and spin-photon entanglement. 6,24,25 Furthermore, the coherence times of these defects are much shorter than those found for NV centers formed during crystal growth. 26 This suggests that a promising approach to producing near-surface NV centers is to incorporate the centers during the growth of a thin epitaxial layer. In this work, we study the optical and spin coherence properties of such growthincorporated centers in an isotopically purified 12 C film.Promisingly, we find the spectral diffusion of the zero-phonon line (ZPL) is significantly improved, when compared to implanted samples. Moreover, we find that the long spin coherence times expected for 12 C diamond persist even for NV centers within 100 nm of the surface. These properties indicate that NV centers in such thin layers are promising for applications in both QIP and magnetometry.We used two samples named S1 and S2. The 12 C diamond layers ([ 12 C] = 99.99%) of S1 and S2 were deposited on synthetic type IIa (100) single-crystal diamond plates by microwave plasma assisted chemical vapor deposition (CVD). The growth conditions are described in our Supporting Information. The resulting 12 C layer thickness is 1000 nm for S1, and 525 nm for S2. In sample S1, nitrogen gas was introduced during the last 100 nm ...

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