Ensembles of nitrogen-vacancy (N-$V$) centers in diamonds are widely utilized for magnetometry, magnetic field imaging, and magnetic resonance detection. At zero ambient field, Zeeman sublevels in the N-V centers lose first-order sensitivity to magnetic fields, as they are mixed due to crystal strain or electric fields. In this work, we realize a zero-field (ZF) magnetometer using polarization-selective microwave excitation in a ${}^{13}\mathrm{C}$-depleted crystal sample. We employ circularly polarized microwaves to address specific transitions in the optically detected magnetic resonance and perform magnetometry with a noise floor of $250\mathrm{pT}/\sqrt{\mathrm{Hz}}$. This technique opens the door to practical applications of N-$V$ sensors for ZF magnetic sensing, such as ZF nuclear magnetic resonance and investigation of magnetic fields in biological systems.

Sensing vector magnetic fields is critical to many applications in fundamental physics, bioimaging, and material science. Magnetic field sensors exploiting nitrogen-vacancy (N-$V$) centers are particularly compelling as they offer high sensitivity and spatial resolution even at the nanoscale. Achieving vector magnetometry, however, often requires applying microwaves sequentially or simultaneously, limiting the sensors’ applications under cryogenic temperature. Here, we propose and demonstrate a microwave-free vector magnetometer that simultaneously measures all Cartesian components of a magnetic field using N-$V$ ensembles in diamond. In particular, the present magnetometer leverages the level anticrossing in the triplet ground state at 102.4 mT, allowing the measurement of both longitudinal and transverse fields with a wide bandwidth from zero to the megahertz range. Full vector sensing capability is proffered by modulating fields along the preferential N-$V$ axis and in the transverse plane and subsequent demodulation of the signal. This sensor exhibits a root-mean-square noise floor that approximately equals 300 $\mathrm{pT}/\sqrt{\mathrm{Hz}}$ in all directions. The present technique is broadly applicable to both ensemble sensors and potentially also to single-N-$V$ sensors, extending the vector capability to nanoscale measurements under ambient temperatures.

We present two fiberized vector magnetic-field sensors, based on nitrogen-vacancy (NV) centers in diamond. The sensors feature sub-nT/Hz magnetic sensitivity. We use commercially available components to construct sensors with a small sensor size, high photon collection, and minimal sensor-sample distance. Both sensors are located at the end of optical fibres with the sensor-head freely accessible and robust under movement. These features make them ideal for mapping magnetic fields with high sensitivity and spatial resolution (≤ mm). As a demonstration we use one of the sensors to map the vector magnetic field inside the bore of a ≥100 mT Halbach array. The vector field sensing protocol translates microwave spectroscopy data addressing all diamonds axes and including double quantum transitions to a 3D magnetic field vector.

The nitrogen-vacancy center (NV center) in diamond at magnetic fields corresponding to the ground-state level anticrossing (GSLAC) region gives rise to rich photoluminescence (PL) signals due to the vanishing energy gap between the electron spin states, which enables for a broad variety of environmental couplings to have an effect on the NV center's luminescence. Previous works have addressed several aspects of the GSLAC photoluminescence, however, a comprehensive analysis of the GSLAC signature of NV ensembles in different spin environments at various external fields is missing. Here we employ a combination of experiments and recently developed numerical methods to investigate in detail the effects of transverse electric and magnetic fields, strain, P1 centers, NV centers, and the ${}^{13}\mathrm{C}$ nuclear spins on the GSLAC photoluminescence. Our comprehensive analysis provides a solid ground for advancing various microwave-free applications at the GSLAC, including but not limited to magnetometry, spectroscopy, dynamic nuclear polarization (DNP), and nuclear magnetic resonance (NMR) detection. We demonstrate that not only the most abundant ${}^{14}\mathrm{NV}$ center but the ${}^{15}\mathrm{NV}$ can also be utilized in such applications.

Magnetic impurities in diamond influence the relaxation properties and thus limit the sensitivity of magnetic, electric, strain, and temperature sensors based on nitrogen-vacancy color centers. Diamond samples may exhibit significant spatial variations in the impurity concentrations hindering the quantitative analysis of relaxation pathways. Here, we present a local measurement technique which can be used to determine the concentration of various species of defects by utilizing double electron-electron resonance. This method will help to improve the understanding of the physics underlying spin relaxation and guide the development of diamond samples, as well as offering protocols for optimized sensing.

Energy levels of nitrogen-vacancy centers in diamond were investigated using optically detected magnetic-resonance spectroscopy near the electronic ground-state level anticrossing (GSLAC) at an axial magnetic field around 102.4 mT in diamond samples with a nitrogen concentration of 1 and 200 ppm. By applying radio waves in the frequency ranges from 0 to 40 MHz and from 5.6 to 5.9 GHz, we observed transitions that involve energy levels mixed by the hyperfine interaction. We developed a theoretical model that describes the level mixing, transition energies, and transition strengths between the ground-state sublevels, including the coupling to the nuclear spin of the NV centers' ${}^{14}\mathrm{N}$ and ${}^{13}\mathrm{C}$ atoms. The calculations were combined with the experimental results by fitting the ODMR spectral lines based on a theoretical model, which yielded information about the polarization of nuclear spins. This study is important for the optimization of experimental conditions in GSLAC-based applications, e.g., microwave-free magnetometry and microwave-free nuclear-magnetic-resonance probes.