The NV defect consists of a substitutional nitrogen atom (N) combined with a vacancy (V) in one of the nearest neighboring sites of the diamond crystal lattice [Fig. 1a]. This defect with C3v symmetry behaves as an artificial atom nestled in the diamond matrix and exhibits a broadband photoluminescence (PL) emission with a zero phonon line at 1.945 eV (?ZPL=637nm), allowing for the detection of individual NV defects using optical confocal microscopy at room temperature. Importantly, the NV defect does not suffer from photobleaching nor blinking, as often observed for solid-state emitters like dye molecules or quantum dots under ambient conditions. This perfect photostability enabled the development of highly robust single photon sources operating at room temperature and is currently exploited in biology where NV defects hosted in diamond nanocrystals are used as fluorescent labels.
Figure 1 : (a) Atomic structure of the NV defect in diamond. (b) Energy level scheme. The notation Ii> denotes the state with spin projection ms=i along the NV defect axis. (c) Optically detected electron spin resonance (ESR) spectra recorded for different magnetic field magnitudes applied to a single NV defect in diamond. The ESR transitions are shifted owing to the Zeeman effect, thus providing a quantitative measurement of the magnetic field projection along the NV defect quantization axis. These spectra are recorded by monitoring the NV defect PL intensity while sweeping the frequency of the microwave (MW) field. Spectra for different magnetic fields are shifted vertically for clarity.
Another essential feature of the NV defect is that its ground level is a spin triplet state, whose sub-levels are split in energy by spin-spin interaction into a singlet state of spin projection ms=0 and a doublet ms=±1, separated by D=2.87 GHz in the absence of a magnetic field [Fig. 1b]. Here, ms denotes the spin projection along the intrinsic quantization axis of the NV defect corresponding to the axis joining the nitrogen and the vacancy ( crystal axis). The defect can be optically excited through a spin conserving transitions to an excited level, which is also a spin triplet. Once optically excited, the NV defect can relax either through the same radiative transition producing a broadband red PL, or through a secondary path involving non radiative intersystem crossing (ISC) to singlet states. This process plays a crucial role in the NV defect spin dynamics. Indeed, while optical transitions are mainly spin conserving (?ms =0), non-radiative ISCs to the singlet state are strongly spin selective as the shelving rate from the ms =0 sublevel is much smaller than those from ms=±1. Conversely, the NV defect decays preferentially from the lowest singlet state towards the ground state ms=0 sublevel. These spin-selective processes provide a high degree of electron spin polarization into ms=0 through optical pumping. Furthermore, since ISCs are non radiative, the NV defect PL intensity is significantly higher when the ms=0 state is populated. Such a spin-dependent PL response enables the detection of electron spin resonance (ESR) on a single defect by optical means. Indeed, when a single NV defect, initially prepared in the ms=0 state through optical pumping, is driven to the ms=±1 spin state by applying a resonant microwave field, a drop in the PL signal is observed, as depicted in Fig. 1c. Over the last years, this property has been extensively used in the context of diamond-based quantum information processing where the NV defect is explored as a solid-state spin qubit.
For magnetometry applications, the principle of the measurement is similar to the one used in optical magnetometers based on the precession of spin-polarized atomic gases. The applied magnetic field is evaluated through the detection of Zeeman shifts of the NV defect electron spin sublevels. Indeed, when a magnetic field is applied in the vicinity of the NV defect, the degeneracy of ms=±1 spin sublevels is lifted by the Zeeman effect, leading to the appearance of two resonance lines in the ESR spectrum [Fig. 1c]. A single NV defect therefore behaves as a magnetic field sensor with an atomic-sized detection volume.
This property forms the cornerstone of the DIADEMS project.
More details can be found in recent review articles
L. Rondin, J.-P. Tetienne, T. Hingant, J.-F. Roch, P. Maletinsky, and V. Jacques, Magnetometry with nitrogen-vacancy defects in diamond.
Rep. Prog. Phys. 77, 056503 (2014).
R. Schirhagl, K. Chang, M. Loretz, and C. L. Degen,?Nitrogen-Vacancy Centers in Diamond: Nanoscale Sensors for Physics and Biology.
Annual Reviews in Physical Chemistry 65, 83–105 (2014).