The negatively charged silicon vacancy (SiV) color center in diamond has recently proven its suitability for bright and stable single photon emission. However, its electronic structure so far has remained elusive. We here explore the electronic structure by exposing single SiV defects to a magnetic field where the Zeeman effect lifts the degeneracy of magnetic sublevels. The similar response of single centers and a SiV ensemble in a low strain reference sample proves our ability to fabricate almost perfect single SiVs, revealing the true nature of the defect's electronic properties. We model the electronic states using a group-theoretical approach yielding a good agreement with the experimental observations. Furthermore, the model correctly predicts polarization measurements on single SiV centers and explains recently discovered spin selective excitation of SiV defects. Negatively charged silicon vacancy (SiV − ) color centers in diamond show a typical room-temperature zero phonon line (ZPL) at 738 nm which splits into a four line fine structure centered at about 737 nm when cooled down to liquid helium temperature [1][2][3]. The origin of the fine structure splitting is attributed to a split ground and excited state [1]. One mechanism that can account for the level splitting is spin-orbit (SO) coupling, like it is present for the excited state in negatively charged nitrogen-vacancy (NV − ) centers [4]. Alternatively, Clark et al. [1] and Moliver [5] suggest a tunnel splitting whereas Goss et al.[6] assume a Jahn-Teller (JT) effect in addition to SO coupling to lift the orbital degeneracy between the electronic states which account for the presumed optical transition 2 E u → 2 E g . To form doubly degenerate 2 E many-body wave functions, at least a trigonal defect symmetry is required [7,8]. The molecular structure of the SiV center was predicted using density functional theory (DFT) to show a rather unique split vacancy configuration, exhibiting a D 3d symmetry [9]. Yet, polarization [10,11] and uniaxial stress measurements [2] evidenced lower symmetrical point groups such as C 2 or D 2 symmetry. Still, all these experimental evidences were obtained using samples that possess strongly strained environments for the defect centers. In this letter, however, we present evidence for the predicted D 3d symmetry by performing spectroscopy on SiV centers in low-strain samples.Recently published EPR measurements showed that the presumed neutral charge state SiV 0 is a S = 1 system [12]. This suggests that its negative counterpart SiV − is a paramagnetic S = 1/2 system, although this has not been confirmed by independent EPR measurements so far. Very recently, we reported direct spin-selective population of the SiV − excited states under a magnetic field, resulting in a spin-tagged resonance fluorescence pattern [13], suggesting that the SiV − shows effectively S = 1/2. In the present letter, we experimentally explore the electronic states of the SiV center by measuring Zeeman splittings and polarization orientation of t...

Among a variety of layered materials used as building blocks in van der Waals heterostructures, hexagonal boron nitride (hBN) appears as an ideal platform for hosting optically-active defects owing to its large bandgap ($\sim 6$~eV). Here we study the optical response of a high-purity hBN crystal under green laser illumination. By means of photon correlation measurements, we identify individual defects emitting a highly photostable fluorescence under ambient conditions. A detailed analysis of the photophysical properties reveals a high quantum efficiency of the radiative transition, leading to a single photon source with very high brightness ($\sim 4\times 10^6$ counts.s$^{-1}$). These results illustrate how the wide range of applications offered by hBN could be further extended to photonic-based quantum information science and metrology

We study, both theoretically and experimentally, the free induction decay (FID) of the electron spin associated with a single nitrogen-vacancy defect in high-purity diamond, where the main source of decoherence is the hyperfine interaction with a bath of 13 C nuclear spins. In particular, we report a systematic study of the FID signal as a function of the strength of a magnetic field oriented along the symmetry axis of the defect. On average, an increment of the coherence time by a factor of √ 5/2 is observed at high magnetic field in diamond samples with a natural abundance of 13 C nuclear spins, in agreement with numerical simulations and theoretical studies. Further theoretical analysis shows that this enhancement is independent of the concentration of nuclear-spin impurities. By dividing the nuclear-spin bath into shells and cones, we theoretically identify the nuclear spins responsible for the observed dynamics. IntroductionColor centers in solids have emerged as good candidates for quantum information processing as they provide optical access on demand to a quantum degree of freedom [1]. Among them, the nitrogen-vacancy (NV) color center in diamond can be manipulated with full control and has remarkable properties for many applications ranging from high-sensitivity and highresolution magnetometry [2][3][4][5][6], to quantum information processing [7][8][9][10][11][12] and imaging in life sciences [13][14][15]. Most of these applications rely on the long coherence time of the NV defect electronic spin, which is mainly limited by magnetic interactions with a bath of paramagnetic impurities inside the diamond matrix. Therefore, efforts have aimed at creating diamond samples with a low concentration of impurities [16,17], controlling the implantation of single NV defects [18][19][20][21][22], manipulating and controlling the dynamics of their spin bath [23][24][25][26][27] and understanding the interaction between the central spin and its environment, both experimentally and theoretically [28][29][30][31][32][33][34].The decoherence of a central spin in the presence of a spin bath has been addressed using several approaches [35][36][37][38][39][40]. Here we study, both experimentally and theoretically, the free induction decay (FID) of the electronic spin associated with a single NV defect in diamond. In particular, we have carried out a statistical study on the coherence time, T * 2 , as a function of the strength of a magnetic field oriented along the NV defect symmetry axis. Our results indicate an increment of the coherence time at large magnetic fields, in agreement with our numerical simulations and theoretical studies. In addition, we study the coherence time for different concentrations of the spin bath and identify the main features of the central spin dynamics. By dividing the bath into shells and cones, we analyze the contribution to decoherence of each impurity with respect to its position relative to the central spin. These results, which complement recent works [34,41], might be particularly usefu...

BackgroundStable and non-toxic fluorescent markers are gaining attention in molecular diagnostics as powerful tools for enabling long and reliable biological studies. Such markers should not only have a long half-life under several assay conditions showing no photo bleaching or blinking but also, they must allow for their conjugation or functionalization as a crucial step for numerous applications such as cellular tracking, biomarker detection and drug delivery.ResultsWe report the functionalization of stable fluorescent markers based on nanodiamonds (NDs) with a bifunctional peptide. This peptide is made of a cell penetrating peptide and a six amino acids long β-sheet breaker peptide that is able to recognize amyloid β (Aβ) aggregates, a biomarker for the Alzheimer disease. Our results indicate that functionalized NDs (fNDs) are not cytotoxic and can be internalized by the cells. The fNDs allow ultrasensitive detection (at picomolar concentrations of NDs) of in vitro amyloid fibrils and amyloid aggregates in AD mice brains.ConclusionsThe fluorescence of functionalized NDs is more stable than that of fluorescent markers commonly used to stain Aβ aggregates such as Thioflavin T. These results pave the way for performing ultrasensitive and reliable detection of Aβ aggregates involved in the pathogenesis of the Alzheimer disease.Electronic supplementary materialThe online version of this article (10.1186/s12951-018-0385-7) contains supplementary material, which is available to authorized users.

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