ABSTRACT
The attention the world of quantum science generated around diamond defects was just the beginning. As soon as their properties started to be studied more deeply, new scenarios opened up, not only in the field of quantum information technologies, but also in other areas of physics and the life sciences. Emerging applications, for example, have been identified in biotechnology and nanomedicine: diamond nanoparticles hosting luminescent centres have been studied for targeted drug delivery [7-9], fluorescence biomarking [10-12], and long-term tracking and imaging in vivo [13, 14]. Due to their non-cytotoxicity, diamond nanoparticles can be functionalised to carry specific substances inside living cells, while at the same time the luminescent centres hosted in the particles can be optically identified and followed in the target tissues. Another application for colour centres in diamond nanoparticles is the measurement of weak magnetic fields with nanoscale spatial resolution [15, 16]. Magnetic fields can alter the fluorescence properties of certain types of colour centres, which may enable the optical detection of magnetic domains and even of individual electron or nuclear spins in biological molecules. Such a variety of promising applications fuels the interest for this topic and constitutes the main motivation to reach a better understanding of diamond colour centres. 4.2 NitrogeN-relateD ceNtres iN DiamoNDCrystallographic defects disrupt the tetrahedral periodicity of carbon atoms in the diamond lattice. They are addressed as intrinsic or extrinsic, according to their nature and origin [17-19]. Here we focus on extrinsic defects.Extrinsic defects are mostly made of substitutional and interstitial impurities of foreign elements embedded in the carbon matrix. They are present in natural and synthetic diamond as isolated atoms or organised in small atomic clusters. There are hundreds of types of luminescent extrinsic impurities that can form in diamond, and a significant fraction of them have been analysed in detail such that their charge and spin states are known under equilibrium conditions [2]. We focus our attention on a particular subclass of extrinsic diamond defects: nitrogen-related defects and specifically the NV centre, which seems to be the most promising from a technological perspective.Nitrogen-related colour centres (Fig. 4.1) are among the most common and the most studied impurities in diamonds. Nitrogen as a diamond impurity was first identified in 1959 by W. Kaiser and W. L. Bond of Bell Telephone Laboratories [20].
