Chemical vapour deposition diamond single crystals with nitrogen-vacancy centres: a review of material synthesis and technology for quantum sensing applications.

Diamond hosts a wide variety of colour centres that have demonstrated outstanding optical and spin properties. Among them, the nitrogen-vacancy (NV) centre is by far the most investigated owing to its superior characteristics, which promise the development of highly sophisticated quantum devices, in particular for sensing applications. Nevertheless, harnessing the potential of these centres mainly relies on the availability of high quality and purity diamond single crystals that need to be specially designed and engineered for this purpose. Plasma assisted chemical vapour deposition (CVD) has become a key enabling technology in this field of research. Nitrogen can indeed be directly doped in situ into a high crystalline quality diamond matrix in a controlled way, allowing the production of single isolated centres or ensembles that can potentially be integrated into a device. In this paper we will provide an overview of the requirements for synthesizing 'quantum-grade' diamond films using CVD. These include the reduction of impurities and surrounding spins that limit coherence times, the control of NV density in a wide range of concentrations as well as their spatial localization within the diamond. Enhancing the charge state and preferential orientation of the colour centres is also discussed. These improvements in material fabrication have contributed to positioning diamond as one of the most promising solid-state quantum systems and the first industrial applications in sensing are just starting to emerge. [ABSTRACT FROM AUTHOR]