  Diamond is a host for 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. The plasma assisted Chemical Vapour Deposition (CVD) has become a
key enabling technology in this field of research. Nitrogen can indeed be
directly in-situ doped 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 on the requirements for synthesizing quantum-grade diamond films by
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 position diamond
as one of the most promising solid-state quantum system and the first
industrial applications in sensing are just starting to emerge.
