Nitrogen-vacancy centres (NVCs) in diamond have electronic and nuclear spins which can be optically initialised and read out [1]. The spin coherence times can be over one minute [2], and optical entanglement has been demonstrated between NVCs [3]. Arrays of single NVCs have been created with good coherence [4-9]. These elements make NVCs a strong platform for quantum computing, quantum communication and quantum sensing. The main techniques for making arrays of NVC are ion implantation [7-10], laser writing [4, 5] and electron-beam writing [6]; the strengths and weaknesses of these techniques will be discussed.  

It has been proposed that single NVC in micron-sized diamonds could test the quantum nature of gravity [11] by creating macroscopic spatial superpositions of a microdiamond [12, 13]. This could in turn lead to a new class of sensitive quantum sensors.  

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