**1. Introduction**

Diamond, composed of carbon elements, is an allotrope of graphene. Diamond, which is the hardest substance on earth, possesses high electron and hole mobility, the highest known thermal conductivity, extremely low thermal coefficient of thermal expansion, and a wide band gap and optical transparency window spanning from the near ultraviolet to the far infrared (IR), and chemical inertness, which make it a unique and desirable solid-state material for several forefront technological applications, especially in the quantum field [1, 2].

In 1963, it was discovered that diamond nanoparticles can be synthesized by the detonation of carbon-based explosives [2]. Since then, a new pathway has been opened to synthesize diamonds. High-temperature-high-pressure, chemical vapor deposition (CVD), and other methods for synthesizing diamond are being developed. It was not until 1965 that Dyer et al. discovered the nitrogen-vacancy center (NV center) with good optical properties and long coherence time of electron spin to rapidly push diamond to the research upsurge, including catalysis, biomedicine, quantum computing, etc [3]. The negatively charged NV centers in diamond have great potential applications in quantum sensing and quantum communication due to their unique long coherence time.

Nowadays, the method of synthesizing nitrogen-doped diamond has been developed rapidly. In the synthesis process of diamond, high-pressure-high-temperature (HPHT) and chemical vapor deposition methods will be introduced. After diamond synthesis, femtosecond laser, electron irradiation, and ion implantation methods can be used for nitrogen doping. However, the current approaches still cannot synthesize NV centers with high permutations. Not only that, the concentration of NV centers is not very high, and the coherence time is only on the order of milliseconds [4]. Therefore, we have to face a lot of challenges to overcome, and the NV centers still requires further studies.
