Enhancing Fluorescence of Shallow Nitrogen-Vacancy Centers in Diamond by Surface Coating with Titanium Oxide Layers

We present an enhancement of the fluorescence of shallow ( $ 10 nm) nitrogen-vacancy (NV $^-$ ) centers by using atomic layer deposition to deposit titanium oxide layers on the diamond surface. In this way, the shallow NV $^-$ center charge states were stabilized, leading to the increasing fluorescence intensity of about 2 times. This surface coating technique could produce a protective layer of controllable thickness without any damages to the solid-state quantum system surface, which might be an approach to the further passivation or packaging techniques for the solid-state quantum devices.