Single SiV$^-$ centers in low-strain nanodiamonds with bulk-like spectral properties and nano-manipulation capabilities.

The controlled interfacing of individually well-isolated atoms is one of the outstanding challenges for bottom-up assemblies of complex quantum systems. Here, we establish SiV$^-$ centers in nanodiamonds as a new potential building block for solid-state based quantum technology. Utilizing an atomic force microscope cantilever we demonstrate controlled nano-manipulation including dipole rotation and cluster decomposition, as well as 1D- and 2D-alignment with $\approx 10$ nm accuracy. We employ H-plasma treatment in combination with resonant and off-resonant excitation to isolate single SiV$^-$ centers per nanodiamond with improved inhomogeneous ensemble linewidth and stable optical transitions. Our methods allow us to observe fine-structure and to elaborate analytical strain model where we experimentally extract the ratio between strain coefficients of excited and ground states. The observed strain values are as low as best values in low-strain bulk diamond. Together with good polarization contrast and excellent spectral stability under resonant excitation our work paves the way for indistinguishable photon emission and should facilitate the development of new complex quantum systems.