  Nanodiamonds (NDs) hosting optically active defects are an important
technical material for applications in quantum sensing, biological imaging, and
quantum optics. The negatively charged silicon vacancy (SiV) defect is known to
fluoresce in molecular sized NDs (1 to 6 nm) and its spectral properties depend
on the quality of the surrounding host lattice. This defect is therefore a good
probe to investigate the material properties of small NDs. Here we report
unprecedented narrow optical transitions for SiV colour centers hosted in
nanodiamonds produced using a novel high-pressure high-temperature (HPHT)
technique. The SiV zero-phonon lines were measured to have an inhomogeneous
distribution of 1.05 nm at 5 K across a sample of numerous NDs. Individual
spectral lines as narrow as 354 MHz were measured for SiV centres in
nanodiamonds smaller than 200 nm, which is four times narrower than the best
SiV line previously reported for nanodiamonds. Correcting for apparent spectral
diffusion yielded a homogeneous linewith of about 200 MHz, which is close to
the width limit imposed by the radiative lifetime. These results demonstrate
that the direct HPHT synthesis technique is capable of producing nanodiamonds
with high crystal lattice quality, which are therefore a valuable technical
material.
