  We investigate the optical properties of silicon-vacancy (SiV) and
germanium-vacancy (GeV) color centers in nanodiamonds under hydrostatic
pressure up to 180 GPa. The nanodiamonds were synthetized by Si or Ge-doped
plasma assisted chemical vapor deposition and, for our experiment, pressurized
in a diamond anvil cell. Under hydrostatic pressure we observe blue-shifts of
the SiV and GeV zero-phonon lines by 17 THz (70 meV) and 78 THz (320 meV),
respectively. These measured pressure induced shifts are in good agreement with
ab initio calculations that take into account the lattice compression based on
the equation of state of diamond and that are extended to the case of the
tin-vacancy (SnV) center. This work provides guidance on the use of
group-IV-vacancy centers as quantum sensors under extreme pressures that will
exploit their specific optical and spin properties induced by their intrinsic
inversion-symmetric structure.
