  The low dielectric losses of an isotropically pure single crystal $^{28}$Si
sample were determined at a temperature of 20 mK and at powers equivalent to
that of a single photon. Whispering Gallery Mode (WGM) analysis revealed large
Quality Factors of order $2\times10^6$ (dielectric loss $\sim 5\times10^{-7}$)
at high powers, degrading to $7\times10^5$ (dielectric loss $\sim
1.4\times10^{-6}$) at single photon energy. A very low-loss narrow line width
paramagnetic spin flip transition was detected with extreme sensitivity in
$^{28}$Si, with very small concentration below $10^{11}$~cm$^{-3}$ (less than
10 parts per trillion) and g-factor of $1.995\pm0.008$. Such determination was
only possible due to the low dielectric photonic losses combined with the long
lifetime of the spin transition (low magnetic loss), which enhances the
magnetic AC susceptibility. Such low photonic loss at single photon energy
combined with the narrow line width of the spin ensemble, indicate that single
crystal $^{28}$Si could be an important crystal for future cavity QED
experiments.
