  We report the study of interactions between cavity photons and paramagnetic
Cr$^{3+}$ spins in a ruby (Cr$^{3+}$:Al$_2$O$_3$) Whispering Gallery mode (WGM)
resonator. Examining the system at microwave frequencies and millikelvin
temperatures, spin-photon couplings up to 610 MHz or about 5% of photon energy
are observed between the impurity spins and high quality factor ($Q > 10^5$)
WGM. Large tunability and spin-spin interaction allows operation in the strong
coupling regime. The system exhibits behaviour not predicted by the usual
Tavis-Cummings model because of interactions within the two-level spin bath,
and the existence of numerous photonic modes.
