  Paramagnetic ions in solid state crystals form the basis for many advanced
technologies such as lasers, masers, frequency standards, and quantum-enhanced
sensors. One of the most-studied examples is the Cr3+ ion in sapphire (Al2O3),
also known as ruby, which has been intensely studied in the 1950s and 1960s.
However, despite decades of research on ruby, some of its fundamental optical
and spin properties have not yet been characterized at ultra low-temperatures.
In this paper, we present optical measurements on a ruby crystal in a dilution
refrigerator at ultra-low temperatures down to 20 mK. Analyzing the relative
populations of its 4A2 ground state spin levels, we extract a lattice
temperature of 143(7) mK under continuous laser excitation. We perform spin
lattice relaxation T1 measurements in excellent agreement with the direct,
one-phonon model. Furthermore, we perform optically detected magnetic resonance
measurements showing magnetically driven transitions between the ground state
spin levels for various magnetic fields. Our measurements characterize some of
ruby's low temperature spin properties, and lay the foundations for more
advanced spin control experiments.
