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Paramagnetic ions in solid state crystals form the basis for many advancedtechnologies such as lasers, masers, frequency standards, and quantum-enhancedsensors. 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 opticaland spin properties have not yet been characterized at ultra low-temperatures.In this paper, we present optical measurements on a ruby crystal in a dilutionrefrigerator at ultra-low temperatures down to 20 mK. Analyzing the relativepopulations of its 4A2 ground state spin levels, we extract a latticetemperature of 143(7) mK under continuous laser excitation. We perform spinlattice relaxation T1 measurements in excellent agreement with the direct,one-phonon model. Furthermore, we perform optically detected magnetic resonancemeasurements showing magnetically driven transitions between the ground statespin levels for various magnetic fields. Our measurements characterize some ofruby's low temperature spin properties, and lay the foundations for moreadvanced spin control experiments.

Spin thermometry and spin relaxation of optically detected Cr3+ ions in ruby Al2O3