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 ultralow temperatures. In this paper, we present optical measurements on a ruby crystal in a dilution refrigerator at ultralow temperatures down to 20 mK. Analyzing the relative populations of its (4)A(2) ground-state spin levels, we extract a lattice temperature of 143 +/- 7mK under continuous laser excitation. We perform spin-lattice relaxation T-1 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.
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