Spin thermometry and spin relaxation of optically detected Cr 3 + ions in ruby Al 2 O 3

After 150 years of research, the gemstone ruby (Cr${}^{3+}$ in Al${}_{2}$O${}_{3}$) still reveals interesting properties. Here, the authors use its spin properties to conduct all-optical thermometry inside a dilution refrigerator, reaching lattice temperatures as low as 143 mK under continuous, low-power laser excitation. They measure the spin relaxation time down to these unprecedented low temperatures, and demonstrate optically detected magnetic resonance within the rich spin structure of the Cr${}^{3+}$ ions.