Optical investigation of ultra-slow spin relaxation in ¹⁷¹ Yb ³⁺ :Y ₂ SiO ₅ single crystals

Abstract We report a detailed investigation of spin relaxation dynamics in the rare-earth-doped crystal 171 Yb:Y 2 SiO 5 , a promising platform for quantum memory based on atomic frequency combs. The population dynamics among the four ground-state hyperfine levels were studied in the 50 mK–6 K temperature range by optically modifying the spin populations and monitoring their return to thermal equilibrium via absorption spectroscopy. Below 1 K, population rethermalization occurs on timescales of several hours and is governed by direct spin-phonon relaxation. In contrast, the lifetime of individual spin states is strongly reduced by spin-spin interactions, ranging from seconds in low-doped (2 ppm) samples to milliseconds at higher doping (10 ppm). These results provide clear guidelines for optimizing doping concentration and operating temperature, indicating that atomic frequency comb lifetimes on the order of one hour are achievable in 171 Yb:Y 2 SiO 5 crystals with Yb concentrations not much lower than 2 ppm and at temperatures at or below 1 K.