Optical homogeneous and inhomogeneous linewidths in $^{171}$Yb$^{3+}$ : Y$_2$SiO$_5$

Rare earth ions are actively investigated as optically addressable spin systems for quantum technologies thanks to their long optical and spin coherence lifetimes. $^{171}$Yb$^{3+}$, which has 1/2 electron and nuclear spins, recently raised interest for its simple hyperfine structure that moreover can result in long coherence lifetimes at zero magnetic field, an unusual property for paramagnetic rare earth ions. Here, we report on the optical inhomogeneous and homogeneous linewidths in $^{171}$Yb$^{3+}$ : Y$_2$SiO$_5$ (site 2) for different doping concentrations. While inhomogeneous linewidth is not correlated to $^{171}$Yb$^{3+}$ concentration, the homogeneous one strongly decreases between 10 and 2 ppm doping level, reaching 255 Hz at 3 K. This is attributed to a slowing down of $^{171}$Yb$^{3+}$ ground state spin flip-flops.