Spin coherence lifetime extension in Tm$^{3+}$:YAG through dynamical   decoupling
  We report on spin coherence lifetime extension on Tm$^{3+}$:YAG obtained through dynamically decoupling the thulium spins from their magnetic environment. The coherence lifetime reached with a Carr-Purcell-Meiboom-Gill sequence revealed a 450-fold extension [$\sim$$(230 \pm 30)$ ms] with respect to previously measured values. Comparison to a simple theoretical model allowed us to estimate the correlation time of the fluctuations of the ground level transition frequency to $(172 \pm 30)$ $\mu$s at 1.7 K. For attaining efficient decoupling sequences, we developed a strategy inspired by the \emph{zero-first-order Zeeman} effect to minimize the large inhomogeneous broadening of the ground level spin transition. 
M. F. Pascual-Winter, R.-C. Tongning, T. Chaneli\`ere, J.-L. Le   Gou\"et
quant-ph