Entanglement in electron-nuclear spin system as immediate cause of the dependence of dephasing rate 1/T₂ on intensity in the optical Bloch equations

Dephasing rate 1/T₂ in the optical Bloch equations doesn't depend on Rabi frequency $\chi =Ed/\hslash$. It seemed true for various mechanisms responsible for optical dephasing processes. In this paper we show that entanglement of nuclear spin with electronic transition in an atom results in dependence of $1/T_2$ on $\chi$. Rate $1/T_2\left(\chi \right)$ decreases if $\chi$ increases. This effect is able to explain experimental data on free-induction decay measured in Pr₃⁺:LaF₃ crystal.