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 χ=Ed/ℏ. 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/T2 on χ. Rate 1/T2(χ) decreases if χ increases. This effect is able to explain experimental data on free-induction decay measured in Pr3+:LaF3 crystal.