  We propose and demonstrate a robust control scheme by ultrafast nonadiabatic
chirped laser pulse, designed for targeting coherent superpositions of
two-level systems. Robustness against power fluctuation is proved by our
numerical study and a proof-of-principle experiment performed with femtosecond
laser interaction on cold atoms. They exhibit for the final driven dynamics a
cusp on the Bloch sphere, corresponding to a zero curvature of fidelity. This
solution is particularly simple and thus applicable to a wide range of
potential applications.
