  In an atomic ensemble, quantum information is typically carried as single
collective excitations. It is very advantageous if the creation of single
excitations is efficient and robust. Rydberg blockade enables deterministic
creation of single excitations via collective Rabi oscillation by precisely
controlling the pulse area, being sensitive to many experimental parameters. In
this paper, we implement the adiabatic rapid passage technique to the Rydberg
excitation process in a mesoscopic atomic ensemble. We make use of a two-photon
excitation scheme with an intermediate state off-resonant and sweep the laser
frequency of one excitation laser. We find the chirped scheme preserves
internal phases of the collective Rydberg excitation and be more robust against
variance of laser intensity and frequency detuning.
