  We report on the excitation of single molecules via narrow zero-phonon
transitions using short laser pulses. By monitoring the Stokes-shifted
fluorescence, we studied the excited state population as a function of the
delay time, laser intensity, and frequency detuning. A pi-pulse excitation was
demonstrated with merely 500 photons, and 5 Rabi cycles were achieved at higher
excitation powers. Our findings are in good agreement with theoretical
calculations and provide a first step toward coherent manipulation of the
electronic states of single molecules with few photons.
