  We propose an efficient method to filter out single atoms from trapped
ensembles with unknown number of atoms. The method employs stimulated adiabatic
passage to reversibly transfer a single atom to the Rydberg state which blocks
subsequent Rydberg excitation of all the other atoms within the ensemble. This
triggers the excitation of Rydberg blockaded atoms to short lived intermediate
states and their subsequent decay to untrapped states. Using an auxiliary
microwave field to carefully engineer the dissipation, we obtain a nearly
deterministic single-atom source. Our method is applicable to small atomic
ensembles in individual microtraps and in lattice arrays.
