  We study two-photon excitation of Rydberg states of atoms under stimulated
adiabatic passage with delayed laser pulses. We find that the combination of
strong interaction between the atoms in Rydberg state and the spontaneous decay
of the intermediate exited atomic state leads to the Rydberg excitation of
precisely one atom within the atomic ensemble. The quantum Zeno effect offers a
lucid interpretation of this result: the Rydberg blocked atoms repetitively
scattering photons effectively monitor a randomly excited atom which therefore
remains in the Rydberg state. This system can be used for deterministic
creation and, possibly, extraction of Rydberg atoms or ions one at a time. The
sympathetic monitoring via decay of ancilla particles may find wider
applications for state preparation and probing of interactions in dissipative
many-body systems.
