  We propose a scheme for preparing an ensemble of atoms in a maximally
entangled W state by exploiting the Rydberg blockade effect. The success of our
protocol is indicated by the detection of an ion, which thus serves as a herald
for successful entangled state preparation. The fidelity of the preparation
scheme is independent of the number of atoms in the ensemble. Therefore, a
small cloud of atoms in a dipole trap randomly loaded from a background gas can
be reliably prepared in a maximally entangled state despite of atom number
fluctuations.
