  We describe an experimentally straightforward method for preparing an
entangled W state of up to 100 qubits. Our repeat-until-success protocol relies
on detection of single photons from collective spontaneous emission in free
space. Our method allows entanglement preparation in a wide range of qubit
implementations that lack entangling qubit-qubit interactions. We give detailed
numerical examples for entanglement of neutral atoms in optical lattices and of
nitrogen-vacancy centres in diamond. The simplicity of our method should enable
preparation of mesoscopic entangled states in a number of physical systems in
the near future.
