We describe an experimentally straightforward method for preparing anentangled W state of up to 100 qubits. Our repeat-until-success protocol relieson detection of single photons from collective spontaneous emission in freespace. Our method allows entanglement preparation in a wide range of qubitimplementations that lack entangling qubit-qubit interactions. We give detailednumerical examples for entanglement of neutral atoms in optical lattices and ofnitrogen-vacancy centres in diamond. The simplicity of our method should enablepreparation of mesoscopic entangled states in a number of physical systems inthe near future.
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