Witnessing entanglement from N-qubit radiation emission

We show how radiation emitted from a collection of noninteracting two-level particles, which can be treated as qubits, can be used to detect a wide class of entangled states of this system. This result is an example of a macroscopical manifestation of entanglement that can be inferred from direct measurements. Some examples of physical effects directly related to the collective emission of radiation, such as photon echoes and superradiance, are discussed, suggesting some ways of how to experimentally witness entanglement with simple and standard techniques.