  The generation and the characterization of multipartite entangled light is an
important and challenging task in quantum optics. In this paper the
entanglement properties of the light emitted from a planar semiconductor
microcavity are studied. The intracavity scattering dynamics leads to the
emission of light that is described by a fourpartite W state. Its multipartite
correlations are identified by using the method of entanglement witnesses.
Entanglement conditions are derived, which are based on a general witness
constructed from W states. The results can be used to detect entanglement of
light that propagates through lossy and even turbulent media.
