  A study on the cause of the multi-particle entanglement is presented in this
work. We investigate how dot-like single quantum well excitons, which are
independently coupled through a single microcavity mode, evolve into maximally
entangled state as a series of conditional measurements are taken on the cavity
field state. We first show how cavity photon affects the entanglement purity of
the double-exciton Bell state and the triple-exciton W state. Generalization to
multi-excitons W states is then derived analytically. Our results pave the way
for studying the crucial cause of multi-particle collective effect.
