Scheme for Concentration of Unknown Greeberger--Horne--Zeilinger Entangled States via Cavity Decay

We present a scheme for entanglement concentration of an unknown atomic
non-maximally entangled GHZ state via cavity decay. In the scheme, the atom trapped in a cavity is manipulated by laser field, so the maximally entangled GHZ state can be obtained by performing certain operation, which can be realized by illuminating the atom in a cavity. Our method is robust against spontaneous atomic decay.