  A recent paper (Phys. Rev. A 86, 034305 (2012)) proposed an entanglement
concentration protocol (ECP) for less-entangled $N$-atom GHZ state with the
help of the photonic Faraday rotation. It is shown that the maximally entangled
atom state can be distilled from two pairs of less-entangled atom states. In
this paper, we put forward an improved ECP for arbitrary less-entangled N-atom
GHZ state with only one pair of less-entangled atom state, one auxiliary atom
and one auxiliary photon. Moreover, our ECP can be used repeatedly to obtain a
higher success probability. If consider the practical operation and imperfect
detection, our protocol is more efficient. This ECP may be useful in current
quantum information processing.
