  Recently, Briegel et al. present a quantum repeaters protocol using nest
entanglement purification for long distance quantum communication(Physical
Review Letters \textbf{81},5932). In this paper we present a modified scheme
for constructing an optimal quantum repeater using doubly entangled photon
pairs that overcomes the limitations of quantum communication via noisy
channels. Based on the imperfect quantum operations and classical
communications, the polarization entangled quantum channels can be built with a
better efficiency.
