  Recently Xiao et al. proposed a scheme for entanglement purification based on
doubly entangled photon states (Phys. Rev. A 77(2008) 042315). We modify their
scheme for improving the efficiency of entanglement purification. This modified
scheme contains two steps, i.e., the bit-flip error correction and the
entanglement purification of phase-flip errors. All the photon pairs in the
first step can be kept as all the bit-flip errors are corrected. For purifying
the phase-flip errors, a wavelength conversion process is needed. This scheme
has the advantage of high efficiency and it requires the original fidelity of
the entangled state wanted fay lower than other schemes, which makes it more
feasible in a practical application.
