  We present a one-step deterministic entanglement purification protocol with
linear optics and postselection. Compared with the Simon-Pan protocol (Phys.
Rev. Lett. 89, 257901 (2002)), this one-step protocol has some advantages.
First, it can get a maximally entangled pair with only one step, not only
improve the fidelity of less-entangled photon pairs by performing the protocol
repeatedly. Second, it works in a deterministic way, not a probabilistic one,
which will reduce a great deal of entanglement resources. Third, it does not
require the polarization state be entangled, only spatial entanglement is
needed. Moreover, it is feasible with current techniques (Nature 423, 417
(2003)). All these advantages will make this one-step protocol more convenient
than others in the applications in quantum communication.
