  The distribution of entangled states between distant locations will be
essential for the future large scale realization of quantum communication
schemes such as quantum cryptography and quantum teleportation. Because of the
unavoidable noise in the quantum communication channel, the entanglement
between two particles is more and more degraded the further they propagate.
Entanglement purification is thus essential to distill highly entangled states
from less entangled ones. Existing general purification protocols are based on
the quantum controlled-NOT (CNOT) or similar quantum logic operations, which
are very difficult to implement experimentally. Present realizations of CNOT
gates are much too imperfect to be useful for long-distance quantum
communication. Here we present a feasible scheme for the entanglement
purification of general mixed entangled states, which does not require any CNOT
operations, but only simple linear optical elements. Since the perfection of
such elements is very high, the local operations necessary for purification can
be performed with the required precision. Our procedure is within the reach of
current technology and should significantly simplify the implementation of
long-distance quantum communication.
