  We propose an optimal entanglement concentration protocol (ECP) for nonlocal
N-electron systems in a partially entangled pure state, resorting to charge
detection and the projection measurement on an additional electron. For each
nonlocal N-electron system, one party in quantum communication, say Alice first
entangles it with an additional electron, and then she projects the additional
electron into an orthogonal basis for dividing the N-electron systems into two
groups. In the first group, the N parties obtain a subset of $N$-electron
systems in a maximally entangled state directly. In the second group, they
obtain some less-entangled N-electron systems which are the resource for the
entanglement concentration in the next round. By iterating the entanglement
concentration process several times, the present ECP has the maximal success
probability, the theoretical limit of an ECP as it just equals to the
entanglement of the partially entangled state, far higher than others, without
resorting to a collective unitary evolution.
