  We present an efficient way for measuring the entanglement of the atoms.
Through the auxiliary single photons input-output process in cavity quantum
electrodynamics (QED), the concurrence of the atomic entanglement can be
obtained according to the success probability of picking up the singlet states
of the atoms. This protocol has three advantages: First, we do not require the
sophisticated controlled-not (CNOT) gates. Second, the distributed atoms are
not required to intact with each other. Third, the atomic entanglement can be
distributed nonlocally, which provides its important applications in
distributed quantum computation.
