  We study channel fidelity for the high-fidelity approach in the
Knill-Laflamme-Milburn (KLM) scheme. We examine an optimal channel fidelity
$f_{opt}$ and identify the corresponding KLM ancilla state. In the limit of
large $n$, where $2n$ is the number of the ancilla qubits, we find
$f_{opt}=1-{\pi^{2} \over 6n^{2}}+{2\pi^{2} \over 9n^{3}}$. We see that as $n$
increases $f_{opt}$ approaches to 1 slightly faster than $f=1-{2 \over n^{2}}$
which is the channel fidelity computed by Franson et. al. in the limit of large
$n$. We also compute the channel fidelity for the ancilla state that gives a
lower bound of success probability of quantum teleportation.
