  We simulate quantum key distribution (QKD) experimental setups and give out
some improvement for QKD procedures. A new data post-processing protocol is
introduced, mainly including error correction and privacy amplification. This
protocol combines the ideas of GLLP and the decoy states, which essentially
only requires to turn up and down the source power. We propose a practical way
to perform the decoy state method, which mainly follows the idea of Lo's decoy
state. A new data post-processing protocol is then developed for the QKD scheme
with the decoy state. We first study the optimal expected photon number mu of
the source for the improved QKD scheme. We get the new optimal mu=O(1)
comparing with former mu=O(eta), where eta is the overall transmission
efficiency. With this protocol, we can then improve the key generation rate
from quadratic of transmission efficiency O(eta2) to O(eta). Based on the
recent experimental setup, we obtain the maximum secure transmission distance
of over 140 km.
