  Decoy state method closes source security loophole in quantum key
distribution (QKD) using laser source. In this method, accurate estimates of
the detection rates of vacuum and single photon events plus the error rate of
single photon events are needed to give a good enough lower bound of the secret
key rate. Nonetheless, the current estimation method for these detection and
error rates, which uses three types of photon intensities, is accurate up to
about 1% relative error. Here I report an experimentally feasible way that
greatly improves these estimates and hence increases the one-way key rate of
the BB84 QKD protocol with unbiased bases selection by at least 20% on average
in realistic settings. The major tricks are the use of more than three types of
photon intensities plus the fact that estimating bounds of the above detection
and error rates is numerically stable although these bounds are related to the
inversion of a high condition number matrix.
