  We propose an efficient quantum key distribution protocol based on the
photon-pair generation from parametric down-conversion (PDC). It uses the same
experimental setup as the conventional protocol, but a refined data analysis
enables detection of photon-number splitting attacks by utilizing information
from a built-in decoy state. Assuming the use of practical detectors, we
analyze the unconditional security of the new scheme and show that it improves
the secure key generation rate by several orders of magnitude at long
distances, using a high intensity PDC source.
