  To beat PNS attack, decoy state quantum key distribution (QKD) based on
coherent state has been studied widely. We present a decoy state QKD protocol
with modified coherent state (MCS). By destruction quantum interference, MCS
with fewer multi-photon events can be get, which may improve key bit rate and
security distance of QKD. Through numerical simulation, we show about 2-dB
increment on security distance for BB84 protocol.
