  In this paper, we investigate the long-standing gap of quantum key rate
between the Weak Coherent Pulse (WCP) and Heralded Single Photon Sources(HSPS)
implementation of quantum cryptographical protocol. We prove that, by utilizing
the Heralded Pair Coherent State (HPCS) photon sources, such a gap can be
actually filled in both BB84 and SARG quantum key distribution. Thus, a
universal photon source which achieves the up-to-date optimal key rate for each
transmission distance is obtained.
