  We propose an efficient phase-encoding quantum secret key generation scheme
with heralded narrow-band single photons. The key information is carried by the
phase modulation directly on the single-photon temporal waveform without using
any passive beam splitters or optical switches. We show that, when the
technique is applied to the conventional fiber-based phase-encoding BB84 and
differential phase shift (DPS) quantum key distribution schemes, the key
generation efficiencies can be improved by a factor of 2 and 3, respectively.
For N(>3)-period DPS systems, the key generation efficiency can be improved by
a factor of N. The technique is suitable for quantum memory-based long-distance
fiber communication system.
