  Signal state preparation in quantum key distribution schemes can be realized
using either an active or a passive source. Passive sources might be valuable
in some scenarios; for instance, in those experimental setups operating at high
transmission rates, since no externally driven element is required. Typical
passive transmitters involve parametric down-conversion. More recently, it has
been shown that phase-randomized coherent pulses also allow passive generation
of decoy states and Bennett-Brassard 1984 (BB84) polarization signals, though
the combination of both setups in a single passive source is cumbersome. In
this paper, we present a complete passive transmitter that prepares decoy-state
BB84 signals using coherent light. Our method employs sum-frequency generation
together with linear optical components and classical photodetectors. In the
asymptotic limit of an infinite long experiment, the resulting secret key rate
(per pulse) is comparable to the one delivered by an active decoy-state BB84
setup with an infinite number of decoy settings.
