  Most experimental realizations of quantum key distribution are based on the
Bennett-Brassard 1984 (so-called BB84) protocol. In a typical optical
implementation of this scheme, the sender uses an active source to produce the
required BB84 signal states. While active state preparation of BB84 signals is
a simple and elegant solution in principle, in practice passive state
preparation might be desirable in some scenarios, for instance, in those
experimental setups operating at high transmission rates. Passive schemes might
also be more robust against side-channel attacks than active sources. Typical
passive devices involve parametric down-conversion. In this paper, we show that
both coherent light and practical single photon sources are also suitable for
passive generation of BB84 signal states. Our method does not require any
external-driven element, but only linear optical components and photodetectors.
In the case of coherent light, the resulting key rate is similar to the one
delivered by an active source. When the sender uses practical single photon
sources, however, the distance covered by a passive transmitter might be longer
than the one of an active configuration.
