Passive preparation of BB84 signal states with coherent light

In a typical optical implementation of the Bennett-Brassard 1984 (so-called BB84) quantum key distribution protocol, the sender uses an active source to produce the required 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 devices usually involve parametric down-conversion. Here we show that coherent light is also suitable for passive generation of BB84 signal states. Our method does not require any externally-driven element, but only linear optical components and photodetectors. The resulting key rate is similar to the one delivered by an active source.