  The passive approach to quantum key distribution (QKD) consists of
eliminating all optical modulators and random number generators from QKD
systems, in so reaching an enhanced simplicity, immunity to modulator side
channels, and potentially higher repetition rates. In this work, we provide
finite-key security bounds for a fully passive decoy-state BB84 protocol,
considering a passive QKD source recently presented. With our analysis, the
attainable secret key rate is comparable to that of the perfect parameter
estimation limit, in fact differing from the key rate of the active approach by
less than one order of magnitude. This demonstrates the practicality of fully
passive QKD solutions.
