  A passive quantum key distribution (QKD) transmitter generates the quantum
states prescribed by a QKD protocol at random, combining a fixed quantum
mechanism and a post-selection step. By avoiding the use of active optical
modulators externally driven by random number generators, passive QKD
transmitters offer immunity to modulator side channels and potentially enable
higher frequencies of operation. Recently, the first linear optics setup
suitable for passive decoy-state QKD has been proposed. In this work, we
simplify the prototype and adopt sharply different approaches for BB84
polarization encoding and decoy-state generation. On top of it, we elaborate a
tight custom-made security analysis surpassing an unnecessary assumption and a
post-selection step that are central to the former proposal.
