  We propose a fully-passive twin-field quantum key distribution (QKD) setup
where basis choice, decoy-state preparation and encoding are all implemented
entirely by post-processing without any active modulation. Our protocol can
remove the potential side-channels from both source modulators and detectors,
and additionally retain the high key rate advantage offered by twin-field QKD,
thus offering great implementation security and good performance. Importantly,
we also propose a post-processing strategy that uses mismatched phase slices
and minimizes the effect of sifting. We show with numerical simulation that the
new protocol can still beat the repeaterless bound and provide satisfactory key
rate.
