  The measurement-device-independent quantum key distribution (MDI-QKD)
possesses the highest security among all practical quantum key distribution
protocols. However, existing multi-intensity decoy-state methods may cause
loopholes when modulating light intensities with practical devices. In this
paper, we propose a passive decoy-state MDI-QKD protocol based on a novel
structure of heralded single-photon sources. It does not need to modulate the
light source into different intensities and thus can avoid leaking modulation
information of decoy states to eavesdroppers, enhancing the security of
practical MDI-QKD. Furthermore, by combining the passive decoy states and
biased basis choices, our protocol can exhibit distinct advantages compared
with state-of-the art MDI-QKD schemes even when the finite key-size effect is
taken into account. Therefore, our present passive decoy-state MDI-QKD seems a
promising candidate for practical implementation of quantum key distributions
in the near future.
