  We investigate the single photon scattering by an emitter chirally coupled to
a one-dimensional waveguide. The single-photon transport property is
essentially different from the symmetrical coupling case. The single photons
propagating towards the emitter in opposite directions show different
transmission behaviors, which is a manifestation of the single-photon diode. In
the ideal chiral coupling case, the transmission probability of the single
photon transport in one direction is zero by critical coupling, while in the
opposite direction it is unity. The diode works well only when the
single-photon frequency meets certain conditions. For a two-level emitter, the
diode works well when the single photon is nearly resonant to the emitter. For
a $\Lambda $-type three-level emitter, when the single-photon frequency is
greatly altered, we can adjust the parameters of the external laser to ensure
the diode works well. The latter provides a manner to realize a single-photon
switch, in which the single-photon transmission probability can reach zero or
unity although the emitter's decay is considered.
