  We study the scattering of a single photon propagating along a waveguide in
an arbitrary superposition state two frequencies with a single three-level
$\Lambda$ atom in a superposition of two non-degenerate ground states where the
atom is coupled to a waveguide. We find that the scattering depends on both the
relative phase between the photon frequencies and the relative phase between
the atomic ground states. Our results show that a three-level $\Lambda$ atom
coupled to a waveguide can be used as photon phase filter that could detect the
relative phase between the two frequencies of the photon superposition state.
