  Microring resonators are attractive for low-power frequency conversion via
Bragg-scattering four-wave-mixing due to their comb-like resonance spectrum.
However, conversion efficiency is limited to 50% due to the equal probability
of up- and down-conversion. Here, we demonstrate how two coupled microrings
enable highly directional conversion between the spectral modes of one of the
rings. An extinction between up- and down-conversion of more than 40 dB is
experimentally observed. Based on this method, we propose a design for on-chip
multiplexed single-photon sources that allow localized frequency modes to be
converted into propagating continuous-mode photon wave packets using a single
operation. The key is that frequency conversion works as a switch on both
spatial and spectral degrees of freedom of photons if the microring is
interferometrically coupled to a bus waveguide. Our numerical results show 99%
conversion efficiency into a propagating mode with a wave packet having a 90%
overlap with a Gaussian for a ratio between intrinsic and coupling quality
factors of 400.
