  Frequency encoding of quantum information together with fiber and integrated
photonic technologies can significantly reduce the complexity and resource
requirements for realizing all-photonic quantum networks. The key challenge for
such frequency domain processing of single photons is to realize coherent and
selective interactions between quantum optical fields of different frequencies
over a range of bandwidths. Here, we report frequency-domain Hong-Ou-Mandel
interference with spectrally distinct photons generated from a chip-based
microresonator. We use four-wave mixing to implement an active frequency
beam-splitter and achieve interference visibilities of $0.95 \pm 0.02$. Our
work establishes four-wave mixing as a tool for selective high-fidelity
two-photon operations in the frequency domain which, combined with integrated
single-photon sources, provides a building block for frequency-multiplexed
photonic quantum networks.
