  Hong-Ou-Mandel (HOM) interference unveils a distinct behavior of identical
particles which cannot be distinguished from each other. Especially for bosons,
two separated identical particles passing through a beamsplitter always go
together into one of the output ports, but that is not the case with other
particles including fermions or classical ones. So far many elemental
properties of quantum physics and information have been discovered through the
concatenated HOM effects, which has been demonstrated in photons and recently
in plasmons, atoms and phonons. However, all demonstrations in optical region
employed two particles in different spatial modes. Here we first report the HOM
interference between two photons in a single spatial mode with different
frequencies (energies) by using a partial frequency conversion. The
demonstrated frequency-domain interferometer allows us to replace spatial
optical paths by optical frequency multiplexing, which opens up a distinct
architecture of the quantum interferometry.
