  The indistinguishability of non-identical photons is dependent on detection
system in quantum physics. If two photons with different wavelengths are
indistinguishable for a detection system, there can be two-photon interference
when these two photons are incident to two input ports of a Hong-Ou-Mandel
interferometer, respectively. The reason why two-photon interference phenomena
are different for classical and nonclassical light is not due to interference,
but due to the properties of light and detection system. These conclusions are
helpful to understand the physics and applications of two-photon interference.
