  Hong-Ou-Mandel (HOM) interference between independent photon sources
(HOMI-IPS) is the fundamental block for quantum information processing, such as
quantum gate, Shor's algorithm, Boson sampling, etc. All the previous HOMI-IPS
experiments were carried out in time-domain, however, the spectral information
during the interference was lost, due to technical difficulties. Here, we
investigate the HOMI-IPS in spectral domain using the recently developed fast
fiber spectrometer, and demonstrate the spectral distribution during the HOM
interference between two heralded single-photon sources, and two thermal
sources. This experiment can not only deepen our understanding of HOMI-IPS in
the spectral domain, but also be utilized to improve the visibility by
post-processing spectral filtering.
