Observation of Hong-Ou-Mandel interference by using a frequency-domain beamsplitter

Recently a single-photon frequency converter by a nonlinear optical medium with a strong pump light [1] has been actively studied for quantum information processing [2]. The effective Hamiltonian of the process is described as a frequency-domain beamsplitter, which has two different frequency modes in its input and output. The transmittance is tunable by a pump power for the nonlinear optical effect. Such a frequency-domain beamsplitter is expected to cause an interference of two photons of different colors [3] in the context of a Hong–Ou–Mandel (HOM) experiment. In this study, we first observed the HOM dip in an optical range by using a frequency-domain beamsplitter.