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Frequency-domain Hong–Ou–Mandel interference: Professor Masato Koashi , Photon Science Center

Hong–Ou–Mandel (HOM) interference is one of the most prominent features of quantum indistinguishable particles, and has been used as the core of many quantum information protocols. Since its first observation in 1987, it has been understood as a phenomenon that occurs when two identical bosons are fed to two input ports of a beam splitter. Here, we report the observation of HOM interference exhibited by two photons with different colours. We developed a frequency-domain beam splitter via a second-order nonlinear medium driven by strong coherent light. A photon passing through the device changes its colour probabilistically. When a single pulse containing two photons with different colours was fed to the beam splitter, the pair of output photons showed a tendency to assume the same colour, with a visibility exceeding the classical limit. Combined with wavelength-division multiplexing, our results will pave the way towards the miniaturization of highly integrated optical circuits for quantum information processing.



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