Four-photon quantum interferometry at a telecom wavelength

We report on an experimental demonstration of four-photon quantum interference using telecom-wavelength photons. The realization of multiphoton quantum interference is essential to linear optics quantum information processing and measurement-based quantum computing. We have developed a source that efficiently emits photon pairs in a pure spectrotemporal mode at a telecom-wavelength region, and have demonstrated quantum interference exhibiting reduced fringe intervals that correspond to a reduced de Broglie wavelength of up to the four-photon NOON state. Our result should open a path to practical quantum information processing using telecom-wavelength photons.