Four-Photon Quantum Interferometry at a Telecom Wavelength
  We report the experimental demonstration of four-photon quantum interference using telecom-wavelength photons. Realization of multi-photon 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 the quantum interference exhibiting the reduced fringe intervals that correspond to the 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. 
Masahiro Yabuno, Ryosuke Shimizu, Yasuyoshi Mitsumori, Hideo Kosaka,   and Keiichi Edamatsu
quant-ph