  Optical fibers have been enabling numerous distinguished applications
involving the operation and generation of light, such as soliton transmission,
light amplification, all-optical switching and supercontinuum generation. The
active function of optical fibers in the quantum regime is expected to be
applicable to ultralow-power all-optical signal processing and quantum
information processing. Here we demonstrate the first experimental observation
of optical nonlinearity at the single-photon level in an optical fiber. Taking
advantage of large nonlinearity and managed dispersion of a photonic crystal
fiber, we have successfully measured very small (10^(-7) ~ 10^(-8)) conditional
phase shifts induced by weak coherent pulses that contain one or less than one
photon per pulse on average. In spite of its tininess, the phase shift was
measurable using much (~10^6 times) stronger coherent probe pulses than the
pump pulses. We discuss the feasibility of quantum information processing using
optical fibers, taking into account the observed Kerr nonlinearity accompanied
by ultrafast response time and low induced loss.
