Observation of optical-fibre Kerr nonlinearity at the single-photon level

Optical fibres have proved to be an important medium for manipulating and generating light in applications including soliton transmission1, light amplification2, all-optical switching3 and supercontinuum generation4. In the quantum regime, fibres may prove useful for ultralow-power all-optical signal processing5 and quantum information processing6. Here, we demonstrate the first experimental observation of optical nonlinearity at the single-photon level in an optical fibre. Taking advantage of the large nonlinearity and managed dispersion of photonic crystal fibres7,8, we report very small (1 × 10−7 to ∼1 × 10−8 rad) conditional phase shifts induced by weak coherent pulses that contain one or less than one photon per pulse on average. We discuss the feasibility of quantum information processing using optical fibres, taking into account the observed Kerr nonlinearity, accompanied by ultrafast response time and low induced loss. The tiny phase changes introduced by nonlinear optics performed at the single-photon level is reported in a photonic crystal fibre with carefully designed nonlinear and dispersion properties. The approach may prove useful in future quantum information processing schemes.