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 transmission(1), light amplification(2), all-optical switching(3) and supercontinuum generation(4). In the quantum regime, fibres may prove useful for ultralow-power all-optical signal processing(5) and quantum information processing(6). 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. bres(7,8), we report very small (1 x 10(-7) to similar to 1 x 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.