Extraction of a single photon from an optical pulse

Removing a single photon from a pulse is one of the most elementary operations that can be performed on light, having both fundamental significance(1,2) and practical applications in quantum communication(3-9) and computation(10). So far, photon subtraction, in which the removed photon is detected and therefore irreversibly lost, has been implemented in a probabilistic manner with inherently low success rates using low-reflectivity beam splitters(1). Here we demonstrate a scheme for the deterministic extraction of a single photon from an incoming pulse. The removed photon is diverted to a different mode, enabling its use for other purposes, such as a photon number-splitting attack on quantum key distribution protocols(11). Our implementation makes use of single-photon Raman interaction (SPRINT)(12,13) with a single atom near a nanofibre-coupled microresonator. The single-photon extraction probability in our current realization is limited mostly by linear loss, yet probabilities close to unity should be attainable with realistic experimental parameters(13).