  Removing a single photon from a pulse is one of the most elementary
operations that can be performed on light, having both fundamental significance
and practical applications in quantum communication and computation. 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. 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. Our implementation makes use of single-photon Raman
interaction (SPRINT) 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.
