Efficient quantum computing using coherent photon conversion

Single photons make very good quantum information carriers, but current schemes for photonic quantum information processing (QIP) are inefficient. We describe a new scheme, \emph{coherent photon conversion (CPC)}, using classically pumped nonlinearities to generate and process complex multiquanta states\footnote{Published in Nature \textbf{478}, 360 (2011)}. One example based on four-wave mixing provides a full suite of QIP tools for scalable quantum computing from a single, versatile process, including: deterministic multiqubit entanglement gates based on a novel photon-photon interaction, high-quality heralded multiphoton states without higher-order imperfections, and robust, high-efficiency detection. Using photonic crystal fibres, we present observations of quantum correlations from a four-colour nonlinear process suitable for CPC and study the feasibility of reaching the deterministic regime with current technology. The scheme could also be implemented in optomechanical, electromechanical and superconducting systems.