The paper presents a novel approach to achieving strong nonlinear interactions at the single-photon level using nano-scale surface plasmons (SPs) on conducting nanowires. The authors demonstrate that this system can act as a nonlinear two-photon switch for incident photons propagating along the nanowire, which can be coherently controlled using quantum optical techniques. They further discuss how this interaction can be tailored to create a single-photon transistor, where the presence or absence of a single incident photon in a "gate" field is sufficient to control the propagation of subsequent "signal" photons. The key advantages of this system include its robustness to strong coupling, the large Purcell factor, and the ability to achieve strong nonlinear optical phenomena at the single-photon level. The paper also explores the integration of SP devices with low-loss dielectric waveguides and other microphotonic devices, making it feasible for large-scale, integrated photonic devices. Finally, the authors outline potential applications in quantum information science, such as efficient single-photon detection and the preparation of Schrödinger cat states of photons.The paper presents a novel approach to achieving strong nonlinear interactions at the single-photon level using nano-scale surface plasmons (SPs) on conducting nanowires. The authors demonstrate that this system can act as a nonlinear two-photon switch for incident photons propagating along the nanowire, which can be coherently controlled using quantum optical techniques. They further discuss how this interaction can be tailored to create a single-photon transistor, where the presence or absence of a single incident photon in a "gate" field is sufficient to control the propagation of subsequent "signal" photons. The key advantages of this system include its robustness to strong coupling, the large Purcell factor, and the ability to achieve strong nonlinear optical phenomena at the single-photon level. The paper also explores the integration of SP devices with low-loss dielectric waveguides and other microphotonic devices, making it feasible for large-scale, integrated photonic devices. Finally, the authors outline potential applications in quantum information science, such as efficient single-photon detection and the preparation of Schrödinger cat states of photons.