A single-photon transistor using nano-scale surface plasmons is described, which utilizes strong coupling between individual optical emitters and tightly confined surface plasmon (SP) modes on conducting nanowires. This system enables a nonlinear two-photon switch for photons propagating along the nanowire, with quantum optical techniques for coherent control. The interaction can be tailored to create a single-photon transistor, where the presence or absence of a single photon in a "gate" field controls the propagation of subsequent "signal" photons. In analogy with electronic transistors, a photonic transistor uses a small optical "gate" field to control another optical "signal" field via nonlinear interactions. The fundamental limit is the single-photon transistor, where signal propagation is controlled by the presence or absence of a single photon in the gate field. Nonlinear devices have applications in optical communication, computation, and quantum information processing, but their practical realization is challenging due to weak single-photon nonlinearities. A new method for strong coupling between light and matter has been proposed and demonstrated, using SPs on nanowires to achieve strong interaction with optical emitters. This process, though linear, allows for nonlinear optical phenomena where individual photons strongly interact. The system can be used to implement a single-photon transistor, combining plasmonics with quantum optics for unprecedented control over light interactions. SPs are electromagnetic modes confined to the surface of a conductor-dielectric interface, enabling sub-wavelength confinement and applications in waveguiding, enhanced transmission, and imaging. SPs on nanowires can strongly interact with optical emitters, leading to efficient single-photon generation. The SP modes form a one-dimensional continuum, with strong confinement and guiding even for small nanowire radii. The Purcell factor, which determines the strength of spontaneous emission into SPs, can be very large, enabling strong nonlinear interactions. A single emitter strongly coupled to SP modes can act as a saturable mirror, reflecting photons efficiently at low power but saturating at high power. The nonlinear response of the system is demonstrated by considering multi-photon input states, where the emitter's response saturates, preventing efficient reflection of multiple photons. The system exhibits strong photon bunching at low power and anti-bunching at higher times, indicating the emitter's ability to control photon statistics. A three-level emitter can be used to create a single-photon transistor, where a gate photon controls the propagation of signal photons. The emitter's internal state is conditioned on the presence or absence of a gate photon, allowing for efficient switching. The system can store photons in the emitter's metastable state, enabling conditional control over signal photons. The transistor's operation is limited by the time over which an undesired spin flip can occur, with the emitter reflecting a large number of photons before an unwanted flip happens. The system can be integrated with low-loss dielectric waveguides for long-distance transport, enabling practical applications in photonicA single-photon transistor using nano-scale surface plasmons is described, which utilizes strong coupling between individual optical emitters and tightly confined surface plasmon (SP) modes on conducting nanowires. This system enables a nonlinear two-photon switch for photons propagating along the nanowire, with quantum optical techniques for coherent control. The interaction can be tailored to create a single-photon transistor, where the presence or absence of a single photon in a "gate" field controls the propagation of subsequent "signal" photons. In analogy with electronic transistors, a photonic transistor uses a small optical "gate" field to control another optical "signal" field via nonlinear interactions. The fundamental limit is the single-photon transistor, where signal propagation is controlled by the presence or absence of a single photon in the gate field. Nonlinear devices have applications in optical communication, computation, and quantum information processing, but their practical realization is challenging due to weak single-photon nonlinearities. A new method for strong coupling between light and matter has been proposed and demonstrated, using SPs on nanowires to achieve strong interaction with optical emitters. This process, though linear, allows for nonlinear optical phenomena where individual photons strongly interact. The system can be used to implement a single-photon transistor, combining plasmonics with quantum optics for unprecedented control over light interactions. SPs are electromagnetic modes confined to the surface of a conductor-dielectric interface, enabling sub-wavelength confinement and applications in waveguiding, enhanced transmission, and imaging. SPs on nanowires can strongly interact with optical emitters, leading to efficient single-photon generation. The SP modes form a one-dimensional continuum, with strong confinement and guiding even for small nanowire radii. The Purcell factor, which determines the strength of spontaneous emission into SPs, can be very large, enabling strong nonlinear interactions. A single emitter strongly coupled to SP modes can act as a saturable mirror, reflecting photons efficiently at low power but saturating at high power. The nonlinear response of the system is demonstrated by considering multi-photon input states, where the emitter's response saturates, preventing efficient reflection of multiple photons. The system exhibits strong photon bunching at low power and anti-bunching at higher times, indicating the emitter's ability to control photon statistics. A three-level emitter can be used to create a single-photon transistor, where a gate photon controls the propagation of signal photons. The emitter's internal state is conditioned on the presence or absence of a gate photon, allowing for efficient switching. The system can store photons in the emitter's metastable state, enabling conditional control over signal photons. The transistor's operation is limited by the time over which an undesired spin flip can occur, with the emitter reflecting a large number of photons before an unwanted flip happens. The system can be integrated with low-loss dielectric waveguides for long-distance transport, enabling practical applications in photonic