1. Department of Physics,
2. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA
3. Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843, USA
4. P.N. Lebedev Physical Institute RAS, Leninskiy prospect 53, Moscow, 119991, Russia
5. These authors contributed equally to this work.

Correspondence to: H. Park^1,2M. D. Lukin¹ Correspondence and requests for materials should be addressed to M.D.L. (Email: lukin@fas.harvard.edu) and H.P. (Email: Hongkun_Park@harvard.edu).

Control over the interaction between single photons and individual optical emitters is an outstanding problem in quantum science and engineering. It is of interest for ultimate control over light quanta¹, as well as for potential applications such as efficient photon collection², single-photon switching³ and transistors⁴, and long-range optical coupling of quantum bits^{5, 6}. Recently, substantial advances have been made towards these goals, based on modifying photon fields around an emitter using high-finesse optical cavities^{2, 3, 5, 6, 7, 8}. Here we demonstrate a cavity-free, broadband approach for engineering photon–emitter interactions^{4, 9} via subwavelength confinement of optical fields near metallic nanostructures^{10, 11, 12, 13}. When a single CdSe quantum dot is optically excited in close proximity to a silver nanowire, emission from the quantum dot couples directly to guided surface plasmons in the nanowire, causing the wire's ends to light up. Non-classical photon correlations between the emission from the quantum dot and the ends of the nanowire demonstrate that the latter stems from the generation of single, quantized plasmons. Results from a large number of devices show that efficient coupling is accompanied by more than 2.5-fold enhancement of the quantum dot spontaneous emission, in good agreement with theoretical predictions.

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