Nature Phys. http://doi.org/wwf (2014)

Quantum technology applications rely on the fine control of light–matter interaction on the single-photon level. To obtain this level of control it is necessary to achieve a regime in which a light-emitting source radiates into a distinct light mode with very high probability. This cooperative coupling between the light emitter and the field radiated by the single photon can be reached, for example, by combining emitters that are polarizable with systems inducing large field enhancement, such as surface plasmons on metals. Following this idea, Sebastian Slama and co-authors now demonstrate experimentally cooperative coupling between the fluorescence emission of ultracold rubidium atoms and surface plasmons propagating on a gold substrate. A laser pulse excites electric dipole oscillations in Rb atoms while they move towards the gold surface. These dipole oscillations consequently couple with surface plasmons on the gold. The surface plasmons then decay into freely propagating photons, which are used to detect successful coupling. The demonstrated cooperative coupling is efficient, with Purcell factors as high as 4.9 for atomic dipoles oscillating normally to the gold surface, above what is currently obtainable in cold atom experiments.