Many quantum information processing protocols require efficient transfer of quantum information from a flying photon to a stationary quantum system 1-3 . To transfer information, a photon must first be absorbed by the quantum system. This can be achieved, with a probability close to unity, by an atom residing in a high-finesse cavity 1 . However, it is unclear whether a photon can be absorbed effectively by an atom in a free space. Here, we report on an observation of substantial extinction of a light beam by a single 87 Rb atom through focusing light to a small spot with a single lens. The measured extinction values can be directly compared to the predictions of existing free-space photon-atom coupling models 4-6 . Our result should open a new perspective on processing quantum information carried by light using atoms, in particular for experiments that require strong absorption of single photons by an atom in free space.Strong interaction between light and matter is essential for successful operation of many quantum information protocols such as quantum networking 1,2 , entanglement swapping between two distant atoms 3,7,8 and implementation of elementary quantum gates 9 . These protocols consider quantum states of localized carriers (nodes), such as atoms, ions or even atomic ensembles, that exchange information through a quantum channel with the help of 'flying' qubits (photons). The quantum channels can be implemented via well-defined photonic modes that couple the nodes with high efficiency. For example, in the original proposal for quantum networks 1 , atoms were placed in high-finesse cavities that not only provide a strong interaction between a photon and an atom, but also ensure that most of the spontaneously emitted photons are collected into the same mode. Experimental advances in atom-photon cavity quantum electrodynamics indeed enabled the information exchange between an atom and single photons in this configuration to be carried out with high efficiency 10-14 . However, scaling such a scheme to many localized nodes is experimentally difficult, because managing the losses and coupling of the intracavity field of high-Q cavities to propagating modes of flying qubits is already quite challenging.In an attempt to avoid the complications connected with cavities, an interface between stationary and flying qubits in a simpler free-space configuration could be considered, where the quantum channel is defined, for example, by a Gaussian mode of a single-mode optical fibre, and a single atom is strongly coupled to this mode with the help of a large-numerical-aperture lens. Indeed, the common model describing the interaction of a monochromatic plane wave with a two-level atom predicts a scattering cross-section of σ = 3l 2 /2π. This area is close to a diffraction-limited spot size of a lens with a large numerical aperture (NA), hence suggesting a high coupling efficiency for such a system. Coupling efficiency here refers to the absorption probability of a flying photon by a stationary quantum system. For a fr...

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We report on a direct measurement of a phase shift on a weak coherent beam by a single 87Rb atom in a Mach-Zehnder interferometer. By strongly focusing the probe mode to the location of the atom, a maximum phase shift of about 1 degree is observed experimentally.