Strong interaction between light and a single trapped atom without the need for a cavity

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 Rb-87 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.