Strong Interaction Between Light and a Single Trapped Atom Without a Cavity

We measured the extinction of a focused light beam by a single 87Rb atom localized in an optical dipole trap and found a value of 9.7\% for a focused Gaussian beam resonantly interacting with an atomic two-level system. Various models describing interaction of an atom with a focused light field are compared to explain our experimental result. Our experiment suggests that a strong coupling may be achieved without cavity assistance. This opens new perspectives for an efficient quantum state transfer of a photonic qubit into an atom. Furthermore, the strong effecitive nonlinerity of the atom may be used to efficiently implement a phase gate between two photons.