Analysis of a single-atom dipole trap

We describe a simple experimental technique which allows us to store a single $^{87}\mathrm{Rb}$ atom in an optical dipole trap. Due to light-induced two-body collisions during the loading stage of the trap the maximum number of captured atoms is locked to one. This collisional blockade effect is confirmed by the observation of photon antibunching in the detected fluorescence light. The spectral properties of single photons emitted by the atom were studied with a narrow-band scanning cavity. We find that the atomic fluorescence spectrum is dominated by the spectral width of the exciting laser light field. In addition we observe a spectral broadening of the atomic fluorescence light due to the Doppler effect. This allows us to determine the mean kinetic energy of the trapped atom corresponding to a temperature of $105\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$. This simple single-atom trap is the key element for the generation of atom-photon entanglement required for future applications in quantum communication and a first loophole-free test of Bell's inequality.