Coupling a photon and an atom in free space
The interaction of a single photon with a single atom in free space is probably one of the most fundamental processes in quantum optics. If the atom is initially in the excited state and the field in the vacuum state, the system will decay to the atom in the ground state and the photon in a wave packet in the far field. This spontaneous emission process is a unitary evolution. Thus, the process should also be able to run backwards starting with the photon in the far field and the atom unexcited – leading to a fully excited atom every single time the experiment is done. We are currently setting up an experiment to show that the probability for the absorption of the single photon can indeed be one . In the presentation potential complications and counter measures are discussed. If successful, the experimental set-up can be used to do non linear optics at the single photon level and to implement quantum gates for quantum information processing. This free space approach is an alternative to other schemes using an optical resonator or a nano optical plasmonic antenna to reach maximum coupling between an atom and a photon.
 R. Maiwald, G. Leuchs, D. Leibfried, J. Britton, J.C. Bergquist, D.J. Wineland
"Stylus ion trap for enhanced access and sensing",
Nature Physics 5, 551 (2009) (arXiv: 0810.2647)