Neutral atoms stored in optical traps are strong candidates for a physical realization of a quantum logic device 1,2 . Far off-resonance optical traps provide conservative potentials and excellent isolation from the environment, and they may be arranged to produce arbitrary arrays of traps, where each trap is occupied by a single atom that can be individually addressed [3][4][5][6] . At present, significant effort is being expended on developing two-qubit gates based on coupling individual Rydberg atoms in adjacent optical microtraps [7][8][9] . A major challenge associated with this approach is the reliable generation of single-atom occupancy in each trap, as the loading efficiency in the past experiments has been limited to 50% (refs 4,7,8,10-12). Here we report a loading efficiency of 82.7% in an optical microtrap. We achieve this by manipulating the collisions between pairs of trapped atoms through tailored optical fields and directly observing the resulting single atoms in the trap.Deterministic control of single neutral atoms is a long-standing goal in atomic physics. Not only would it represent a milestone in scientists' ability to control the microscopic world, but also because it would enable a neutral-atom-based quantum logic device 7-9 . Two approaches have successfully led to direct observation of subPoissonian number distributions of atoms in optical microtraps, without consecutive atom sorting 13 . In the first, the Mott insulator transition of a Bose-Einstein condensate provides an efficient route for high occupancy of individual atoms in optical lattices where atoms can tunnel between adjacent lattice sites [14][15][16][17] . The second approach, which may be applied in arbitrary geometries 18,19 , is to employ light-assisted collisions 4,[10][11][12] . This method makes use of the change in the atom-atom interaction that arises when light drives one of the atoms undergoing a collision to the electronic excited state. In the case of light with a frequency below resonance (red detuned), the atom pair is excited to an attractive potential leading to the atoms forming a molecule and/or gaining a large amount of kinetic energy. In each case, both atoms are lost, leading to a maximal 50% chance of ending with one atom in the trap, depending on whether the initial atom number is even or odd 10,12 . However, a process where only one atom is lost as a result of a two-body collision would lead to deterministic preparation of a single atom in a given site. In the past, it has been shown that various forms of collisional trap loss can be suppressed by the application of optical control fields, and in particular, the use of blue-detuned light to effect so-called optical shielding 20,21 . In this Letter, we study light-assisted collisions at the single-event level. We prepare individual pairs of atoms in an optical microtrap and expose them to near-resonant light. We directly observe that light-assisted collisions between these atoms can lead to only one atom being lost. By choice of the frequency and intensi...

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We study the dynamics of atoms in optical traps when exposed to laser cooling light that induces light-assisted collisions. We experimentally prepare individual atom pairs and observe their evolution. Due to the simplicity of the system (just two atoms in a microtrap) we can directly simulate the pair's dynamics, thereby revealing detailed insight into it. We find that often only one of the collision partners gets expelled, similar to when using blue detuned light for inducing the collisions. This enhances schemes for using light-assisted collisions to prepare individual atoms and affects other applications as well. PACS numbers:Optically trapped cold atoms provide an exciting platform for studying the change in atom-atom interactions due to absorption or emission of light. The advantage of such a system is that the atoms are isolated from their surroundings and basic physical effects in photochemistry can be studied without interference from processes induced by the environment. Recently this has led to controlled formation of ultra-cold molecular gases [1,2] and in the past, photo-association spectroscopy has provided detailed knowledge about atom-atom interaction potentials [3,4].Typically, experiments on photo-association or lightassisted collisions of cold atoms are conducted on large samples. Studying microscopic processes at the individual event level reveals information hidden in ensemble averages of larger samples [5,6]. Pioneering work in studying individual light-assisted collisions was conducted using small samples of atoms in a high gradient MagnetoOptical Trap (MOT) [7,8]. Those studies observed that up to 10% of loss events manifested themselves as just one of the collision partners being lost from the MOT.Of particular interest to many modern experiments in atomic physics are light-assisted collisions or photoassociation events induced by near-resonant light. This phenomenon is of fundamental interest [9] and plays a crucial role in modern laser cooling experiments. In addition, light-assisted collisions have been employed to isolate individual atoms in optical microtraps [10][11][12][13][14], to redistribute atoms loaded into an array of traps [15], and to perform parity number measurement of atoms in optical lattices [16,17]. Isolation experiments that used bluedetuned light to induce collisions have achieved high efficiency, whereas experiments using red detuned light have reported efficiencies of about 50%. For several of these applications it is assumed that both atoms of the pair are lost from the optical microtrap when they undergo light-assisted collisions.Here we revisit the ejection of atoms from a far off resonance optical trap due to light-assisted collisions induced by red-detuned laser cooling light. We implement the idealized collision experiment in which only two atoms are trapped so we can observe individual atom loss events. A numerical model of the complex dynamics of the expelling process agrees well with our experiment. We find that the light-assisted collisions can lead to j...