An optical conveyor belt for single neutral atoms

Using optical dipole forces we have realized controlled transport of a single or any desired small number of neutral atoms over a distance of a centimeter with sub-micrometer precision. A standing wave dipole trap is loaded with a prescribed number of cesium atoms from a magneto-optical trap. Mutual detuning of the counterpropagating laser beams moves the interference pattern, allowing us to accelerate and stop the atoms at preselected points along the standing wave. The transportation efficiency is close to 100 %. This optical ”singleatom conveyor belt” represents a versatile tool for future experiments requiring deterministic delivery of a prescribed number of atoms on demand. PACS: 32.80.Lg, 32.80.Pj, 42.50.Vk Quantum engineering of microscopic systems requires manipulation of all degrees of freedom of isolated atomic particles. The most advanced experiments are implemented with trapped chains of ions [1,2,3]. Neutral atoms are more difficult to control because of the weaker interaction of induced electric or paramagnetic dipoles with inhomogeneous electromagnetic fields. Optical dipole traps [4] could provide a level of control similar to ion traps, since they store neutral atoms in a nearly conservative potential with long coherence times [5]. The variety of different dipole traps allows for an individual design, depending on the specific experimental demands [6]. Here we use a time-varying standing wave optical dipole trap to displace atoms by macroscopic distances on the order of a centimeter with sub-micrometer precision [7]. A similar technique of moving optical lattices has been applied for the acceleration of large ensembles of atoms in [8,9]. Time-dependent magnetic fields can also be used for controlled transport of clouds of neutral atoms as has been demonstrated with a micro-fabricated device [10]. Recently, techniques have been developed to load a dipole trap with single atoms only [11,12]. In our case, we have combined controlled manipulation of the trapping potential with deterministic loading of a dipole trap with a prescribed small number of atoms [7]. These atoms are then transported with high efficiency over macroscopic distances and observed by positionsensitive fluorescence detection in the dipole trap. Here we describe the transportation technique in detail. We also analyze the dependency of the measured transportation efficiency on both the transportation distance and the acceleration. This is followed by a brief discussion of possible applications and alternative approaches. 1 The standing-wave dipole trap Our dipole trap (Fig.1) consists of two counter-propagating Gaussian laser beams with equal intensities and optical frequencies ν1 and ν2 producing a positionand time-dependent dipole potential