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Consistent isolation of individual atoms using cold collisions

Mikkel F. Andersen, Tzahi Grünzweig, Andrew J. Hilliard, M. McGovern, Y. H. Fung

Published Jan 1, 2011• 0 citations
Publisher

Abstract

The fact that a single neutral atom stored in an optical micro-trap is very well isolated from the environment makes it attractive for application in a quantum logic device. Scaling such a device requires a consistent and efficient method of loading a single atom into the micro-trap. By inducing light-assisted collisions between trapped pairs of atoms we observe the outcome of individual collision events. This allowed us to devise a method that enables a loading efficiency of 83% of a single atom into a micro-trap. Furthermore, we are able to accurately count the small number of atoms using fluorescence imaging at high densities while minimizing loss through light assisted collisions.

DOI
10.1364/iqec.2011.i94

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The fact that a single neutral atom stored in an optical micro-trap is very well isolated from the environment makes it attractive for application in a quantum logic device. Scaling such a device requires a consistent and efficient method of loading a single atom into the micro-trap. By inducing light-assisted collisions between trapped pairs of atoms we observe the outcome of individual collision events. This allowed us to devise a method that enables a loading efficiency of 83% of a single atom into a micro-trap. Furthermore, we are able to accurately count the small number of atoms using fluorescence imaging at high densities while minimizing loss through light assisted collisions.
The fact that a single neutral atom stored in an optical micro-trap is very well isolated from the environment makes it attractive for application in a quantum logic device. Scaling such a device requires a consistent and efficient method of loading a single atom into the micro-trap. By inducing light-assisted collisions between trapped pairs of atoms we observe the outcome of individual collision events. This allowed us to devise a method that enables a loading efficiency of 83% of a single atom into a micro-trap. Furthermore, we are able to accurately count the small number of atoms using fluorescence imaging at high densities while minimizing loss through light assisted collisions.
The detailed control achieved over single optically trapped neutral atoms makes them candidates for applications in quantum metrology and quantum information processing. The last few decades have seen different methods developed to optimize the preparation efficiency of single atoms in optical traps. Here we review the near-deterministic preparation of single atoms based on light-assisted collisions and describe how this method can be implemented in different trap regimes. The simplicity and versatility of the method makes it feasible to be employed in future quantum technologies such as a quantum logic device.
We review light assisted collisions in a high-density far-off resonant optical trap (FORT). By tuning the parameters of the light that induces the collisions, the effects of the collisions can be controlled. Trap loss can be suppressed even at high atomic densities, allowing us to count the atoms using fluorescence detection. When only two atoms are trapped, individual loss events reveal new information about the process, and the simplicity of the system allows for a numerical simulation of the dynamics. By optimizing the experimental parameters, we implement an efficient method to prepare single atoms in the FORT. Our methods can be extended to load quantum registers for quantum information processing.
In this thesis we study: the dynamics leading to a near-deterministic preparation of single atoms in a microscopic optical dipole trap; the non destructive imaging and counting of trapped atoms; and a system for sorting single atoms between optical dipole traps using a high numerical aperture lens and a spatial light modulator. \n \nWe near-deterministically prepare trapped single Rubidium atoms by exposing them to a tailored light field which induces light assisted collisions. By preparing pairs of Rubidium atoms in an optical micro-trap we can study the outcome of light assisted collisions at the single event level. Using this knowledge we tailor the light such that each collision releases only enough energy for one atom of a colliding atom-pair to escape the trap. This thereby yields an 83% single atom loading efficiency. \n \n \nWe demonstrate a method to count small numbers of atoms held in a deep, microscopic optical dipole trap by collecting fluorescence from atoms exposed to a standing wave of light that is blue-detuned from resonance. While scattering photons, the atoms are cooled by a Sisyphus mechanism that results from the spatial variation in light intensity. The use of a small blue detuning limits the losses due to light assisted collisions, thereby making the method suitable for counting several atoms in a microscopic volume. \n \nWe create two overlapping one-dimensional optical lattices using a single laser beam, a spatial light modulator and a high numerical aperture lens. These lattices have the potential to trap single atoms, and using the dynamic capabilities of the spatial light modulator may shift and sort atoms to a minimum atom-atom separation of 1.52 µm. We show how a simple feedback circuit can compensate for the spatial light modulator's intensity modulation.
We investigate the use of light assisted collisions for the deterministic preparation of individual atoms in a microtrap. Blue detuned light is used in order to ensure that only one of the collision partners is lost from the trap. We obtain a 91% loading efficiency of single 85 Rb atoms. This can be achieved within a total preparation time of 542 ms. A numerical model of the process quantitatively agrees with the experiment giving an in-depth understanding of the dynamics of the process and allowing us to identify the factors that still limit the loading efficiency. The fast loading time in combination with the high efficiency may be sufficient for loading quantum registers at the size required for competitive quantum computing.
We show that controlled inelastic collisions can improve the single atom loading efficiency in the collisional blockade regime of optical microtraps. A collisional loss process where only one of the colliding atoms are lost, implemented during loading, enables us to kick out one of the atoms as soon as a second atom enters the optical microtrap. When this happens faster than the pair loss, which has limited the loading efficiency of previous experiments to about 50%, we experimentally observe an enhancement to 80%. A simple analytical theory predicts the loading dynamics. Our results opens up an efficient and fast route for loading individual atoms into optical tweezers and arrays of microtraps that are too tight for easy implementation of the method reported in [1,2]. The loading of tight traps with single atoms is a requirement for their applications in future experiments in quantum information processing and few-body physics.