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arXiv:1011.5538 · Optics Letters

Counting atoms in a deep optical microtrap

Matthew McGovern, Andrew J. Hilliard, Tzahi Grünzweig, Mikkel F. Andersen

Published Mar 16, 2011• 21 citations
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Abstract

We 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.

DOI
10.1364/ol.36.001041

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•Jan 1, 2011•0 citations
We 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.
We 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.
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 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.
Experiments are performed on loading cold atoms into a far-off resonance optical dipole trap that is spatially separated from the laser-cooling region. The atoms are delivered using a far-off resonant optical dipole guide and enter the trap through a switchable blue-detuned light sheet or ``trap door.'' This scheme provides a means to transfer atoms from a dissipative to a non-dissipative trap without a significant loss of phase-space density. We study the dynamics of atoms within the trap and measure their lifetime. Preliminary results on multiple loading are presented. As the technique does not rely on near-resonant light, the atom number limit characteristic of laser-cooling experiments can, in principle, be exceeded.
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.
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.