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International Conference on Atomic Physics (ICAP)

Single Atom Manipulation in a Microscopic Dipole Trap

G. Reymond, Nicolas Schlosser, Philippe Grangier

Published Apr 1, 2003• 0 citations
Publisher

Abstract

We describe the operation of a very small optical dipole trap that is designed to store and manipulate individual atoms. Due to the very small dipole trap volume, a “collisional blockade ” mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We demonstrate experimentally the existence of this regime, and we describe also the “weakloading ” and “strong loading ” regimes outside the blockade range. In addition, we describe methods to measure the oscillation frequencies of a single atom in the trap with a high accuracy, and a “release and recapture ” method designed for temperature measurements at the single atom level. The measured “single atom temperature ” is 35 µK, that is in the sub-Doppler regime. Finally, by exploiting the extremely high density observed in the strong loading regime, we present preliminary results for evaporative cooling, where typically 30 atoms at 200 µK are forced to evaporate into about 10 atoms at 15 µK. 1

DOI
10.1142/9789812705099_0008

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We describe the operation of a very small optical dipole trap that is designed to store and manipulate individual atoms. Due to the very small dipole trap volume, a “collisional blockade ” mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We demonstrate experimentally the existence of this regime, and we describe also the “weakloading ” and “strong loading ” regimes outside the blockade range. In addition, we describe methods to measure the oscillation frequencies of a single atom in the trap with a high accuracy, and a “release and recapture ” method designed for temperature measurements at the single atom level. The measured “single atom temperature ” is 35 µK, that is in the sub-Doppler regime. Finally, by exploiting the extremely high density observed in the strong loading regime, we present preliminary results for evaporative cooling, where typically 30 atoms at 200 µK are forced to evaporate into about 10 atoms at 15 µK. 1
We present a brief overview of optical trapping experiments of individual neutral atoms. Then we describe in more details an experiment using a very small optical dipole trap, that is designed to store and manipulate individual atoms. Due to the very small dipole trap volume, a "collisional blockade" mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We study this regime experimentally, and we describe methods to measure the oscillation frequencies and the temperature of a single atom in the trap with a high accuracy.
We analyze the operating regimes of a very small optical dipole trap, loaded from a magneto-optical trap, as a function of the atom loading rate, i.e., the number of atoms per second entering the dipole trap. We show that, when the dipole trap volume is small enough, a "collisional blockade" mechanism locks the average number of trapped atoms on the value 0.5 over a large range of loading rates. We also discuss the "weak loading" and "strong loading" regimes outside the blockade range, and we demonstrate experimentally the existence of these three regimes.
We have realized a very small optical dipole trap that is designed to store and manipulate individual atoms. Due to the very small dipole-trap volume, a 'collisional blockade' mechanism locks the average number of trapped atoms at a value of 0.5 over a large range of loading rates. Here we describe methods to characterize the motion of a single trapped atom, and we also demonstrate the possibility of trapping two atoms individually at a short distance apart. Finally, we study theoretically the possibility to perform 'conditional quantum logic' by scattering photons from two such atoms.
We describe a simple experimental technique which allows us to store a single 87Rb^{87}\mathrm{Rb}87Rb atom in an optical dipole trap. Due to light-induced two-body collisions during the loading stage of the trap the maximum number of captured atoms is locked to one. This collisional blockade effect is confirmed by the observation of photon antibunching in the detected fluorescence light. The spectral properties of single photons emitted by the atom were studied with a narrow-band scanning cavity. We find that the atomic fluorescence spectrum is dominated by the spectral width of the exciting laser light field. In addition we observe a spectral broadening of the atomic fluorescence light due to the Doppler effect. This allows us to determine the mean kinetic energy of the trapped atom corresponding to a temperature of $105\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$. This simple single-atom trap is the key element for the generation of atom-photon entanglement required for future applications in quantum communication and a first loophole-free test of Bell's inequality.
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.
Recently, we have experimentally demonstrated a continuous loading mechanism for an optical dipole trap from a guided atomic beam [M. Falkenau, V. V. Volchkov, J. R\"uhrig, A. Griesmaier, and T. Pfau, Phys. Rev. Lett. 106, 163002 (2011)]. The observed evolution of the number of atoms and temperature in the trap are consequences of the unusual trap geometry. In the present paper, we develop a model based on a set of rate equations to describe the loading dynamics of such a mechanism. We consider the collision statistics in the nonuniform trap potential that leads to two-dimensional evaporation. The comparison between the resulting computations and experimental data allows to identify the dominant loss process and suggests ways to enhance the achievable steady-state atom number. Concerning subsequent evaporative cooling, we find that the possibility of controlling axial and radial confinement independently allows faster evaporation ramps compared to single beam optical dipole traps.