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Journal of Physics B Atomic Molecular and Optical Physics

Simulating gravity-assisted loading of laser-cooled atoms into a hollow-core photonic-bandgap fiber

Taehyun Yoon, Michal Bajcsy

Published Apr 15, 2020• 2 citations
Publisher

Abstract

Abstract We numerically simulate gravity-assisted loading of laser-cooled atoms into a hollow-core fiber. For this, we compute the trajectories of falling atoms and test the success or failure of loading with a range of atoms’ initial positions and velocities. By overlapping these results with the position and velocity distribution of the trapped atomic cloud, the number of loaded atoms is estimated without a random sampling process. We present our predictions for the atomic cloud’s loading efficiency into a photonic-bandgap fiber with a 7.5 μ m-diameter hollow core for various experimental conditions, such as the initial temperature, the cloud position and size, and the dipole trap power, and compare the predictions to experimental data. Our approach reduces the computational resources and time compared to the Monte-Carlo approach used in previously reported works.

DOI
10.1088/1361-6455/ab8946

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Abstract We numerically simulate gravity-assisted loading of laser-cooled atoms into a hollow-core fiber. For this, we compute the trajectories of falling atoms and test the success or failure of loading with a range of atoms’ initial positions and velocities. By overlapping these results with the position and velocity distribution of the trapped atomic cloud, the number of loaded atoms is estimated without a random sampling process. We present our predictions for the atomic cloud’s loading efficiency into a photonic-bandgap fiber with a 7.5 μ m-diameter hollow core for various experimental conditions, such as the initial temperature, the cloud position and size, and the dipole trap power, and compare the predictions to experimental data. Our approach reduces the computational resources and time compared to the Monte-Carlo approach used in previously reported works.
We present a thorough experimental investigation of the loading process of laser-cooled atoms from a magneto-optical trap into an optical dipole trap located inside a hollow-core photonic bandgap fiber, followed by propagation of the atoms therein. This, e.g., serves to identify limits to the loading efficiency and thus optical depth which is a key parameter for applications in quantum information technology. Although only limited access in 1D is available to probe atoms inside such a fiber, we demonstrate that a detailed spatially resolved characterization of the loading and trapping process along the fiber axis is possible by appropriate modification of probing techniques combined with theoretical analysis. Specifically, we demonstrate the loading of up to $2.1\ifmmode\times\else\texttimes\fi{}{10}^{5}$ atoms with a transfer efficiency of 2.12.1%2.1 during the course of 50 ms and a peak loading rate of $4.7\ifmmode\times\else\texttimes\fi{}{10}^{3}$ atoms ${\mathrm{ms}}^{\ensuremath{-}1}$ resulting in a peak atomic number density on the order of ${10}^{12}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. Furthermore, we determine the evolution of the spatial density (profile) and ensemble temperature as it approaches its steady-state value of $T=1400\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{K}$, as well as loss rates, axial velocity and acceleration. The spatial resolution along the fiber axis reaches a few millimeters, which is much smaller than the typical fiber length in experiments. We compare our results to other fiber-based as well as free-space optical dipole traps and discuss the potential for further improvements.
Cold atoms trapped and guided in hollow-core photonic crystal fibers provide a scalable diffraction-free setting for atom–light interactions for quantum technologies. However, due to the mismatch of the depth and spatial extension of the trapping potential from free space to the fiber, the number of cold atoms in the fiber is mainly determined by the loading process from free space to waveguide confinement. Here, we provide a numerical study of the loading dynamics of cold atoms into a hollow-core photonic crystal fiber. We use the Monte Carlo method to simulate the trajectories of an ensemble of cold atoms from free space trapping potential to optical potential inside a hollow-core fiber and calculate the temperature, loading efficiency, and geometry of the ensemble. We also study the noise sources that cause heating and a loss of atoms during the process. Our result could be used to design and optimize the loading process of cold atoms into a hollow-core fiber for cold atom experiments.
We report loading of laser-cooled cesium atoms into a hollow-core photonic-band gap fiber and confining the atoms in the fiber's $7\text{\ensuremath{-}}\ensuremath{\mu}\mathrm{m}$-diameter core with a magic-wavelength dipole trap at $\ensuremath{\sim}935$ nm. The use of the magic wavelength removes the ac Stark shift of the 852 nm optical transition in cesium caused by the dipole trap in the fiber core and suppresses the inhomogeneous broadening of the atomic ensemble that arises from the radial distribution of the atoms. This opens the possibility to continuously probe the atoms over timescales of a millisecond---approximately 1000 times longer than what was reported in previous works, because the dipole trap does not have to be modulated. We describe our atom-loading setup and its unique features and present spectroscopy measurements of the cesium's D2{D}_{2}D2​ line in the continuous-wave dipole trap with up to $1.7\ifmmode\times\else\texttimes\fi{}{10}^{4}$ loaded inside the hollow-core fiber.
We describe the loading of laser-cooled rubidium atoms into a single-mode hollow-core photonic-crystal fiber. Inside the fiber, the atoms are confined by a far-detuned optical trap and probed by a weak resonant beam. We describe different loading methods and compare their trade-offs in terms of implementation complexity and atom-loading efficiency. The most efficient procedure results in loading of  30~30 30,000 rubidium atoms, which creates a medium with an optical depth of  180~180 180 inside the fiber. Compared to our earlier study [1] this represents a sixfold increase in the maximum achieved optical depth in this system.