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arXiv:2101.07544 · Physical Review A

Preparation of one-dimensional chains and dense cold atomic clouds with a high numerical aperture four-lens system

Antoine Glicenstein, Giovanni Ferioli, Ludovic Brossard, Yvan R. P. Sortais, Daniel Barredo, Florence Nogrette, Igor Ferrier-Barbut, Antoine Browaeys

Published Apr 1, 2021• 23 citations
arXivHTMLPDF

Abstract

We report the efficient and fast $(\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}\mathrm{Hz})$ preparation of randomly loaded one-dimensional (1D) chains of individual $^{87}\mathrm{Rb}$ atoms and of dense atomic clouds trapped in optical tweezers using an upgraded experimental platform. This platform is designed for the study of atomic ensembles featuring either ordered or disordered distributions of the atomic positions. It is composed of two high-resolution optical systems perpendicular to each other, enhancing observation and manipulation capabilities. The setup includes a dynamically controllable telescope, which we use to vary the tweezer beam waist. A $\mathrm{\ensuremath{\Lambda}}$-enhanced gray molasses on the D1 line enhances the loading of the traps from a magneto-optical trap. Using these tools, we prepare chains of up to $\ensuremath{\sim}100$ atoms separated by $\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ by retroreflecting the tweezer light, hence producing a 1D optical lattice with strong transverse confinement. Dense atomic clouds with peak densities up to ${n}_{0}\ensuremath{\sim}{10}^{15}$ $\mathrm{atoms}/{\mathrm{cm}}^{3}$ are obtained by compression of an initial cloud. This high density results in interatomic distances smaller than $\ensuremath{\lambda}/(2\ensuremath{\pi})$ for the D2 optical transitions, making it ideal to study light-induced interactions in dense samples.

DOI
10.1103/physreva.103.043301

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We report the efficient and fast (\ensuremath∼2Hz)(\ensuremath{\sim}2\phantom{\rule{0.16em}{0ex}}\mathrm{Hz})(\ensuremath∼2Hz) preparation of randomly loaded one-dimensional (1D) chains of individual 87Rb^{87}\mathrm{Rb}87Rb atoms and of dense atomic clouds trapped in optical tweezers using an upgraded experimental platform. This platform is designed for the study of atomic ensembles featuring either ordered or disordered distributions of the atomic positions. It is composed of two high-resolution optical systems perpendicular to each other, enhancing observation and manipulation capabilities. The setup includes a dynamically controllable telescope, which we use to vary the tweezer beam waist. A \ensuremathΛ\mathrm{\ensuremath{\Lambda}}\ensuremathΛ-enhanced gray molasses on the D1 line enhances the loading of the traps from a magneto-optical trap. Using these tools, we prepare chains of up to \ensuremath∼100\ensuremath{\sim}100\ensuremath∼100 atoms separated by \ensuremath∼1\ensuremathμm\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}\ensuremath∼1\ensuremathμm by retroreflecting the tweezer light, hence producing a 1D optical lattice with strong transverse confinement. Dense atomic clouds with peak densities up to n0\ensuremath∼1015{n}_{0}\ensuremath{\sim}{10}^{15}n0​\ensuremath∼1015 atoms/cm3\mathrm{atoms}/{\mathrm{cm}}^{3}atoms/cm3 are obtained by compression of an initial cloud. This high density results in interatomic distances smaller than \ensuremathλ/(2\ensuremathπ)\ensuremath{\lambda}/(2\ensuremath{\pi})\ensuremathλ/(2\ensuremathπ) for the D2 optical transitions, making it ideal to study light-induced interactions in dense samples.
The main objective of this research is to study a particular case of light-matter interaction: the scattering of light in dense atomic samples.The regime of high atomic densities is represented by samples where the distance between neighboring atoms is on the order of the wavelength associated with the atomic transition.In this regime, short-range atomic interactions cannot be neglected, and in these systems, we expect to observe significant modifications in collective effects such as subradiance or superradiance.Overall, the propagation of light through this type of system is not well-known to the scientific community, and the existing models work to describe very specific situations.To pave the way for achieving our overall goal, this work proposes an experimental setup to obtain a dense cloud of 88 Sr, which is the chemical species we work with in our laboratory.For this purpose, atoms previously trapped using a Magneto-Optical Trap are transferred to a Crossed Optical Dipole Trap.During the optimization and characterization of the optical trap, problems were encountered in producing an image with quantitative information of the atomic ensemble and in transferring a large number of atoms in a very small trapping volume.These problems were resolved with the implementation of a Phase Contrast Imaging system and the implementation of an Optical Molasses as an additional cooling step.Under these conditions, we obtained a cloud that can be suitable for conducting studies on collective effects, transitioning from the dilute to the dense regime.The initial experiments, once our atomic ensemble was prepared, involve measuring the coherent transmission of a low-intensity beam through the cloud.For this purpose, incident light near the resonance of a dipolar transition of the type J = 0 ↔ J = 1 of 88 Sr is used, interacting with a homogeneous region of the sample.Simulations of this type of process can be performed using the Coupled Dipole Model, which describes the overall response of light scattering by the sample considering dipole-dipole interactions mediated by light.With this work, we aim to advance the understanding of density effects by comparing the coherent optical response of the dense cloud with the theory based on the Coupled Dipole Model.
Optical tweezer arrays of neutral atoms have emerged as a versatile and powerful platform for quantum simulation and quantum information processing. In this thesis, I present the realization of an experimental platform for quantum simulation based on individually trapped strontium atoms in optical tweezer arrays. The experiment combines precise laser cooling, optical control, and high-resolution imaging to enable the manipulation and high fidelity detection of single atoms. I developed and optimized a two-stage three-dimensional magneto-optical trap (3D-MOT) that produces cold and dense samples of strontium atoms from a thermal beam. In the f irst stage, atoms are captured and precooled using the dipole-allowed 1S0 → 1P1 transition at 461nm (which has a linewidth of !=2ω ↑ 32MHz). The second stage employs the 1S0 → 3P1 narrow intercombination transition at 689nm (with a linewidth of !=2ω ↑7.5kHz) to further cool and compress the atomic cloud. By optimizing the trapping parameters, I achieved clouds with temperatures around 5μK and densities above 1010cm→3, providing ideal initial conditions for e!cient loading into optical tweezers. A 3↑3 array of 360μK deep optical tweezers is generated by spatially modulating an 813.4nm laser beam using two crossed acousto-optic deflectors. The deflected beams are relayed and focused into the science chamber using a high–numericalaperture objective, which simultaneously serves as the collection optics for atomic f luorescence. The fluorescence light emitted by the atoms is imaged onto a scientific qCMOS camera, enabling site-resolved detection of individual atoms. I implemented and optimized the in-trap cooling dynamics to prevent atom loss during imaging and to minimize residual thermal motion. In addition, by precisely tuning lightassisted collisions, multiple atoms occupancies are removed from the traps and a single-atom regime is reached in the array, with a filling fraction of about 50%. I have demonstrated a single-atom detection fidelity of 99,986(1)% and a survival probability of 97(2)% between subsequent image acquisition, comparable to other current state-of-the-art experiments. To characterize the platform, I carried out release-and-recapture experiments, from which it is possible to measure a temperature for the individually trapped atoms of 12.95(5)μK. These results demonstrate the successful implementation of a robust and high-fidelity platform for optical trapping and imaging of single strontium atoms. The apparatus provides the foundation for future quantum simulation experiments with alkalineearth atoms excited to the Rydberg state, o"ering precise control over individual particles and the ability to scale towards larger, programmable atomic arrays.
Abstract We demonstrate programmable control over the spatial distribution of ultra-cold atoms confined in an optical lattice. The control is facilitated through a combination of spatial manipulation of the magneto-optical trap and atomic population shelving to a metastable state. We first employ the technique to load an extended (5 mm) atomic sample with uniform density in an optical lattice clock (OLC), reducing atomic interactions and realizing remarkable frequency homogeneity across the atomic cloud. We also prepare multiple spatially separated atomic ensembles, and realize multi-ensemble clock operation within the standard one-dimensional (1D) OLC architecture. Leveraging this technique, we prepare two oppositely spin-polarized ensembles that are independently addressable, offering a platform for implementing spectroscopic protocols for enhanced tracking of local oscillator phase. Finally, we demonstrate a relative fractional frequency instability at one second of 2.4 ( 1 ) × 10 − 17 between two ensembles, useful for characterization of intra-lattice differential systematics.
We present the experimental production and characterization of a dense cold atomic cloud of 88Sr^{88}\text{Sr}88Sr atoms, optimized for the future studies of light transport in highly dense regimes. Using narrow-line molasses on the 689 nm transition, combined with a far off-resonant optical dipole trap, we achieve spatial densities as high as 7.9×1013 atoms/cm37.9 \times 10^{13} \, \text{atoms/cm}^37.9×1013atoms/cm3 and optical depths up to 64. This approach stands out from previous methods by integrating narrow-line molasses with an optical dipole trap, enabling high-density samples without relying on evaporative cooling. Unlike traditional absorption imaging, which becomes inaccurate in such dense regimes, we demonstrate that phase-contrast imaging (PCI) can reliably reconstruct the in-situ density profile even for highly spatially and optically dense samples. The use of a spatial light modulator instead of a fixed phase plate in the PCI setup provides enhanced flexibility and control of imaging parameters, making this imaging technique robust against imaging artifacts and adaptable to varying experimental conditions. Moreover, we derive theoretical conditions for reliable PCI operation in dense regimes and validate these experimentally, showing excellent agreement with time-of-flight measurements even at the highest densities. Our results establish a robust method for producing and characterizing dense atomic clouds.
We report on the realization of a platform for trapping and manipulating individual 88Sr atoms in optical tweezers. A first cooling stage based on a blue shielded magneto-optical trap (MOT) operating on the |1S0⟩→|1P1⟩ transition at 461 nm enables us to trap approximately 4 × 106 atoms at a temperature of 6.8 mK. Further cooling is achieved in a narrow-line red MOT using the |1S0⟩→|3P1⟩ intercombination transition at 689 nm, bringing 5 × 105 atoms down to 5μK and reaching a density of 4 × 1010 cm3. Atoms are then loaded into 813 nm tweezer arrays generated by crossed acousto-optic deflectors and tightly focused onto the atoms with a high-numerical-aperture objective. Through light-assisted collision processes we achieve the collisional blockade, which leads to single-atom occupancy with a probability of about 50%. The trapped atoms are detected via fluorescence imaging with a fidelity of 99.986(6)%, while maintaining a survival probability of 97(2)%. The release-and-recapture measurement provides a temperature of 12.92(5)μK for the atoms in the tweezers, and the ultra-high-vacuum environment ensures a vacuum lifetime higher than 7 min. These results demonstrate a robust alkaline-earth tweezer platform that combines efficient loading, cooling, and high-fidelity detection, providing the essential building blocks for scalable quantum simulation and quantum information processing with Sr atoms.
We model the efficiency of loading atoms of various species into a one-dimensional optical lattice from a cold ensemble, taking into account the initial cloud temperature and size, the lattice laser properties affecting the trapping potential, and the atomic parameters. Stochastic sampling and dynamical evolution are used to simulate the transfer, leading to estimates of transfer efficiency for varying trap depth and profile. Tracing the motion of the atoms also enables the evaluation of the equilibrium temperature and site occupancy in the lattice. The simulation compares favorably against a number of experimental results and is used to compute an optimum lattice-waist-to-cloud-radius ratio for a given optical power.