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Cooling Lithium and Cesium Single Atoms in Optical Tweezers

Saumitra S. Phatak

Published Jul 8, 2026• 0 citations
Publisher

Abstract

This dissertation advances optical tweezer platforms for neutral-atom quantum processors by addressing three interconnected bottlenecks: imaging survival, motional ground-state preparation, and narrow-line cooling, through experimental and theoretical work on single lithium and cesium atoms.For Li, the lightest stable alkali atom, this work demonstrates a per-image survival probability of 99.950(2)% and a state-detection fidelity of 99.97(2)% across 2000 consecutive 15 ms images, a record for any trapped neutral atom at the time of publication. This was achieved by applying Λ-enhanced gray-molasses (Λ-GM) cooling continuously on the D1 line while simultaneously imaging with the same beam, and represents the first demonstration of loading lithium into a tweezer directly from a MOT. To explain this performance, the dissertation develops a unified Lindblad master-equation formalism for laser cooling in a tightly confined harmonic trap, encompassing polarization-gradient, gray-molasses, EIT, Λ-GM, and single-photon sideband cooling within a single theoretical framework, with 1D simulations that correctly predict steady-state temperature, excited-state fraction, and cooling efficiency across these techniques.For133133Cs, the work exploits the forbidden electric-quadrupole transition 6S1/2→5D5/26S1/2​→5D5/2​ at 685 nm (Γ ≈ 2π × 117.6 kHz) to demonstrate background-free imaging and narrow-line sideband cooling for the first time in an alkali atom. By positioning the atom at the intensity node of a Laguerre-Gaussian vortex beam, the drive is enhanced while cascade fluorescence at 852 nm is optically isolated from the excitation light, enabling both high-fidelity background-free detection and single-photon sideband cooling to near the motional ground state.Beyond the core cooling and imaging results, the dissertation includes a comprehensive experimental techniques chapter (fiber coupling, AOM design, polarimetry, laser locking, optical pumping, and a Gaussian-process optimization framework), a chapter on quadrupole transition theory and orbital-angular-momentum selection rules, and a concluding survey of the competitive quantum computing landscape across neutral-atom, superconducting, trapped-ion, photonic, spin, and topological qubit platforms. Together, this work establishes the Cs and Li cooling/imaging toolbox as a foundation for future assembly of ultracold LiCs polar molecules in optical tweezer arrays.

DOI
10.25394/pgs.32867018

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This dissertation advances optical tweezer platforms for neutral-atom quantum processors by addressing three interconnected bottlenecks: imaging survival, motional ground-state preparation, and narrow-line cooling, through experimental and theoretical work on single lithium and cesium atoms.For Li, the lightest stable alkali atom, this work demonstrates a per-image survival probability of 99.950(2)% and a state-detection fidelity of 99.97(2)% across 2000 consecutive 15 ms images, a record for any trapped neutral atom at the time of publication. This was achieved by applying Λ-enhanced gray-molasses (Λ-GM) cooling continuously on the D1 line while simultaneously imaging with the same beam, and represents the first demonstration of loading lithium into a tweezer directly from a MOT. To explain this performance, the dissertation develops a unified Lindblad master-equation formalism for laser cooling in a tightly confined harmonic trap, encompassing polarization-gradient, gray-molasses, EIT, Λ-GM, and single-photon sideband cooling within a single theoretical framework, with 1D simulations that correctly predict steady-state temperature, excited-state fraction, and cooling efficiency across these techniques.For133133Cs, the work exploits the forbidden electric-quadrupole transition 6S1/2→5D5/26S1/2​→5D5/2​ at 685 nm (Γ ≈ 2π × 117.6 kHz) to demonstrate background-free imaging and narrow-line sideband cooling for the first time in an alkali atom. By positioning the atom at the intensity node of a Laguerre-Gaussian vortex beam, the drive is enhanced while cascade fluorescence at 852 nm is optically isolated from the excitation light, enabling both high-fidelity background-free detection and single-photon sideband cooling to near the motional ground state.Beyond the core cooling and imaging results, the dissertation includes a comprehensive experimental techniques chapter (fiber coupling, AOM design, polarimetry, laser locking, optical pumping, and a Gaussian-process optimization framework), a chapter on quadrupole transition theory and orbital-angular-momentum selection rules, and a concluding survey of the competitive quantum computing landscape across neutral-atom, superconducting, trapped-ion, photonic, spin, and topological qubit platforms. Together, this work establishes the Cs and Li cooling/imaging toolbox as a foundation for future assembly of ultracold LiCs polar molecules in optical tweezer arrays.
We have imaged lithium-6 thousands of times in an optical tweezer using Λ-enhanced gray molasses cooling light. Despite being the lightest alkali metal, with a recoil temperature of 3.5 μK, we achieve an imaging survival of 0.999 50(2), which sets the new benchmark for low-loss imaging of neutral atoms in optical tweezers. Lithium is loaded directly from a magneto-optical trap into a tweezer with an enhanced loading rate of 0.7. We cool the atom to 70 μK and present a new cooling model that accurately predicts steady-state temperature and scattering rate in the tweezer. These results pave the way for ground state preparation of lithium en route to the assembly of the LiCs molecule in its ground state.
We demonstrate background-free imaging and sideband cooling of a single 133Cs^{133}\mathrm{Cs}133Cs atom via the narrow-line $6{S}_{1/2}\ensuremath{\rightarrow}5{D}_{5/2}$ electric quadrupole transition in a 1064 nm optical tweezer. The 5D5/25{D}_{5/2}5D5/2​ state decays through the 6P3/26{P}_{3/2}6P3/2​ state to the ground state, emitting an 852 nm wavelength photon that allows for background-free imaging. By encoding both spin and orbital angular momentum onto the 685 nm excitation light, we achieve background-free fluorescence histograms with 99.58(3)% imaging fidelity by positioning the atom at the dark center of a vortex beam. Tuning the tweezer polarization ellipticity realizes a magic trap for the stretched $|F=4,{m}_{F}=4\ensuremath{\rangle}\ensuremath{\rightarrow}|{F}^{\ensuremath{'}}=6,{m}_{F}^{\ensuremath{'}}=6\ensuremath{\rangle}$ cycling transition. Using a Gaussian beam, we cool to $5\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{K}$ in a 1.1 mK trap and outline a strategy for ground-state cooling. We compare cooling performance across different sideband regimes, while also exploring how the orbital angular momentum of structured light controls the selection rules for quadrupole transitions. These results expand the toolbox for high-fidelity quantum control and cooling in alkali-atom tweezer arrays.
Cooling atoms to the ground-state of optical tweezers is becoming increasingly important for high-fidelity imaging, cooling, and molecular assembly. While extensive theoretical work has been conducted on cooling in free space, fewer studies have focused on cooling in bound states. In this work, we present a unified formalism for optical cooling mechanisms in neutral atom tweezers, including resolved and unresolved sideband cooling with different trapping potentials, polarization gradient cooling, gray molasses cooling, ΛΛΛ-enhanced gray molasses cooling, and Raman sideband cooling. We perform simulations and demonstrate good agreement with a simplified spin model. We derive and discuss the fundamental limits of each cooling mechanism and propose new strategies for achieving ground-state cooling in optical tweezers. Our findings provide valuable insights into optimizing cooling schemes for neutral atoms in optical tweezers, paving the way for minimizing thermal decoherence in Rydberg and molecular gates and improving efficiencies of molecular assembly.
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.
Laser cooling of single atoms in optical tweezers is a prerequisite for neutral atom quantum computing and simulation. Resolved sideband cooling comprises a well-established method for efficient motional ground-state preparation, but typically requires careful cancellation of light shifts in so-called magic traps. Here, we study a novel laser cooling scheme which overcomes such constraints, and applies when the ground state of a narrow cooling transition is trapped stronger than the excited state. We demonstrate our scheme, which exploits sequential addressing of red sideband transitions via frequency chirping of the cooling light, at the example of 88Sr^{88}\mathrm{Sr}88Sr atoms and report ground-state populations compatible with recent experiments in magic tweezers. The scheme also induces light-assisted collisions, which are key to the assembly of large atom arrays. Our work enriches the toolbox for tweezer-based quantum technology, also enabling applications for tweezer-trapped molecules and ions that are incompatible with resolved sideband cooling conditions.
Engineering controllable, strongly interacting many-body quantum systems is at the frontier of quantum simulation and quantum information processing. Arrays of laser-cooled neutral atoms in optical tweezers have emerged as a promising platform because of their flexibility and the potential for strong interactions via Rydberg states. Existing neutral atom array experiments utilize alkali atoms, but alkaline-earth atoms offer many advantages in terms of coherence and control, and also open the door to new applications in precision measurement and time keeping. In this Letter, we present a technique to trap individual alkaline-earth-like ytterbium (Yb) atoms in optical tweezer arrays. The narrow ^{1}S_{0}-^{3}P_{1} intercombination line is used for both cooling and imaging in a magic-wavelength optical tweezer at 532 nm. The low Doppler temperature allows for imaging near the saturation intensity, resulting in a very high atom detection fidelity. We demonstrate the imaging fidelity concretely by observing rare (<1 in 10^{4} images) spontaneous quantum jumps into and out of a metastable state. We also demonstrate stochastic loading of atoms into a two-dimensional, 144-site tweezer array. This platform will enable advances in quantum information processing, quantum simulation, and precision measurement. The demonstrated narrow-line Doppler imaging may also be applied in tweezer arrays or quantum gas microscopes using other atoms with similar transitions, such as erbium and dysprosium.