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arXiv:2206.00144 · Physical Review A

High-fidelity low-loss state detection of alkali-metal atoms in optical tweezer traps

Matthew N. H. Chow, Bethany Little, Yuan‐Yu Jau

Published Sep 7, 2023• 18 citations
arXivHTMLPDF

Abstract

We demonstrate the discrimination of ground-state hyperfine manifolds of a cesium atom in an optical tweezer using a simple probe beam with $99.{91}_{\ensuremath{-}0.02}^{+0.02}$% detection fidelity and 0.9(2)% detection-driven loss of bright-state atoms. Our detection infidelity of $0.{09}_{\ensuremath{-}0.02}^{+0.02}$% is an order of magnitude better than previously published low-loss readout results for alkali-metal atoms in optical tweezers. We achieve these results by identifying and mitigating an extra depumping mechanism due to stimulated Raman transitions induced by trap light in the presence of probe light. In this work, complex optical systems and stringent vacuum pressures are not required, enabling straightforward adoption of our techniques on contemporary experiments.

DOI
10.1103/physreva.108.032407

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Physical Review A•Mar 1, 1996•88 citations
We demonstrate the discrimination of ground-state hyperfine manifolds of a cesium atom in an optical tweezer using a simple probe beam with 99.91\ensuremath−0.02+0.0299.{91}_{\ensuremath{-}0.02}^{+0.02}99.91\ensuremath−0.02+0.02​% detection fidelity and 0.9(2)% detection-driven loss of bright-state atoms. Our detection infidelity of 0.09\ensuremath−0.02+0.020.{09}_{\ensuremath{-}0.02}^{+0.02}0.09\ensuremath−0.02+0.02​% is an order of magnitude better than previously published low-loss readout results for alkali-metal atoms in optical tweezers. We achieve these results by identifying and mitigating an extra depumping mechanism due to stimulated Raman transitions induced by trap light in the presence of probe light. In this work, complex optical systems and stringent vacuum pressures are not required, enabling straightforward adoption of our techniques on contemporary experiments.
Characterizing the trap-light-induced frequency shifts of optical Zeeman transitions is essential for the precise control of the interacting dynamics between optically trapped single atoms and external light fields. The closed hyperfine transition, for example, 6 S 1/2 | F = 4〉 → 6 P 3/2 | F ′ = 5〉 of the cesium (Cs) atom, is commonly used because of its high transition strength. However, within this hyperfine transition line, different Zeeman transitions experience distinct light shifts due to diverse susceptibilities to the light field, which are usually sorted as dynamic scalar, vector, and tensor polarizabilities. Here, we report an efficient method for measuring the light shifts of optical Zeeman transitions using single neutral atoms confined in an optical tweezer (OT). The method employs a weak probe beam together with an auxiliary optical pumping beam to manipulate the atomic population in ground states. In this way, population changes in a specific Zeeman state are mapped onto a different ground-state hyperfine manifold, allowing the optical spectrum of the Zeeman transition to be reconstructed through standard state-detection techniques. By using the method, we demonstrate the spectrum measurement of the open Zeeman transition 6 S 1/2 | F = 4, m F = 4〉 → 6 P 3/2 | F ′ = 5, m F ′ = 3〉 for a single Cs atom trapped in a π polarized OT. We further perform a systematic characterization of the tweezer-induced light shifts for both this open transition and the closed Zeeman transition 6 S 1/2 | F = 4, m F = 4〉 → 6 P 3/2 | F ′ = 5, m F ′ = 5〉 over a wide range of trap depths. The influence of the tensor polarizability in deep trap depths is experimentally observed.
Recent advances in quantum simulation based on neutral atoms have largely benefited from high-resolution, single-atom sensitive imaging techniques. A variety of approaches have been developed to achieve such local detection of atoms in optical lattices or optical tweezers. For alkaline-earth and alkaline-earth-like atoms, the presence of narrow optical transitions opens up the possibility of performing novel types of Sisyphus cooling, where the cooling mechanism originates from the capability to spatially resolve the differential optical level shifts in the trap potential. Up to now, it has been an open question whether high-fidelity imaging could be achieved in a "repulsive Sisyphus" configuration, where the trap depth of the ground state exceeds that of the excited state involved in cooling. Here, we demonstrate high-fidelity (99.971(1)%) and high-survival (99.80(5)%) imaging of strontium atoms using repulsive Sisyphus cooling. We use an optical lattice as a pinning potential for atoms in a large-scale tweezer array with up to 399 tweezers and show repeated, high-fidelity lattice-tweezer-lattice transfers. We furthermore demonstrate loading the lattice with approximately 10 000 atoms directly from the MOT and scalable imaging over >10 000 lattice sites with a combined survival probability and classification fidelity better than 99.2%. Our lattice thus serves as a locally addressable and sortable reservoir for continuous refilling of optical tweezer arrays in the future.
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 demonstrate the lossless state-selective detection of a single rubidium 87 atom trapped in an optical tweezer. This detection is analogous to the one used on trapped ions. After preparation in either a dark or a bright state, we probe the atom internal state by sending laser light that couples an excited state to the bright state only. The laser-induced fluorescence is collected by a high numerical aperture lens. The single-shot fidelity of the detection is 98.6±0.2% and is presently limited by the dark count noise of the detector. The simplicity of this method opens new perspectives in view of applications to quantum manipulations of neutral atoms.
We have confined magneto-optically trapped cesium atoms to densities of nearly 1012{10}^{12}1012 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ by directing most of the population to a hyperfine level which does not interact with the trapping light. This compression represents almost an order of magnitude improvement on the best density we have been able to attain in a conventional magneto-optical trap. In previous work such improvements in density have only been reported for much lighter sodium. We discuss and demonstrate the difficulties in applying the technique to a heavy alkali-metal atom and the limits that are imposed on the best combination of density and temperature that can be produced. We present a simple model that accurately reproduces the density behavior, and justify its basic assumptions by Monte Carlo simulation. \textcopyright{} 1996 The American Physical Society.