Imaging a $^{6}\mathrm{Li}$ Atom in an Optical Tweezer 2000 Times with $\mathrm{\ensuremath{\Lambda}}$-Enhanced Gray Molasses

Imaging a ${}^{6}{Li}$ Atom in an Optical Tweezer 2000 Times with $Λ$ -Enhanced Gray Molasses

Karl N. Blodgett, David Peana, Saumitra S. Phatak, Lane M. Terry, Maria Paula Montes, and Jonathan D. Hood

Phys. Rev. Lett. 131, 083001 – Published 21 August 2023

Abstract

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 $3.5 μ K$ , 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 $70 μ K$ 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.

Received 4 May 2023
Accepted 31 July 2023

DOI:https://doi.org/10.1103/PhysRevLett.131.083001

Physics Subject Headings (PhySH)

Cooling & trapping Light-matter interaction Optical pumping Spontaneous emission Stark effect

Atoms

Rotating wave approximation

Atomic, Molecular & Optical

Authors & Affiliations

Karl N. Blodgett ¹, David Peana¹, Saumitra S. Phatak ², Lane M. Terry ¹, Maria Paula Montes², and Jonathan D. Hood ^1,2,*

¹Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA
²Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, USA

^*hoodjd@purdue.edu

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Vol. 131, Iss. 8 — 25 August 2023

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Images

Figure 1
Low-loss $Λ$ -GM imaging of ${}^{6}{Li}$ . (a) Atom survival in an optical tweezer with (blue) and without (orange) active cooling light vs consecutive images. The data with continuous GM cooling have a $1 / e$ decay time of 30.0(8) s, corresponding to 2000 images. The atom without active cooling light applied is heated due to off-resonant scattering of the tweezer light and has a $1 / e$ time of 5.13(7) s. The inset is the D1 level structure with $Λ$ -GM parameters labeled. (b) Histogram of fluorescence from a single ${}^{6}{Li}$ atom trapped in a 1064 nm optical tweezer. This histogram represents an averaged image (3000 shots) of a single region of interest with a signal threshold of 65 photon counts and a loading rate of 0.68(3). Fitting a Gaussian distribution to both peaks and integrating the 0 atom (1 atom) fraction above (below) the signal threshold gives an imaging fidelity of 0.999 7(2). The insets show the cropped region of interest of an image of 0 (left) or 1 (right) atom. (c) Single-shot image of five occupied adjacent tweezer sites. (d) Simplified picture of $Λ$ -GM in a tweezer. A dark state $| D, n ⟩$ couples to the bright state $| B, n - 1 ⟩$ with one less motional quanta. Spontaneous emission then pumps the population to the dark state $| D, n - 1 ⟩$ when in the Lamb-Dicke regime.
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Figure 2
Single-atom imaging. 1064 nm tweezer light (light red) is focused down to $\sim 1 μ m$ at the center of the double-sided glass octagon cell. Three retroreflected laser beams containing $Λ$ -GM light (dark red) are sent to the tweezer-trapped atom(s). Fluorescent photons are collected back through the same objective and imaged onto an EMCCD camera. MOT and mini-MOT coils are shown; shim coils are omitted for clarity.
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Figure 3
Properties of ${}^{6}{Li}$ in a tweezer. (a) Atom loading vs trap depth (mK). For imaging, trap depth is ramped to a constant value of 3.4 mK. (b) Survival vs trap depth after 533 consecutive image cycles. (c) Photons collected vs trap depth during 15 ms image. In (b) and (c), $Λ$ -GM parameters are not changed as a function of trap depth. (d) Release-and-recapture survival probability under different cooling scenarios. Light blue: optimized cooling for low temperature. Dark blue: optimized cooling for survival. Green: hold in tweezer for 1 s (no cooling light applied). Solid lines are Monte Carlo simulations used to extract a temperature.
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Figure 4
Simulation of $Λ$ -GM in a tweezer. (a) $Λ$ -GM in a harmonic potential. In step 1, spontaneous emission pumps the bright state $| B, n ⟩$ to the dark state $| D, n ⟩$ with the same motional level. In step 2, the dark state is transferred to the bright state with a net loss of one motional quanta due to the three-level system formed by the $Ω$ and $η_{LD} Ω$ terms. (b) Simulation of steady-state temperature as a function of cooling power and trapping frequency for $δ_{1} = δ_{2} = 20 MHz$ . The dashed line is where the bright state shift equals the trapping frequency. (c) Temperature versus the cooling and repumper detunings. $Λ$ -GM is seen on the diagonal where $Δ = 0 MHz$ . Normal GM is seen away from that condition. (d) Plot of population of steady state in the excited state. The experimental optimum is 0.006 excited, giving a scattering rate consistent with experiment.
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Physical Review Letters

Imaging a Li6 Atom in an Optical Tweezer 2000 Times with Λ-Enhanced Gray Molasses

Karl N. Blodgett, David Peana, Saumitra S. Phatak, Lane M. Terry, Maria Paula Montes, and Jonathan D. Hood

Phys. Rev. Lett. 131, 083001 – Published 21 August 2023

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Imaging a ${}^{6}{Li}$ Atom in an Optical Tweezer 2000 Times with $Λ$ -Enhanced Gray Molasses