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arXiv:2408.15359 · Physical Review Letters

Quantifying Light-Assisted Collisions in Optical Tweezers across the Hyperfine Spectrum

Steven K. Pampel, Matteo Marinelli, Mark O. Brown, José Paulo D'Incao, Cindy A. Regal

Published Jan 10, 2025• 6 citations
arXivHTMLPDF

Abstract

We investigate the role of hyperfine structure in resonant-dipole interactions between two atoms cotrapped in an optical tweezer. Two-body loss rates from light-assisted collisions (LACs) are measured across the ^{87}Rb hyperfine spectrum and connected to properties of molecular photoassociation potentials via a semiclassical model. To obtain our results, we introduce an imaging technique that leverages repulsive LACs to detect two atoms in a trap, thereby circumventing parity constraints in tweezers. Our findings offer key insights for exploiting hyperfine structure in laser-induced collisions to control cold atoms and molecules in a broad range of quantum science applications.

DOI
10.1103/physrevlett.134.013202

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Journal of Physics B Atomic Molecular and Optical Physics•Jan 14, 1995•24 citations
We investigate the role of hyperfine structure in resonant-dipole interactions between two atoms cotrapped in an optical tweezer. Two-body loss rates from light-assisted collisions (LACs) are measured across the ^{87}Rb hyperfine spectrum and connected to properties of molecular photoassociation potentials via a semiclassical model. To obtain our results, we introduce an imaging technique that leverages repulsive LACs to detect two atoms in a trap, thereby circumventing parity constraints in tweezers. Our findings offer key insights for exploiting hyperfine structure in laser-induced collisions to control cold atoms and molecules in a broad range of quantum science applications.
We study light-assisted collisions in an ensemble containing a small number ($\ensuremath{\sim}$3) of cold 87{}^{87}87Rb atoms trapped in a microscopic dipole trap. Using our ability to operate with one atom exactly in the trap, we measure the one-body heating rate associated with a near-resonant laser excitation, and we use this measurement to extract the two-body loss rate associated with light-assisted collisions when a few atoms are present in the trap. Our measurements indicate that the two-body loss rate can reach surprisingly large values $\ensuremath{\beta}>{10}^{\ensuremath{-}8}$ cm3{}^{3}3 s${}^{\ensuremath{-}1}$ and varies rapidly with the trap depth and the parameters of the excitation light.
We study light-assisted collisions in an ensemble containing a small number (~3) of cold Rb87 atoms trapped in a microscopic dipole trap. Using our ability to operate with one atom exactly in the trap, we measure the one-body heating rate associated to a near-resonant laser excitation, and we use this measurement to extract the two-body loss rate associated to light-assisted collisions when a few atoms are present in the trap. Our measurements indicate that the two-body loss rate can reach surprisingly large values beta>10^{-8} cm^{3}.s^{-1} and varies rapidly with the trap depth and the parameters of the excitation light.
Measurements of the temperature dependence of trap loss collisions of laser-cooled rubidium atoms are presented. A significant suppression of the rate of collisions between ground- and excited-state atoms is observed for temperatures below $\\sim$70 mu\\mumuK. In such inelastic collisions, the atomic excitation takes place at long range and must survive against spontaneous decay to short range. Our results are consistent with recent predictions for the low temperature suppression of these collisions due to reduced survival of atomic excitation at low collision velocities.^ We report on measurements of trap loss collisions between ultracold Rb atoms which are confined in a magneto-optical trap. Both isotopes sp85\\sp{85}sp85Rb and sp87\\sp{87}sp87Rb have been studied over wide range of trap laser intensities and detunings. The trap loss collision rates exhibit variations over 3 orders of magnitude. We observe significant differences between the isotopes. At large detunings the ability of the trap to recapture products of the hyperfine-changing collisions is significantly diminished. This finding is supported by our numerical simulations of the recapture process. The trap loss rates due to collisions with room temperature background gas have also been measured.^ Using laser light tuned to a repulsive molecular potential, we have been able to suppress inelastic ground-state hyperfine changing collisions between ultracold sp87\\sp{87}sp87Rb atoms. Adiabatic excitation to the repulsive curve alters the atomic trajectories and prevents the atoms from approaching close enough for the hyperfine change to occur. Experimental results show suppressions up to $\\sim$50% and are in reasonable agreement with a simple Landau-Zener model.^ Finally, we have observed a significant cooperative effect in ultracold trap loss collisions induced by two separate lasers. One laser, tuned close to the atomic resonance, excites the atom pair to an attractive potential at long range. The resulting acceleration and deflection give rise to an enhancement in the collisional flux of ground-state atoms reaching short range, as probed by a second laser. Enhancements up to a factor of $\\sim$3 have been observed. ^
We present a new approach to investigating Rydberg molecules by demonstrating the formation and characterization of individual Rb^{*}Cs Rydberg molecules using optical tweezers. By employing single-atom detection of Rb and Cs, we observe molecule formation via correlated loss of both species and study the formation dynamics with single-particle resolution. We control the interatomic distances by manipulating the relative wave function of atom pairs using the tweezer intensity, optimizing the coupling to molecular states and exploring the effect of the tweezer on these states. Additionally, we demonstrate molecule association with atoms trapped in separate tweezers, paving the way for state-selective assembly of polyatomic molecules. The observed binding energies, molecular alignment, and bond lengths are in good agreement with theory. Our approach is broadly applicable to Rydberg tweezer platforms, expanding the range of available molecular systems and enabling the integration of Rydberg molecules into existing quantum science platforms.
Chemical reactions typically proceed via stochastic encounters between reactants. Going beyond this paradigm, we combined exactly two atoms in a single, controlled reaction. The experimental apparatus traps two individual laser-cooled atoms [one sodium (Na) and one cesium (Cs)] in separate optical tweezers and then merges them into one optical dipole trap. Subsequently, photoassociation forms an excited-state NaCs molecule. The discovery of previously unseen resonances near the molecular dissociation threshold and measurement of collision rates are enabled by the tightly trapped ultracold sample of atoms. As laser-cooling and trapping capabilities are extended to more elements, the technique will enable the study of more diverse, and eventually more complex, molecules in an isolated environment, as well as synthesis of designer molecules for qubits.
Inelastic atomic collisions constitute loss mechanisms for neutral atom traps, and the study of trap-loss rates may provide detailed information on the long-range interaction between atoms and on the dynamics of very slow collisions. Gallagher and Pritchard (1989) have given a simple two-state model for such collisions which includes a radiative transition or a single curve crossing at short range representing, for example, fine-structure-changing collisions. In order to better describe the more complicated case found in alkali metals, we have extended this model to include several excited hyperfine structure levels with curve crossings at long range. We find that collisions in a laser field that are capable of expelling atoms from the trap can be suppressed in some laser frequency regions and enhanced in others, in comparison with the predictions of the simple two-state model. Using this model to represent radiative escape via the 2 u potential of alkali dimer molecules we are able to greatly improve agreement between theory and experiment, including new measurements on 85 Rb reported here.