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arXiv:2001.07770 · Physical Review A

Sawtooth-wave adiabatic passage in a magneto-optical trap

John P. Bartolotta, Murray Holland

Published May 27, 2020• 10 citations
arXivHTMLPDF

Abstract

We investigate theoretically the application of sawtooth-wave adiabatic passage (SWAP) in a one-dimensional (1D) magneto-optical trap (MOT). As opposed to related methods that have been previously discussed, our approach utilizes repeated cycles of stimulated absorption and emission processes to achieve both trapping and cooling, thereby reducing the adverse effects that arise from photon scattering. Specifically, we demonstrate this method's ability to cool, slow, and trap particles with fewer spontaneously emitted photons, with higher forces, and in less time when compared to a traditional MOT scheme that utilizes the same narrow linewidth transition. We calculate the phase-space compression that is achievable and characterize the resulting system equilibrium cloud size and temperature.

DOI
10.1103/physreva.101.053434

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We investigate theoretically the application of sawtooth-wave adiabatic passage (SWAP) in a one-dimensional (1D) magneto-optical trap (MOT). As opposed to related methods that have been previously discussed, our approach utilizes repeated cycles of stimulated absorption and emission processes to achieve both trapping and cooling, thereby reducing the adverse effects that arise from photon scattering. Specifically, we demonstrate this method's ability to cool, slow, and trap particles with fewer spontaneously emitted photons, with higher forces, and in less time when compared to a traditional MOT scheme that utilizes the same narrow linewidth transition. We calculate the phase-space compression that is achievable and characterize the resulting system equilibrium cloud size and temperature.
We demonstrate sawtooth wave adiabatic passage (SWAP) in a grating magneto-optical trap (MOT) operating on the 1^11S0_00​ →\rightarrow→ 3^33P1_11​ transition of neutral 88^{88}88Sr. From numerical simulations of SWAP using our laser beam geometry, we find that SWAP provides greater cooling than triangle wave frequency modulation despite the complex polarization environment of a grating MOT. The simulation is confirmed by our experimental results, where we demonstrate a factor of two improvement in transfer efficiency between our 1^11S0_00​ →\rightarrow→ 1^11P1_11​ grating MOT and our 1^11S0_00​ →\rightarrow→ 3^33P1_11​ grating MOT. We trap up to 3×1063\times10^63×106 88^{88}88Sr atoms in the 1^11S0_00​ →\rightarrow→ 3^33P1_11​ grating MOT, at an average temperature of 4.9 μμμK with a lifetime of approximately 0.7 s. Our results show that SWAP is effective in non-orthogonal laser beam geometries, allowing greater duty cycles or higher atom number in sensors based on narrow-line grating MOTs.
Sawtooth Wave Adiabatic Passage (SWAP) laser cooling was recently demonstrated using a narrow-linewidth single-photon optical transition in atomic strontium and may prove useful for cooling other atoms and molecules. However, many atoms and molecules lack the appropriate narrow optical transition. Here we use such an atom, 87Rb, to demonstrate that two-photon Raman transitions with arbitrarily-tunable linewidths can be used to achieve 1D SWAP cooling without significantly populating the intermediate excited state. Unlike SWAP cooling on a narrow transition, Raman SWAP cooling allows for a final 1D temperature well below the Doppler cooling limit (here, 25 times lower); and the effective excited state decay rate can be modified in time, presenting another degree of freedom during the cooling process. We also develop a generic model for Raman Landau-Zener transitions in the presence of small residual free-space scattering for future applications of SWAP cooling in other atoms or molecules.
We improve the efficiency of sawtooth-wave adiabatic-passage cooling for strontium atoms in three dimensions and combine it with standard narrow-line laser cooling. With this technique, we create strontium magneto-optical traps with $6\ifmmode\times\else\texttimes\fi{}{10}^{7}$ bosonic 88Sr^{88}\mathrm{Sr}88Sr ($1\ifmmode\times\else\texttimes\fi{}{10}^{7}$ fermionic 87Sr^{87}\mathrm{Sr}87Sr) atoms at phase-space densities of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ ($1.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$). Our method is simple to implement and is faster and more robust than traditional cooling methods.
The results of simulations of cooling based on sawtooth-wave adiabatic passage (SWAP) are presented, including the possibility of population leaking to states outside of the cycling transition. The amount of population leaking can be substantially suppressed compared to Doppler cooling, which could be useful for systems that are difficult to repump back to the cycling transition. The suppression of the leaked population was more effective when simulating the slowing of a beam than in cooling a thermal distribution. As expected, calculations of the leaked population versus branching ratio of spontaneous emission show that the suppression is more effective for narrow linewidth transitions. In this limit, using SWAP to slow a beam may be worth pursuing even when the branching ratio out of the cycling transition is greater than 10%.
Abstract Sawtooth Wave Adiabatic Passage (SWAP) laser cooling was recently demonstrated using a narrow-linewidth single-photon optical transition in atomic strontium and may prove useful for cooling other atoms and molecules. However, many atoms and molecules lack the appropriate narrow optical transition. Here we use such an atom, 87Rb, to demonstrate that two-photon Raman transitions with arbitrarily-tunable linewidths can be used to achieve 1D SWAP cooling without significantly populating the intermediate excited state. Unlike SWAP cooling on a narrow transition, Raman SWAP cooling allows for a final 1D temperature well below the Doppler cooling limit (here, 25 times lower); and the effective excited state decay rate can be modified in time, presenting another degree of freedom during the cooling process. We also develop a generic model for Raman Landau–Zener transitions in the presence of small residual free-space scattering for future applications of SWAP cooling in other atoms or molecules.
We provide a theoretical analysis for a recently demonstrated cooling method. Two-level particles undergo successive adiabatic transfers upon interaction with counterpropagating laser beams that are repeatedly swept over the transition frequency. We show that particles with narrow linewidth transitions can be cooled to near the recoil limit. This cooling mechanism has a reduced reliance on spontaneous emission compared to Doppler cooling, and hence shows promise for application to systems lacking closed cycling transitions, such as molecules.