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Monte Carlo simulation proposal for narrow-line magneto-optical traps

Bruno Santos

Published Oct 16, 2023• 0 citations
Publisher

Abstract

In this work, we proposed and implemented a Monte Carlo simulation to estimate experimental quantities of narrow-line magneto-optical traps (nMOTs).Our model relies on sampling the atoms' movement under laser light and a quadrupole magnetic field as a Markov chain.We assume that the involved electronic transition can be modelled as a four-level system and split into three independent two-level systems, which is only valid for nMOTs in the power-broadened regime.We were able to estimate quantities for three nMOT arrangements at different laboratories.The first nMOT traps dysprosium atoms on an electronic transition with narrowness η = 43.8,which is not an ideal value since it is slightly larger.Nevertheless, we could obtain estimated quantities that were more accurate than the theoretical predictions.The other two nMOTs trap strontium atoms on an electronic transition with narrowness η = 1.6.We could also obtain estimated quantities close to the experimental measures in this case.

DOI
10.11606/d.76.2023.tde-03012024-095738

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•Jan 1, 2014•48 citations
In this work, we proposed and implemented a Monte Carlo simulation to estimate experimental quantities of narrow-line magneto-optical traps (nMOTs).Our model relies on sampling the atoms' movement under laser light and a quadrupole magnetic field as a Markov chain.We assume that the involved electronic transition can be modelled as a four-level system and split into three independent two-level systems, which is only valid for nMOTs in the power-broadened regime.We were able to estimate quantities for three nMOT arrangements at different laboratories.The first nMOT traps dysprosium atoms on an electronic transition with narrowness η = 43.8,which is not an ideal value since it is slightly larger.Nevertheless, we could obtain estimated quantities that were more accurate than the theoretical predictions.The other two nMOTs trap strontium atoms on an electronic transition with narrowness η = 1.6.We could also obtain estimated quantities close to the experimental measures in this case.
We realize narrow-linewidth magneto-optical traps of 46^{46}46Ti, 48^{48}48Ti and 50^{50}50Ti atoms based on a 1040 nm-wavelength transition, cooling atoms to a minimum temperature in one dimension of Tz=990(20)T_z=990(20)Tz​=990(20) nK and a three-dimensional temperature of T3D=1.28(7)T_\mathrm{3D}=1.28(7)T3D​=1.28(7) μμμK. Atoms are pre-cooled in a broad-line magneto-optical trap and then transferred with about 25% efficiency to the narrow-line trap. We operate the narrow-line trap in two stages over 85 ms. First, a single cooling beam, blue-detuned from the narrow-linewidth atomic resonance, optically pumps and traps the atoms on a two-dimensional surface where the Zeeman shift from the applied spherical quadrupole magnetic field brings the light nearly to resonance. Second, four additional beams, counter-propagating in the transverse directions, cool and compress the atoms in all dimensions. The high magnetic moment of the laser cooling state makes the dynamics of the narrow-line titanium trap similar to those of other magnetic atoms. We measure the lifetime of the excited state of the transition to be τ=8.2(9)τ=8.2(9)τ=8.2(9) μμμs, indicating a transition linewidth of γ/2π=20(2)γ/2π=20(2)γ/2π=20(2) kHz, and also measure isotope shifts on the narrow-line transition. We use Stern-Gerlach separation on the ultracold Ti gas to measure the mJm_JmJ​-distribution in the narrow-line magneto-optical trap, finding over 98% of the atoms in the stretched spin state.
The theoretical description of the external degrees of freedom of atoms trapped inside a magneto-optical trap (MOT) often relies on the decoupling of the evolution of the internal and external degrees of freedom. That is possible thanks to much shorter timescales typically associated with the first ones. The electronic structure of alkaline-earth atoms, on the other hand, presents ultra-narrow transitions and metastable states that makes such an approximation invalid in the general case. In this article, we report on a model based on open Bloch equations for the evolution of the number of atoms in a magneto-optical trap. With this model we investigate the loading of the strontium blue magneto-optical trap under different repumping schemes, either directly from a Zeeman slower, or from an atomic reservoir made of atoms in a metastable state trapped in the magnetic quadrupolar field. The fluorescence observed on the strong 461~nm transition is recorded and quantitatively compared with the results from our simulations. The comparison between experimental results and calculations within our model allowed to identify the existence of the decay paths between the upper level of the repumping transition and the dark strontium metastable states, which could not be explained by electric dipole transition rates calculated in the literature. Moreover, our analysis pinpoints the role of the atomic movement in limiting the efficiency of the atomic repumping of the Sr metastable states.
We characterize the properties of a new form of magneto-optical trap (MOT) that relies on non-equilibrium population dynamics associated with narrow-linewidth optical transitions. We demonstrate this mechanism using the 7.5 kHz linewidth transition in both bosonic and fermionic strontium isotopes. In contrast to standard narrow-line MOTs based on imbalances in equilibrium scattering rates, our system benefits from a less complex laser system, a larger capture fraction from higher temperature samples, robustness against experimental perturbations and the ability to operate in the presence of large inhomogeneous broadening of the atomic transition frequency.
The theoretical description of the external degrees of freedom of atoms trapped inside a magnetooptical trap (MOT) often relies on the decoupling of the evolution of the internal and external degrees of freedom.That is possible thanks to much shorter timescales typically associated with the first ones.The electronic structure of alkaline-earth atoms, on the other hand, presents ultra-narrow transitions and metastable states that makes such an approximation invalid in the general case.In this article, we report on a model based on open Bloch equations for the evolution of the number of atoms in a magneto-optical trap.With this model we investigate the loading of the strontium blue magnetooptical trap under different repumping schemes, either directly from a Zeeman slower, or from an atomic reservoir made of atoms in a metastable state trapped in the magnetic quadrupolar field.The fluorescence observed on the strong 461 nm transition is recorded and quantitatively compared with the results from our simulations.The comparison between experimental results and calculations within our model allowed to identify the existence of the decay paths between the upper level of the repumping transition and the dark strontium metastable states, which could not be explained by electric dipole transition rates calculated in the literature.Moreover, our analysis pinpoints the role of the atomic movement in limiting the efficiency of the atomic repumping of the Sr metastable states.
We present our technique to create a magneto-optical trap (MOT) for dysprosium atoms using the narrow-line cooling transition at 626 nm to achieve suitable conditions for direct loading into an optical dipole trap. The MOT is loaded from an atomic beam via a Zeeman slower using the strongest atomic transition at 421 nm. With this combination of two cooling transitions we can trap up to 2.0·10(8) atoms at temperatures down to 6 μK. This cooling approach is simpler than present work with ultracold dysprosium and provides similar starting conditions for a transfer to an optical dipole trap.
We present our technique to create a magneto-optical trap for dysprosium atoms using the narrow-line cooling transition at 626 \,nm to achieve suitable conditions for direct loading into an optical dipole trap. The magneto-optical trap is loaded from an atomic beam via a Zeeman slower using the strongest atomic transition at 421 \,nm. With this combination of two cooling transitions we can trap up to 2.0⋅1082.0\cdot10^82.0⋅108 atoms at temperatures down to 6 μ\, \muμK. This cooling approach is simpler than present work with ultracold dysprosium and provides similar starting conditions for a transfer to an optical dipole trap.