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Physical Review A

Observation of sub-Doppler temperatures in bosonic magnesium

T. E. Mehlstäubler, Karsten Moldenhauer, Matthias Riedmann, N. Rehbein, J. Friebe, Ernst Maria Rasel, Wolfgang A. Ertmer

Published Feb 12, 2008• 25 citations
Publisher

Abstract

Temperatures below the Doppler limit of $1.9\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ have been observed in magnesium. The strong cooling transition was modified by a coherent two-color excitation exploiting the longer lifetime of an upper level. We developed a theoretical model to describe the light forces and cooling originating from the induced quantum interference in a three-level system. Time-of-flight measurements verified temperatures of $500\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ in a one-dimensional (1D) molasses in accordance with our theoretical model. By implementing this scheme in a 3D magneto-optical trap with a single ir beam, temperatures as low as $1\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ could be realized. For ideal conditions we extrapolate to temperatures of $50\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$. With cooling times of about $1\phantom{\rule{0.3em}{0ex}}\mathrm{ms}$, a fast and efficient cooling scheme was realized, particularly attractive for optical frequency standards.

DOI
10.1103/physreva.77.021402

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Temperatures below the Doppler limit of 1.9mK1.9\phantom{\rule{0.3em}{0ex}}\mathrm{mK}1.9mK have been observed in magnesium. The strong cooling transition was modified by a coherent two-color excitation exploiting the longer lifetime of an upper level. We developed a theoretical model to describe the light forces and cooling originating from the induced quantum interference in a three-level system. Time-of-flight measurements verified temperatures of 500\ensuremathμK500\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}500\ensuremathμK in a one-dimensional (1D) molasses in accordance with our theoretical model. By implementing this scheme in a 3D magneto-optical trap with a single ir beam, temperatures as low as 1mK1\phantom{\rule{0.3em}{0ex}}\mathrm{mK}1mK could be realized. For ideal conditions we extrapolate to temperatures of 50\ensuremathμK50\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}50\ensuremathμK. With cooling times of about 1ms1\phantom{\rule{0.3em}{0ex}}\mathrm{ms}1ms, a fast and efficient cooling scheme was realized, particularly attractive for optical frequency standards.
Standard sub-Doppler cooling methods are not applicable in bosonic magnesium due to the non-degenerate ground state. However, the Doppler limit can be surpassed by coupling the excited state of the cooling transition to the level1D2and cooling via a coherent two-photon transition. This process modifies the population in1P1, similar to electromagnetically induced transparency, as well as the resulting force profile in optical molasses. These modifications can be used to obtain a steeper slope in the force profile around v=0 and hence lower temperatures as compared to doppler cooling on the strong transition only.
We report on progress of generating ultracold atoms of magnesium. Magnesium belongs to the few elements suited for a neutral atom optical frequency standard. Its unique properties such as a very low black body radiation shift, the occurrence of fermionic and bosonic isotopes of suitable abundance, and the existence of a magic wavelength make it a very attractive candidate for future optical clocks. With our experiment, we determined for the first time the absolute frequency of the intercombination transition 31S0? 33P1of24Mg with a Ramsey-Bord Enterferometer, which was linked to a primary Cs Frequency standard of the PTB via a fiber femtosecond laser. Cooling ensembles of magnesium in the non-magnetic ground state to microkelvins is difficult as standard sub-Doppler cooling schemes are not applicable. We investigate novel methods to lower the temperatures of magnesium atoms. One promising method is based on a coherent two photon-process which provides higher velocity selectivity. Alternatively, standard sub-Doppler cooling schemes are applicable to the long-lived excited state, which in addition can be confined in magnetic traps. Article not available.
Sub-Doppler laser cooling in a magneto-optical trap for thulium atoms at a wavelength of 410.6 nm has been experimentally studied. Without any dedicated molasses period of sub-Doppler cooling, the cloud of 3 × 106 atoms at a temperature of 25(5) μK was observed. The measured temperature is significantly lower than the Doppler limit of 240 μK for the cooling transition at 410.6 nm. The high efficiency of the sub-Doppler cooling process is due to a near-degeneracy of the Landé g-factors of the lower 4f 136s 2 (J = 7/2) and the upper 4f 125d 3/26s 2 (J = 9/2) cooling levels.
Following the bichromatic sub-Doppler cooling scheme on the D1{D}_{1}D1​ line of 40{}^{40}40K recently demonstrated in Fernandes et al. [Europhys. Lett. 100, 63001 (2012)], we introduce a similar technique for 7{}^{7}7Li atoms and obtain temperatures of $60\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{K}$ while capturing all of the $5\ifmmode\times\else\texttimes\fi{}{10}^{8}$ atoms present from the previous stage. We investigate the influence of the detuning between the the two cooling frequencies and observe a threefold decrease of the temperature when the Raman condition is fulfilled. We interpret this effect as arising from extra cooling due to long-lived coherences between hyperfine states. Solving the optical Bloch equations for a simplified $\ensuremath{\Lambda}$-type three-level system we identify the presence of an efficient cooling force near the Raman condition. After transfer into a quadrupole magnetic trap, we measure a phase space density of $\ensuremath{\sim}$${10}^{\ensuremath{-}5}$. This laser cooling offers a promising route for fast evaporation of lithium atoms to quantum degeneracy in optical or magnetic traps.
Abstract We present a theoretical approach to achieve sub-Doppler cooling of magnesium fluoride (MgF) molecules by tuning the AC Stark shift with a blue-detuned laser. We study three blue-detuned magneto–optical trapping (MOT) schemes by using the Bayesian optimization method with the optical Bloch equations. We perform a comprehensive analysis of the relationship between the force in the MOT and the velocities and positions of the MgF molecules. Monte–Carlo simulations show that our MOT schemes can achieve a temperature as low as 28 μK and a density as high as 4.0 × 108 cm−3 at the conditions of a ratio of two laser intensities of 2:7, a detuning of 3Γ and a polarization configuration of σ −–σ +. These results provide a clear way for transferring a large number of MgF molecules into a conservative trap to enhance the subsequent cooling such as evaporative or sympathetic cooling.
Magnesium atoms are cooled in a magneto-optical trap (MOT) using the 3s2 S0 → 3s3p P1 resonance transition. Magnesium is a simple atom which offers a unique possibility for comparison to the Doppler theory of laser cooling. We measure trap parameters as a function of laser-intensity, -detuning and magnetic field gradient. We find the main features well accounted for by Doppler theory, but temperature measurements gave significant higher values than predicted by the Doppler theory. We also observe radiation pressure effects within the MOT that limit the maximum achievable density. Comparing our results to predictions by models in the literature shows a good agreement. Recently we have improved our set-up and now trap about 150 × 106 atoms.