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arXiv:2602.21306

Optical repumping and atom number balancing in a two-color MOT

Shubha Deutschle, Lőrinc Sárkány, Milán János Negyedi, József Fortágh, Andreas Günther, Philippe Wilhelm Courteille

quant-phphysics.atom-phSubmitted Feb 24, 2026• Announced Feb 26, 2026• 0 citations
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Abstract

We study a novel repumping transition for $^{88}$Sr atoms trapped in a 'blue' magneto-optical trap. We show that, while the repumping efficiency is about three orders of magnitude smaller than for traditional schemes, it is sufficient for recycling all atoms, provided the repumping laser beams are arranged to form a 'green' magneto- optical trap (MOT) helping to cool and confine the atoms and preventing their loss. Our main findings are: (i) that the green MOT configuration is able to trap 10 times more atoms in the blue MOT than using the green transition merely as a repump, and (ii) that the atom numbers in the two-color MOT can be balanced through experimental control parameters. The interest of this scheme lies in its capability of reaching low temperature and its suitability for continuous atomic beam generation.

Comments
13 pages, 7 figures

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We study a novel repumping transition for 88^{88}88Sr atoms trapped in a 'blue' magneto-optical trap. We show that, while the repumping efficiency is about three orders of magnitude smaller than for traditional schemes, it is sufficient for recycling all atoms, provided the repumping laser beams are arranged to form a 'green' magneto- optical trap (MOT) helping to cool and confine the atoms and preventing their loss. Our main findings are: (i) that the green MOT configuration is able to trap 10 times more atoms in the blue MOT than using the green transition merely as a repump, and (ii) that the atom numbers in the two-color MOT can be balanced through experimental control parameters. The interest of this scheme lies in its capability of reaching low temperature and its suitability for continuous atomic beam generation.
We demonstrate a magneto-optical trap (MOT) with counter-propagating two-color cooling beams in a cesium 6S1/2 6P3/2 8S1/2 (852.3+794.6 nm) atomic system. Based on the conventional MOT due entirely to the 852.3 nm cooling laser’s scattering forces, we replace one of the six 852.3 nm cooling beams with a 794.6 nm cooling beam. Our two-color MOT can efficiently cool and trap atoms from the red to blue detuning sides of two-photon resonance without pre-cooling. The technique is promising for the direct generation of correlated photon pairs in a two-color MOT based on diamond-configuration four-wave mixing.
We demonstrate a novel dual color magneto—optical trap (MOT), which uses two sets of overlapping laser beams to cool and trap 87 Rb atoms. The volume of cold cloud in the dual color MOT is strongly dependent on the frequency difference of the laser beams and can be significantly larger than that in the normal MOT with single frequency MOT beams. Our experiment shows that the dual color MOT has the same loading rate as the normal MOT, but much longer loading time, leading to threefold increase in the number of trapped atoms. This indicates that the larger number is caused by reduced light induced loss. The dual color MOT is very useful in experiments where both high vacuum level and large atom number are required, such as single chamber quantum memory and Bose—Einstein condensation (BEC) experiments. Compared to the popular dark spontaneous-force optical trap (dark SPOT) technique, our approach is technically simpler and more suitable to low power laser systems.
We demonstrate a novel dual color magneto–optical trap (MOT), which uses two sets of overlapping laser beams to cool and trap 87 Rb atoms. The volume of cold cloud in the dual color MOT is strongly dependent on the frequency difference of the laser beams and can be significantly larger than that in the normal MOT with single frequency MOT beams. Our experiment shows that the dual color MOT has the same loading rate as the normal MOT, but much longer loading time, leading to threefold increase in the number of trapped atoms. This indicates that the larger number is caused by reduced light induced loss. The dual color MOT is very useful in experiments where both high vacuum level and large atom number are required, such as single chamber quantum memory and Bose–Einstein condensation (BEC) experiments. Compared to the popular dark spontaneous-force optical trap (dark SPOT) technique, our approach is technically simpler and more suitable to low power laser systems.
Abstract A magneto-optical trap (MOT) at narrow transition offers lower temperature and hence is the key for producing a high phase density atomic cloud and subsequently quantum degeneracy with a large number of atoms for many elements. In this paper, we describe the loading of 87Rb atoms in the MOT using a narrow open transition (5S 1 / 2 → 6P 3 / 2 ) at 420 nm (i.e. blue MOT) from the routinely implemented MOT using a broad cyclic transition (5S 1 / 2 → 5P 3 / 2 ) at 780 nm (i.e. IR MOT). Using the four times narrower transition, we have trapped around 108 atoms in the MOT with a typical temperature of around 54 μ K. We have also studied the behavior of the blue MOT with various parameters such as hold time, detuning, and power of trapping and repumper beams.
We explore the efficacy of two single-repumping schemes, 5 s 5 p P 2 3 − 5 p 2 P 2 3 ( 481 nm ) and 5 s 5 p P 2 3 − 5 s 5 d D 2 3 ( 497 nm ) , for a magneto-optical trap (MOT) of Sr atoms. We reveal that the enhancement in the MOT lifetime is limited to 26.9(2) for any single-repumping scheme. Our investigation indicates that the primary decay path from the 5 s 5 p P 1 1 state to the 5 s 5 p P 0 3 state proceeds via the 5 s 4 d D 1 3 state, rather than through the upper states accessed by the single-repumping lasers. We estimate that the branching ratio for the 5 s 5 p P 1 1 → 5 s 4 d D 1 3 → 5 s 5 p P 0 3 decay path is 1 : 3.9 ( 1.5 ) × 10 6 and the decay rate for the transition from the 5 s 5 p P 1 1 state to the 5 s 4 d
We demonstrate a magneto-optical trap(MOT) with counter-propagating two-color cooling b(’ams in a cesium6S1/2 — 6P3/2 — 8S!/2(852.3 + 794.6 nm) atomic system. Based on the conventional MOT due entirely to the852.3 nm cooling laser5 s scattering forces, we replace one of the six 852.3 inn cooling beams witli a 794.6 11111 cooling beam. Our two-color MOT can efficiently cool and trap atoms from the red to blue detuning sides of two-photon resonance without pre-cooling. The technique is promising for the direct generation of correlated photon pairs in a two-color MOT based on diamond-configuration four-wave mixing.