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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

MOT loading enhancement with stimulated light forces

Tara Cubel Liebisch, E. Blanshan, Elizabeth A. Donley, John E. Kitching

Published May 1, 2011• 0 citations
Publisher

Abstract

We demonstrate atom number enhancement in a magneto-optical trap (MOT) by use of bichromatic cooling to slow an atomic beam that is loaded into a MOT. Bichromatic cooling employs stimulated emission to apply strong cooling forces that are not limited by spontaneous emission. We demonstrate a factor of 3.5 increase in atom number captured from the atomic beam for a 1.5 cm cooling length. For a 1.5 cm effective cooling length, our technique yields MOT atom number enhancement that is about three times higher than the enhancement achieved via spontaneous emission.

DOI
10.1109/fcs.2011.5977757

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We demonstrate atom number enhancement in a magneto-optical trap (MOT) by use of bichromatic cooling to slow an atomic beam that is loaded into a MOT. Bichromatic cooling employs stimulated emission to apply strong cooling forces that are not limited by spontaneous emission. We demonstrate a factor of 3.5 increase in atom number captured from the atomic beam for a 1.5 cm cooling length. For a 1.5 cm effective cooling length, our technique yields MOT atom number enhancement that is about three times higher than the enhancement achieved via spontaneous emission.
We decelerated an atomic beam of 87{}^{87}87Rb using a stimulated-emission slowing technique that employs a bichromatic standing light wave of high intensity, and increased the atom number in a magneto-optical trap (MOT) of low capture velocity by up to a factor of 14, and the load rate into the MOT by a factor of 20. We performed the slowing over distances under 1.5 cm with 5.2 mW of total power in the bichromatic beams. The average stimulated force was 2.2 times the maximum spontaneous emission force, and could be made even stronger in a smaller apparatus.
For cold samples of laser-cooled atoms to be useful in emerging technologies such as compact atomic clocks and sensors, it is necessary to achieve small sample sizes while retaining a large number of cold atoms. Achieving large atom numbers in a small system is a major challenge for producing miniaturized laser-cooled atomic clocks, since the number of captured atoms in a vapor-cell magneto-optical trap (MOT) scales as the fourth power of the laser beam diameter. This strong dependence on size is fundamentally set by the maximum spontaneous light force ħkγ/2, where ħk is the photon momentum and γ/2 is the maximum spontaneous photon scatter rate of a saturated transition of linewidth γ. We are attempting to surmount the limit imposed by spontaneous emission by using bichromatic cooling - a technique that uses stimulated emission to slow the atoms. We have built a table-top experiment that uses stimulated-emission bichromatic cooling to pre-cool rubidium atoms and dramatically enhance the trappable atom number in a small MOT. The apparatus lets us test how bichromatic cooling scales with miniaturization. Here we report on our first experimental results of cooling a thermal beam of rubidium atoms down to MOT capture velocities.
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.
We demonstrate enhancement in the number of trapped cesium atoms in a magneto-optical trap (MOT) using a control laser that illuminates only a small faction of the capture region of the trap without interacting with the cold cloud of atoms. The enhancement is maximized when the laser is slightly blue detuned with respect to the cooling transition. Trap loading curves point to approximately a twofold increase in the capture rate, which as a consequence results in the increase in the steady state number of trapped atoms. Enhanced loading is confirmed by MOT loading and decay curves taken under the modulation of the control laser beam. Optical pumping of the inaccessible Zeeman states into the stretched states is suggested as a possible mechanism.
We have performed continuous slowing of an atomic beam to load a magneto-optical trap (MOT) on axis. To avoid the undesirable effects of the strong slowing laser on the performance of the MOT, a dark spot in the center of the slowing laser beam is used. We varied the spot size and the slowing-laser frequency and intensity to optimize the number of trapped atoms. Our ``dark-spot'' slowing increased the number of trapped atoms by a factor of 30, allowing us to trap $\ensuremath{\sim}{10}^{9}$ atoms in the MOT.