Intense continuous magneto-optical source of cold ⁸⁷Rb atoms

In most atomic fountain frequency standards an ensemble of ultra-cold caesium (or rubidium) atoms is prepared by loading a magneto-optical trap (MOT) directly from thermal caesium (or rubidium) atomic vapours. As long as an MOT can capture only atoms with velocities below about 30 m/s, while the mean thermal velocity is about 300 m/s, this way of loading is not very efficient. The typical number of atoms in such a ‘vapour’ MOT is usually below 10⁸ and is limited by the pressure of caesium (or rubidium) thermal vapours, which cannot be much higher than the residual gas pressure inside the vacuum chamber of the fountain. For ⁸⁷Rb atoms the situation is even worse because of the presence of the second, more abundant, isotope ⁸⁵Rb, which is not used in the measurements.

Fig.1: A schematic diagram of the LVIS magneto-optical source of cold atoms

A much more efficient way to load the MOT in a fountain is to use a beam of slow atoms generated from an additional magneto-optical source of cold atoms. The design of such a continuous low velocity intense source (LVIS) of cold atoms^[1] is shown above. It consists of an MOT with a dark channel in it, through which the slow atoms are extracted.  Our LVIS source differs from the standard arrangement mainly in that it uses all independent laser beams, instead of using retro-reflected laser beams. The exact balance of the intensities of the counter-propagating laser beams in such a source essentially increases of the flux of cold atoms^[2]. Our LVIS source is producing a continuous flux of ⁸⁷Rb atoms of 10⁹ atoms per second at a velocity of 8.5 m/s and a total laser power of 60 mW. The loading curve of the main MOT of the rubidium fountain from the LVIS source is shown below. Another major advantage of the source is that it makes it possible to load an optical molasses directly (red curve in Fig. 2), without using the MOT stage of cooling. It has been shown that about 10^{9 87}Rb atoms can be collected in the optical molasses in just 200 ms, a good starting point for an intense rubidium fountain frequency standard.

Fig. 2: The loading curve of the main MOT (black curve) and optical molasses (red curve)
of the Rb fountain from the LVIS source of cold atoms

References

1. Z.T. Lu et al., Phys. Rev. Lett. 77, 3331 (1996).
2. Y. B. Ovchinnikov, Opt. Comm. 249, 473 (2005).