Rubidium and cesium in one fountain: a new tool for the search of the time variation of the fine structure constant

We have built in experiment to perform a test of the stability of the fine structure constant /spl alpha/=e/sup 2///spl nabla/c. This test is based on high accuracy comparisons between atomic microwave frequency standards using different atomic species. We have first developed a /sup 87/Rb fountain with a relative accuracy of 2/spl times/10/sup -15/ that we compare with BNM-LPTF Cs fountains. This comparison leads to a test of the stability of the fine structure constant at the level of 8/spl times/10/sup -15/ yr/sup -1/. We are now developing a dual fountain that will operate with /sup 87/Rb and /sup 133/Cs simultaneously. Using this device, we expect to improve the accuracy of Rb and Cs fountains and bring the resolution of the test of the stability of /spl alpha/ in the 10/sup -16/ yr/sup -1/ range. The Cs part of this device is giving satisfactory preliminary results.