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 α=e²/∇c. This test is based on high accuracy comparisons between atomic microwave frequency standards using different atomic species. We have first developed a ⁸⁷Rb fountain with a relative accuracy of 2×10^-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 8×10^-15 yr^-1. We are now developing a dual fountain that will operate with ⁸⁷Rb and ¹³³Cs 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 α in the 10^-16 yr^-1 range. The Cs part of this device is giving satisfactory preliminary results