Search for Variations of Fundamental Constants using Atomic Fountain Clocks

Over five years, we have compared the hyperfine frequencies of ^133Cs and ^87Rb atoms in their electronic ground state using several laser-cooled ^133Cs and ^87Rb atomic fountains with an accuracy of 10^-15. These measurements set a stringent upper bound to a possible fractional time variation of the ratio between the two frequencies: ddtln (ₑ₁) / (₂ₒ) = (0. 27. 0) 10^-16 yr^-1 (1 uncertainty). The same limit applies to a possible variation of the quantity (ₑ₁/₂ₒ) ^-0. 44, which involves the ratio of nuclear magnetic moments and the fine structure constant.