The mercury-ion optical clock and the search for temporal variation of fundamental constants

The repeated comparison of atomic frequency standards based upon different transitions enables the search for time variation of the fundamental constants that determine the transition frequencies. Over the course of two years we compared the frequency of the ¹⁹⁹Hg⁺ 5d¹⁰6s²S₁2 /(F=0)/5d⁹6s^{2 2}D₅2/(F=2) electric-quadrupole transition at 282 nm with the frequency of the ground-state hyperfine splitting in neutral ¹³³Cs that defines the SI second. These measurements constrain any fractional time variation of the ratioν_Cs/ν_Hg between the two frequencies to be less than ±7×10^-15yr^-1 (1σ uncertainty). According to recent atomic structure calculations, this sets an upper limit to a possible fractional time variation of the product gCs(m_e/m_p)α^6.0 at the same level.