Testing the stability of fundamental constants with the 199Hg+   single-ion optical clock
  Over a two-year duration, we have compared the frequency of the 199Hg+ 5d106s 2S 1/2 (F=0)<-> 5d9 6s2 2D 5/2 (F=2) electric-quadrupole transition at 282 nm with the frequency of the ground-state hyperfine splitting in neutral 133Cs. These measurements show that any fractional time variation of the ratio nu(Cs)/nu(Hg) between the two frequencies is smaller than +/- 7 10^-15 / yr (1 sigma uncertainty). According to recent atomic structure calculations, this sets an upper limit to a possible fractional time variation of g(Cs) m_e / m_p alpha^6.0 at the same level. 
S. Bize, S. A. Diddams, U. Tanaka, C. E. Tanner, W. H. Oskay, R. E.   Drullinger, T. E. Parker, T. P. Heavner, S. R. Jeffert...