Frequency ratio of two optical clock transitions in $^{171}$Yb$^+$ and   constraints on the time-variation of fundamental constants
  Singly-ionized ytterbium, with ultra-narrow optical clock transitions at 467 nm and 436 nm, is a convenient system for the realization of optical atomic clocks and tests of present-day variation of fundamental constants. We present the first direct measurement of the frequency ratio of these two clock transitions, without reference to a cesium primary standard, and using the same single ion of 171Yb+. The absolute frequencies of both transitions are also presented, each with a relative standard uncertainty of $6\times 10^{-16}$. Combining our results with those from other experiments, we report a three-fold improvement in the constraint on the time-variation of the proton-to-electron mass ratio, $\dot{\mu}/{\mu} = 0.2(1.1)\times 10^{-16}$ year$^{-1}$, along with an improved constraint on time-variation of the fine structure constant, $\dot{\alpha}/{\alpha} = -0.7(2.1)\times 10^{-17}$ year$^{-1}$. 
R. M. Godun, P. B. R. Nisbet-Jones, J. M. Jones, S. A. King, L. A. M.   Johnson, H. S. Margolis, K. Szymaniec, S. N. Lea, K....
physics.atom-ph