Frequency Ratio of Two Optical Clock Transitions in <mml:mmultiscripts>Yb+ 171 </mml:mmultiscripts> and Constraints on the Time Variation of Fundamental Constants

Singly ionized ytterbium, with ultranarrow optical clock transitions at 467 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 <mml:mmultiscripts>Yb⁺ 171 </mml:mmultiscripts> . The absolute frequencies of both transitions are also presented, each with a relative standard uncertainty of 6 ×1 0^-16. Combining our results with those from other experiments, we report a threefold improvement in the constraint on the time variation of the proton-to-electron mass ratio, μ ˙ /μ =0.2 (1.1 )×1 0^-16 yr^-1 , along with an improved constraint on time variation of the fine structure constant, α ˙ /α =-0.7 (2.1 )×1 0^-17 yr^-1 .