Frequency ratio of two optical clock transitions in 171Yb+ 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 171Yb+. The absolute frequencies of both transitions are also presented, each with a relative standard uncertainty of 6 × 10 -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, mu-dot/mu = 0.2(1.1) × 10-16 yr-1, along with an improved constraint on time variation of the fine structure constant, alpha-dot/alpha = -0.7(2.1) x 10-17 yr-1.