Accurate measurements of different transition frequencies between atomic levels of the electronic and hyperfine structure over time are used to investigate temporal variations of the fine structure constant $\alpha$ and the proton-to-electron mass ratio $\mu$. We measure the frequency of the $^2S_{ 1/2}\rightarrow { ^2F_{ 7/2}}$ electric octupole (E3) transition in $^{ 171}$Yb$^+$ against two caesium fountain clocks as $f(E3) = 642\,121\,496\,772\,645.36(25)$~Hz with an improved fractional uncertainty of $3.9\times 10^{ -16}$. This transition frequency shows a strong sensitivity to changes of $\alpha$. Together with a number of previous and recent measurements of the $^2S_{ 1/2}\rightarrow { ^2D_{ 3/2}}$ electric quadrupole transition in $^{ 171}$Yb$^+$ and with data from other elements, a least-squares analysis yields $(1/\alpha)(d\alpha/dt)=-0.20(20)\times 10^{ -16}/\mathrm{ yr}$ and $(1/\mu)(d\mu/dt)=-0.5(1.6)\times 10^{ -16}/\mathrm{ yr}$, confirming a previous limit on $d\alpha/dt$ and providing the most stringent limit on $d \mu/dt$ from laboratory experiments.
Categories: physics.atom-ph