Frequency comparisons and absolute frequency measurements of 171Yb+ single-ion optical frequency standards

Abstract

We describe experiments with an optical frequency standard based on a laser cooled ${}^{171}$Yb${}^{+}$ ion confined in a radiofrequency Paul trap. The electric-quadrupole transition from the ${}^2{S}_{1/2}(F=0)$ ground state to the ${}^2{D}_{3/2}(F=2)$ state at the wavelength of 436 nm is used as the reference transition. In order to compare two ${}^{171}$Yb${}^{+}$ standards, separate frequency servo systems are employed to stabilize two probe laser frequencies to the reference transition line centers of two independently stored ions. The experimental results indicate a relative instability (Allan standard deviation) of the optical frequency difference between the two systems of ${\sigma }_y(1000\mathrm{s})=5\cdot 10^{-16}$ only, so that shifts in the sub-hertz range can be resolved. Shifts of several hertz are observed if a stationary electric field gradient is superimposed on the radiofrequency trap field. The absolute optical transition frequency of Yb${}^{+}$ at 688 THz was measured with a cesium atomic clock at two times separated by 2.8 years. A temporal variation of this frequency can be excluded within a $1\sigma$ relative uncertainty of $4.4\cdot 10^{-15}$ yr${}^{-1}$. Combined with recently published values for the constancy of other transition frequencies this measurement provides a limit on the present variability of the fine structure constant $\alpha$ at the level of $2.0\cdot 10^{-15}$ yr${}^{-1}$.

12 pages, 5 figures, Proceedings of MPLP'04, Novosibirsk, August 22.-27., 2004