We describe experiments with an optical frequency standard based on a laser
cooled 171Yb+ ion confined in a radiofrequency Paul trap. The
electric-quadrupole transition from the 2S1/2β(F=0) ground state to the
2D3/2β(F=2) state at the wavelength of 436 nm is used as the reference
transition. In order to compare two 171Yb+ 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 Οyβ(1000s)=5β 10β16 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 1Ο relative uncertainty of 4.4β 10β15 yrβ1. 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 Ξ± at the level of
2.0β 10β15 yrβ1.Comment: 12 pages, 5 figures, Proceedings of MPLP'04, Novosibirsk, August
22.-27., 200