Comparison of two single-ion optical frequency standards at the Hertz level

We describe experimental investigations of an optical frequency standard based on a single laser-cooled ¹⁷¹Yb⁺ ion confined in a radiofrequency Paul trap. The electric quadrupole transition from the ²S₁2 /(F=0) ground state to the ²D₃2/(F=2) state at 435.5 nm is used as the reference transition. The transition was resolved with a Fourier-limited width of approximately 30 Hz. In order to investigate the instability and accuracy of a future frequency standard based on the reference transition, a first experiment was done comparing two laser frequencies stabilized individually to two separate ions in different traps. As a result we found a relative frequency instability σ_y(640 s)=1.5·10^-15 for the difference of the two laser frequencies. We also derived a first estimate Θ(²D₃2/)=(3.9±1.9)ea₀² for the electric quadrupole moment of the ²D₃2/ state.