The electric quadrupole moment of the 199Hg+ 5d96s22d5/2 state is measured to be (D, 5/2) = -2.98(8) × 10-40 C m2. The value was determined by measuring the frequency of the 199Hg+ 5d106s2S1/2 – 5d96s22d5/2 optical clock transition for different applied electric field gradients. An isolated, mechanically stable optical cavity provides a frequency reference for the measurement. We compare the results with theoretical calculations and discuss the implications for the accuracy of an atomic clock based upon this transition. We now expect that the frequency shift caused by the interaction of the quadrupole moment with stray electric-field gradients will not be limit the accuracy of the Hg+ optical clock at the 10-18 level.
Physical Review Letters
ion, laser, optical standard