Optical lattice clock with neutral mercury

Optical lattice clocks offer the possibility to combine accuracy in the 10
^-18
range together with exquisite short stability, 10
^-16
for a measurement time of 1 second or even better [7]. Clocks with such level of accuracy and stability largely outperform the existing primary frequency standard based on the laser-cooled atomic fountain geometry and on an atomic transition in the microwave domain. Optical clocks allow fundamental physics tests with unprecedented accuracy [11] and open the way to new applications such as Earth gravitation potential mapping [12]. The ultimate limitation to the performance of optical lattice clock is still under investigation. Nonetheless, one limiting systematic shifts is clearly identified already: the blackbody radiation shift, the shift of the clock frequency due to the interaction of atoms with the ambient thermal electromagnetic background. At the temperature of 300 K and in fractional terms, this frequency shift is of the order of -5.5×10
^-15
for strontium (Sr) and -2.6×10
^-15
for ytterbium (Yb). Consequently, this effect must be controlled to much better than the percent level for an accuracy of 10
^-17
, a highly challenging task. One motivation for considering mercury (Hg) is its low susceptibility to blackbody radiation [8]. At 300 K, the corresponding fractional frequency shift is only -1.6×10
^-16
, ~16 times smaller than for Yb and ~34 times smaller than for Sr. Hg is also interesting for its high sensitivity to a putative variation of the fine structure constant. Hg has 7 natural isotopes, 6 of them with abundance above 6%, 2 fermions and 5 bosons, which are all candidates for an optical lattice clock. Using Hg for an optical lattice clock remains however a significant technical challenge given that most of the laser wavelengths necessary to manipulate atoms and to probe the clock transition are in the deep ultraviolet range of the electromagnetic spectrum.