Reaching the 10 −15 accuracy range with a Hg optical lattice clock

Following previous work on neutral atom lattice clocks [1–4], we have presented work regarding a 3rd neutral atomic species, namely mercury. We have carried out absolute frequency measurements of the (6s2) 1S 0 — (6s7s) 3P 0 clock transition frequency in 199Hg against a Cs fountain primary reference and achieved a fractional uncertainty of 5.7 parts in 1015, nearly three orders of magnitude lower than previously measured [5].