Cryogenic optical lattice clocks

The accuracy of atomic clocks relies on the superb reproducibility of atomic spectroscopy, which is accomplished by careful control and the elimination of environmental perturbations on atoms. To date, individual atomic clocks have achieved a 10(-18) level of total uncertainties(1,2), but a two-clock comparison at the 10(-18) level has yet to be demonstrated. Here, we demonstrate optical lattice clocks with Sr-87 atoms interrogated in a cryogenic environment to address the blackbody radiation-induced frequency shift(3), which remains the primary source of systematic uncertainty(2,4-6) and has initiated vigorous theoretical(7,8) and experimental(9,10) investigations. The systematic uncertainty for the cryogenic clock is evaluated to be 7.2 x 10(-18), which is expedited by operating two such cryo-clocks synchronously(11,12). After 11 measurements performed over a month, statistical agreement between the two cryo-clocks reached 2.0 x 10(-18). Such clocks' reproducibility is a major step towards developing accurate clocks at the low 10(-18) level, and is directly applicable as a means for relativistic geodesy(13).